18th, Laser Aided Plasma Diagnostics

Sep 24, 2017, 5:00 PM → Sep 28, 2017, 1:00 PM Europe/Prague

LAPD18 Symposium is organized by Institute of Plasma Physics of the Czech Academy of Sciences. It will take place in Hotel DUO in Prague, Czech Republic from Sunday, 24 September 2017 (evening) to Thursday, 28 September 2017.

The LAPD18 Symposium is the continuation of a biennial series that began at Kyushu University in 1983. It brings together physicists and chemists in diverse areas of laser-based plasma diagnostics including the physics of nuclear fusion, laser physics and low-temperature plasma chemistry and physics. The symposium aims to promote cross-pollination of these fields via fruitful discussion, and covers all diagnostics using electromagnetic waves (lasers and microwaves) applied to fusion plasmas, industrial process plasmas, environmental plasmas, plasmas for medical applications, atmospheric plasmas, plasmas in liquids and other plasma applications. Topics on hardware developments related to laser-aided plasma diagnostics are also welcome. Instrumentation developments related to laser-aided plasma diagnostics also received emphasis in the program.

Sunday, September 24

Registration & Welcome Reception

Monday, September 25

Welcome

Convener: Prof. Daniel Den Hartog (University of Wisconsin-Madison)

Akazaki Lecture

Convener: Prof. Daniel Den Hartog (University of Wisconsin-Madison)

1 Scientific and technical challenges on the road towards fusion electricity

The **European Roadmap to the realisation of fusion energy** breaks the quest for fusion energy into eight missions. For each mission, it reviews the current status of research, identifies open issues, proposes a research and development programme and estimates the required resources. It points out the needs to intensify industrial involvement and to seek all opportunities for collaboration outside Europe. **A long-term perspective on fusion is mandatory** since Europe has a leading position in this field and major expectations have grown in other ITER parties on fusion as a sustainable and secure energy source. China, for example, is launching an aggressive programme aimed at fusion electricity production well before 2050. Europe can keep the pace only if it focuses its effort and pursues a pragmatic approach to fusion energy. With this objective the present roadmap has been elaborated. The roadmap covers three periods: The short term which runs parallel to the European Research Framework Programme Horizon 2020, the medium term and the long term. **ITER is the key facility of the roadmap** as it is expected to achieve most of the important milestones on the path to fusion power. Thus, the vast majority of resources proposed for Horizon 2020 are dedicated to ITER and its accompanying experiments. The medium term is focussed on taking ITER into operation and bringing it to full power, as well as on preparing the construction of a demonstration power plant DEMO, which will for the first time supply fusion electricity to the grid. Building and operating DEMO is the subject of the last roadmap phase: the long term. It might be clear that the Fusion Roadmap is tightly connected to the ITER schedule. A number of key milestones are the first operation of ITER (presently foreseen in 2025), the start of the DT operation foreseen in 2035 and reaching the full performance at which the thermal fusion power is 10 times the power put in to the plasma. **DEMO will provide first electricity to the grid.** The Engineering Design Activity will start a few years after the first ITER plasma, while the start of the construction phase will be a few years after ITER reaches full performance. In this way ITER can give viable input to the design and development of DEMO. Because the neutron fluence in DEMO will be much higher than in ITER (atoms in the plasma facing components of DEMO will undergo 50-100 displacements during the full operation life time, compared to only 1 displacement in ITER), it is important to develop and validate materials that can handle these very high neutron loads. For the testing of the materials a dedicated 14 MeV neutron source is needed. This DEMO Oriented Neutron Source (DONES) is therefore an important facility to support the fusion roadmap The presentation will focus on the strategy behind the fusion roadmap and will describe the major challenges that need to be tackled on the road towards fusion electricity. Encouraging recent results will be given to demonstrate the outcome of the focused approach in European fusion research.

Speaker: Prof. Tony Donné (EUROfusion)

Slides 170925.Donne.LAPD18.pdf

Topical

2 Polarimetric Thomson scattering for high Te fusion plasmas

Polarimetric Thomson scattering (TS) is a technique for the analysis of TS spectra in which the electron temperature $T_e$ is determined from the depolarization of the scattered radiation, a relativistic effect noticeable only in very hot ($T_e$ > 10 keV) fusion plasmas. It has been proposed as a complementary technique to supplement the conventional spectral analysis in the ITER CPTS (Core Plasma Thomson Scattering) system for measurements in high $T_e$, low $n_e$ plasma conditions. In this paper we review the characteristics of the depolarized TS radiation with special emphasis to the conditions of the ITER CPTS system and we describe a possible implementation of this diagnostic method suitable to significantly improve the performances of the conventional TS spectral analysis in the high $T_e$ range. By extensive simulations of the expected measurement errors we investigate the benefit of the inclusion in the detection system of an additional channel designed to measure the unpolarized component of the TS radiation for various ITER scenarios. Then we consider the problem of the calibration of the polarimetric response of this diagnostic and finally we describe a preliminary attempt to observe the depolarization of the TS radiation in the HRTS system of JET.

Speaker: Prof. Leonardo Giudicotti (Padova University - Department of Physics and Astronomy)

10:00 AM Coffee Break

Topical

3 Relativistic electron kinetic effects on laser diagnostics in burning plasmas

Several major laser diagnostics are under development for measurement of plasma density, electron temperature, magnetic field and current density in ITER: toroidal interferometer/polarimeter (TIP), poloidal polarimeter (PoPola) and Thomson scattering systems (TS). Each of them is based on the electron response to laser light propagating through the plasma. We examine the effects of relativistic electron thermal motion on the refractive indices and polarization of high-frequency electromagnetic waves (specifically laser light, both directed and scattered). Two different topics are covered: interferometry/polarimetry (I/P) and polarization of Thomson scattered light, unified by the importance of relativistic (quadratic in $v_{Te}/c$) electron kinetic effects. For I/P applications, rigorous analytical results are obtained perturbatively by expansion in powers of the small parameter $\tau = T_{e}/m_{e}c^{2}$, where $T_{e}$ is electron temperature and $m_{e}$ is electron rest mass. There are two physically different sources of linear in $\tau$ thermal corrections which are comparable in magnitude but contribute with opposite signs: non-relativistic (NR) Doppler-like effects and the relativistic electron mass dependence on the velocity. The relativistic mass effect reduces the magnitude of the NR correction for the Cotton-Mouton effect and changes the signs of the corrections to the interferometric phase and Faraday rotation angle. Experimental validation of the analytical models has been made by analyzing data of more than 1200 pulses collected from high-$T_{e}$ JET discharges. Good agreement with the full relativistic model was demonstrated, and disagreement with the cold plasma and NR models. Based on this validation the relativistic analytical expressions are included in the error analysis and design projects of the ITER TIP and PoPola systems. The linear in $\tau$ model has been recently extended from Maxwellian to a more general class of anisotropic electron distributions to account for distortions caused by equilibrium current, ECRH and RF current drive effects. The polarization properties of incoherent Thomson scattered light are being examined as a method of $T_{e}$ measurement relevant to ITER operational regimes. The theory is based on Stokes vector transformation and Mueller matrices formalism. The general approach is subdivided into frequency-integrated and frequency-resolved applications. For each of them, the exact relativistic solutions are presented in the form of Mueller matrix elements averaged over the relativistic Maxwellian distribution function. These exact analytic calculations are performed without any approximations for the full range of incident polarizations, scattering angles, and electron thermal motion from non-relativistic to ultra-relativistic. Newly obtained results for the frequency-resolved case have been verified by comparison with the numerical code developed by L. Giudicotti and co-authors. The results are in a good agreement (< 0.01% deviations) verifying both calculations. The precise analytic expressions provide output much more rapidly than relativistic kinetic numerical codes. The speed will be critical to the development of real time feedback control of burning plasmas. This work was supported by the U. S. Department of Energy

Speaker: Dr Vladimir Mirnov (Department of Physics, University of Wisconsin-Madison, Madison, WI 53706, USA)

Topical

4 Design and commissioning of the collective Thomson scattering diagnostic at Wendelstein 7-X

The collective Thomson scattering (CTS) diagnostic for ion temperature and MHD measurements is being installed at the Wendelstein 7-X (W7-X) stellarator. W7-X is designed for high β operation, with β approximately around 5%. The target electron density of W7-X plasmas is 1.5$\cdot$10$^{20}$ m$^{-3}$ and the target electron temperature in the center is 8-10 keV. The CTS diagnostic has no fundamental limitations on measuring the ion temperature at the W7-X target plasma parameters. The diagnostic utilizes a 140 GHz heating gyrotron as a source for the probing radiation and uses the ECRH quasi-optical transmission lines both for the probing and receiving beams. The exploitation of the receiver in the ECE range of frequencies is one of the challenges of the diagnostic. The quasi-optical transmission line allows measuring at two toroidal locations with different topologies of the magnetic field. The diagnostic is equipped with steerable mirrors in the bean-shaped cross-section, which allows flexible scattering geometries on the low field side, however the plasma center is not accessible at the standard magnetic field of 2.5 T on axis due to absorption of the probing beam. In the triangular cross-section, remote steering antennas are installed both for the receiver and the probe. These antennas are steerable in one dimension, thus making a beam overlap search possible, as well as ion temperature profile measurements including the plasma center. The number of scattering geometries in the triangular cross-sections is limited because of the steering limitations. The scattering spectra will be analyzed using the electrostatic eCTS code, which is integrated into the Bayesian integrated data analysis framework Minerva. This contribution concentrates on the design aspects and commissioning of the CTS diagnostic at Wendelstein 7-X.

Speaker: Dr Dmitry Moseev (Max-Planck-Institut für Plasmaphysik)

Oral

5 Hot and dense plasma probing by soft X-ray lasers

Soft X-ray lasers, due to their short wavelength, high brightness, and good spatial coherence, are excellent sources for the diagnostics of dense plasmas (up to $10^{25}$ cm$^{-3}$) which are relevant to e.g. inertial fusion. We will present several techniques and experimental results obtained at the QSS collisionally pumped 21.2 nm neon-like zinc laser installed at PALS Recearch Center among them plasma density measurement by a double Lloyd mirror interferometer, deflectometer based on Talbot effect, measuring plasma density gradients itself, with a following ray tracing postprocessing. Moreover, the high spatial resolution (~nm scale) plasma images can be obtained when soft X-ray lasers are used.

Speaker: Miroslav Krus (Institute of Plasma Physics of the CAS)

Slides krus@LAPD.pdf

Oral

6 EAST kinetic equilibrium reconstruction combining with Polarimeter-Interferometer internal measurement constraints

Plasma equilibrium reconstruction plays an important role in the tokamak plasma research. With a high temporal and spatial resolution, the POlarimeter-INTerferometer (POINT) system on EAST has provided effective measurements for both the 60 s H-mode operation and $10^2$ s long pulse discharge. Based on internal Faraday rotation measurements provided by the POINT system, the equilibrium reconstruction with a more accurate current profile constraint has been demonstrated successfully on EAST. Experimental diagnostics on EAST also provided the kinetic equilibrium which includes the information of current profile, safety factor profile and pressure profile. Take the pressure information from kinetic EFIT into the equilibrium reconstruction with Faraday rotation constraint, the new equilibrium reconstruction not only provides a more accurate internal current profile but also contains other experimental diagnostics information. It is found that the new equilibrium reconstruction shows a great consistent with sawtooth phenomena and includes the pedestal structure for H-mode plasma, which the magnetic reconstruction doesn’t satisfy. The new improved equilibrium reconstruction is greatly helpful to the future theoretical analysis.

Speaker: Mr Hui Lian (Institute of Plasma Physics, Chinese Academy of Sciences)

Slides 18th_LAPD_lianhui_final.pdf

General

7 Design Advances of the Core Plasma Thomson Scattering Diagnostic for ITER

The Core Plasma Thomson Scattering (CPTS) diagnostic on ITER performs measurements of the electron temperature and density profiles which are critical to the understanding of the ITER plasma. The diagnostic must satisfy the ITER project requirements, which translate to requirements on performance as well as reliability, safety and engineering requirements. The implications of these requirements are particularly challenging for beam dump lifetime, the need for continuous active alignment of the diagnostic during operation, the requirement for low neutron flux through the diagnostic drawer and the protection of the first mirror from plasma deposition. The CPTS design has been evolving over a number of years. One recent improvement is that the collection optics have been modified to include new freeform surfaces. These freeform surfaces introduce extra complexity to the manufacturing of the optics but provide greater flexibility in the design. The greater flexibility introduced allows for example to lower neutron throughput or use fewer surfaces while conserving optical performance. Compared with existing TS systems, the CPTS diagnostic has a relatively small collection solid angle, long scattering lengths, high laser energy with low integration time. Performance assessment have shown that scattering from a 1064 nm laser will be sufficient to meet the measurement requirements, at least for the system at the start of operations. The optical transmission in the lower wavelength region, below 600 nm, is expected to degrade over the ITER lifetime due to neutrons and deposition on the first mirror. For this reason, it is proposed that the diagnostic shall additionally include measurements of TS ‘depolarised light’ and a 1318 nm laser system. These additional techniques have different spectral and polarisation dependencies compared to scattering from a 1064 nm laser and hence these additional measurements introduce greater robustness into the inferred measurements of electron temperature and density.

Speaker: Dr Rory Scannell (UKAEA / CCFE)

12:30 PM Lunch

General

Convener: Prof. Tony Donné (EUROfusion)

8 Laser Aided Diagnostic Systems in ITER

Laser Aided diagnostics play a large role in ITER. This extends from basic control, to advanced control to the physics understanding. As ITER is the first Tokamak ever to be built that that is a Nuclear Facility, it brings with it a large range of challenges. Typically, lasers are used in ITER for several key areas. These include Thomson scattering, line averaged density Interferometers, line averaged density - Interferometer-Polarimeters, internal q-profile Polarimeters, plasm current polarimeters, Erosion monitoring and Tritium monitoring. Lasers are further used to monitor the in-vessel as well as a very important use in the alignment of all the systems. One thing that differentiates ITER and hence the techniques used from those deployed in the past is the fact that the systems have to be intelligent and find ways to self-calibrate and self-align. This is demanding that the design teams are pushing the boundaries of the current designs to reach the level needed for this modern and demanding Tokamak. So far, the ITER team which is extended across the world has been developing many systems to address the above challenges. These include a self-aligning, fringe-hop eliminating scheme for the critical line average density measurements. These also include Thomson Scattering systems that can identify where they are pointing and react to the alignment in real time just to mention but a few. There is also a need to develop interpretation algorithms that take in to account the new phenomena that will be measured as well as redundancy in the algorithms that are used for the critical measurements. One particular issue that is shared amongst all these systems at different levels is the issue of the First mirror. To avoid neutrons escaping the confining boundary, mirrors are needed and these bring with them challenges. These challenges have and are still being addressed with excellent development in recent years. While these systems have undergone already a significant amount of development, there is still a need to push the boundaries further and ensure that the operation of ITER will be made successful by the quality of the systems that provide the important measurements. In this presentation, an outline of the different systems and approaches as well as some areas that need to be explored will be presented. The views and opinions expressed herein do not necessarily reflect those of the ITER Organization.

Speaker: Dr Michael Walsh (ITER Organization, CS 90 046 - 13067 St Paul Lez Durance Cedex, France)

Slides LAPD-Prague-2017-Presentation-MJW_3_VGD3JC_as_presented_final1.pdf

Topical

9 High spatial resolution and dual laser wavelength Thomson scattering system for W7-X stellarator – lessons learned for ITER

Thomson scattering (TS) is one of the main diagnostics in nuclear fusion experiments for electron temperature and –density measurements. Most of the TS systems are realized with pulsed, high power Nd:YAG lasers as light source with a wavelength at λ=1064 nm, together with interference filter polychromators for spectral analysis of the scattered light in the near infrared region (typically between 700-1060 nm). Such a system is able to measure electron temperatures up to approximately 15 keV within error bars of 10%. At higher electron temperatures the broadness together with the relativistic blue shift of the spectrum are the causes of inadmissible error bars. Due to background radiation (line emission and Bremsstrahlung) it is not advisable to extend the interference filters of the polychromators to much shorter wavelengths. As an alternative method an additional Nd:YAG laser with λ=1319 nm, fired directly after the λ=1064 nm Nd:YAG laser, which can be used to measure the scattered light from both lasers with the same polychromator, effectively increasing the observed spectral range. Such a TS system is able to measure electron temperatures up to 25-30 keV. We present the two laser wavelength approach for the Thomson scattering system at the Wendelstein 7-X stellarator. The status of the 1319 nm Nd:YAG laser development and the design of optical components for both high power laser wavelengths (λ=1064 nm and λ=1319 nm) plus a coating for a λ=635 nm adjustment laser are shown.

Speaker: Dr Ekkehard Pasch (Max-Planck-Institut für Plasmaphysik)

Oral

10 LIDAR TS for ITER core plasma - Part 1: Layout and Hardware

The original time-of-flight design of the Thomson scattering diagnostic for the ITER core plasma has been given up by ITER. Summarising the reasons given: “there is no laser, there is no detector, and there is no beam dump”. In this paper we show that these claims, regarding the hardware of the proposed LIDAR TS system, are not valid. The optical layout of the front end has been changed only little in comparison with the latest one considered by ITER. Its labyrinth fulfils the shielding requirements. It is characterised by a very much recessed first mirror. The main change is that it offers an optical collection without any vignetting over the low field side. The full optical layout from laser to detector is described. The entire collection system is a large relay system with no refractive elements inside the port plug. The throughput of the system is defined only by the size and the angle of acceptance of the detectors. This in combination with the fact that the LIDAR system uses only one set of spectral channels for the whole line of sight means that no absolute calibration using Raman scattering from a non-hydrogen isotope gas fill of the vessel is needed. Alignment of the system is easy since the collection optics view the footprint of the laser on the inner wall. As it is the purpose of this paper to show the technological feasibility of the LIDAR system, the above criticized hardware is considered here. To complete the picture, there are two posters in addition to this talk. In Part 2 we demonstrate by numerical simulations that the accuracy of the measurements as required by ITER is maintained throughout the given plasma parameter range. It is shown that the lack of detectors sensitive at wavelengths longer than 900 nm is no problem. In addition, the effect of enhanced background radiation in the wavelength range 400 nm – 500 nm is considered. In Part 3 the recovery of calibration in case of changing spectral transmission of the front end is treated. We also investigate how to improve the spatial resolution at the plasma edge. In this paper we use, simultaneously, two different wavelength pulses from a Nd:YAG laser system Its fundamental wavelength ensures measurements of 3 keV up to more than 40 keV, whereas the injection of the harmonic enables measurements of low temperatures. In this paper we show that this laser source is no show-stopper. The required fast, large sensitive area, MCP photomultipliers are commercially available already now. The use of these detectors together with the described laser source ensures the spatial resolution for the core plasma as specified by ITER. Since the system described is a time-of-flight system, reflection from the beam dump does not hamper the measurements. The beam dump mainly serves as protection of the inner wall against laser damage. We describe situations where we make use of the beam dump development for the ITER edge TS system, and situations where the beam is dumped directly on an inner wall tile.

Speaker: Hans Salzmann (Retired from MPI f. Plasmaphysik, Garching)

Slides Prague_Talk_v.5.pdf

Poster Session #1 Introduction

11 Estimation of reliable range of electron temperature measurements with sets of given optical band-pass filters for KSTAR Thomson scattering system based on synthetic Thomson data

Reliabilities of electron temperature measurements based on Thomson scattering (TS) systems depend on transmittance of the optical band-pass filters in polychromators. We investigate the system performance as a function of electron temperature to determine reliable range of measurements. To achieve the objective, we build a forward model of the KSTAR TS system, and generate synthetic TS data with the prescribed electron temperature and density profiles. The prescribed profiles are compared with the estimated ones to evaluated the system performance.

Speaker: Prof. Young-chul Ghim (Korea Advanced Institute of Science and Technology)

summary IntroductorySlide_KHKim.pdf

12 Spectroscopic Studies of Silicon Plasma Produced by an Nd: YAG Laser

Spectroscopic studies of silicon (Si) plasma produced by the fundamental (1064 nm) and second (532 nm) harmonics of an Nd: YAG laser by placing the target material in open air at atmospheric pressure are presented.. The electron temperature measurements have been estimated using the Boltzmann plot method. The electron temperature as a function of laser irradiance (2 x $10^{10}$ to 6.5 x $10^{10}$ W/cm$^2$) ranges from 11290 to 14120 K (for fundamental), and 11660 to 13750 K (for second) laser respectively. The electron number density is calculated using Stark broadening profile of 288.15 nm (3p4s 1P1 → 3s23p2 1D2)transition. The electron number density as a function of laser irradiance ranges from 2.20 x $10^{16}$ to 1.55 x $10^{17}$/cm$^3$ (for fundamental) and 1.09 x $10^{16}$ to 1.80 x $10^{17}$/cm$^3$ (for second) laser respectively. The spatial distribution of electron temperature and electron number density shows same decreasing trend up to 2.0 mm from the target surface (0.05 mm). It is observed that electron temperature and number density increases as laser irradiance is increased. In addition inverse bremsstrahlung absorption coefficient has been estimated. Apart from that variation of these plasma parameters (electron temperature and number density) along-with variation of distances in the direction of propagation of plasma plume have also been investigated. The plasma parameters calculated based on this research work are within the range already reported in the literature.

Speaker: Dr Hanif Muhammad (National University of Sciences & Technology, Islamabad, Pakistan)

Slides Slide_M_Hanif.pdf Slide_M_Hanif.pptx

13 High resolution Thomson scattering on the COMPASS tokamak - optimisation of data processing

Due to the character of the Thomson scattering (TS) process the intensity of the effective collected signal is reduced by several orders of magnitudes against the probing light source intensity. Thus, precise spectral and absolute calibrations together with optimised data processing are indispensable in order to provide reliable and accurate plasma diagnostic. Since the installation of the TS system on the COMPASS tokamak commercial programming platforms were used for the calculations. New set of routines based on the open source programming language Python is being tested and introduced as a standard tool for performed calibration and TS data processing. Mitigation of stray light impact and optimization of the routines for fitting the detected scattered signal are the main issues proposed in the new method. Comparison of existing and newly developed processing systems is presented.

Speaker: Mr Miroslav Šos (Institute of Plasma Physics of the CAS, Czech Republic)

Poster [Sos]_(2017)_LAPD.pdf

summary VU_final_presentation_18-9-2017_v3.pdf

14 Progress of Thomson scattering diagnostic on HL-2A tokamak

In recent two years, some efforts have been made to promote the performance of incoherent Thomson scattering (TS) diagnostic on HL-2A tokamak. Step motors are used to adjust the reflecting mirrors and focusing lens of the input laser beam optics, by which it is easy to control the laser beam pass through the narrow throats of the lower and upper closed divertors. Spectral calibration has been refined. Hardware of Si-APD detector electronics is improved, which provides two output signal channels. In one channel, only the fast TS signal is output after deducting the influence of plasma light. In the other, both the fast TS signal and the background signal of slow-varying plasma light are output. In this 2017 experiment campaign, the new developed electronics are tested and TS signals can be obtained from the two channels, which are digitized by 1GS-12bit transient recorders. In data processing, the TS pulse shape is fitted with different functions output from the two different channels. The statistical estimation of Te data is also optimized. More channels of fast digitizers and more positions of Te and ne measurement are planned and are in constructions.

Speaker: Dr Yuan Huang (Southwestern Institute of Physics)

Slides 2017LAPD-TS_Huang.pdf

15 A pulse-burst laser system for Thomson scattering on NSTX-U

A pulse-burst laser system has been built for Thomson scattering on NSTX-U, and is currently being integrated into the NSTX-U Thomson scattering diagnostic system. The laser is Nd:YAG operated at 1064 nm, *q*-switched to produce ≥1.5 J pulses with $\sim$20 ns FWHM. It is flashlamp pumped, with dual-rod oscillator (9 mm) and dual-rod amplifier (12 mm). Variable pulse-width drive of the flashlamps is accomplished by IGBT (insulated gate bipolar transistor) switching of electrolytic capacitor banks. Direct control of the laser Pockels cell drive enables optimal pulse energy extraction. The laser will be operated in three modes. The specified base mode is continuous 30 Hz rep rate, and is the standard operating mode of the laser. The base mode will be interrupted to produce a “slow burst” (specified 1 kHz rep rate for 50 ms) or a “fast burst” (specified 10 kHz rep rate for 5 ms). Laser pulsing is halted for a set period (a few minutes) following a burst to allow the YAG rods to cool; this type of operation is called a heat-capacity laser. The laser system has demonstrated compliance with all specified operating modes, and is capable of exceeding design specifications by significant margins, e.g., higher rep rates for longer burst periods. Burst operation of this laser system will be used to capture fast time evolution of the electron temperature and density profiles during events such as ELMs, the L-H transition, and various MHD modes. The material in this presentation is based upon work supported by the U.S. Department of Energy, Office of Science, Office of Fusion Energy Sciences under Award Number DE-SC0015474.

Speaker: Prof. Daniel Den Hartog (University of Wisconsin-Madison)

Poster Den_Hartog_LAPD2017_poster_20170818.pdf

summary Den_Hartog_LAPD2017_poster_slide_20170907.pdf

16 Upgrade to resolution of TCV Thomson scattering diagnostic system and performance analysis

The recently completed MST2 upgrade to the Thomson scattering (TS) system on the TCV Tokamak at the Swiss Plasma Center at EPFL was performed. This upgrades aim was to provide an enhanced spatial and spectral resolution while maintaining the high level of diagnostic stability that is required for physics studies of TCV plasmas. In order to improve the spatial resolution of the system, there were two primary changes to the existing TS that were performed: The addition of 40 new compact 5-channel spectrometers and a modification of the current fiber optic design. The complete redesign of the fiber backplane and installation of all new fiber optics allow for the removal of the observational gaps present in the previous design. This redesign of the fibers plus the increase in total spectrometers from 47 to 89, results in an improved overall spatial resolution of 12 mm in the core and 6 mm at the edge. This new design incorporates fewer larger diameter fibers and thus results in a decrease in the Signal to Noise ratio of approximately 1.25 in the core and 2.0 at the edge. Additional upgrades to acquisition systems, namely the gate integration, keeps the SNR similar to that of the post upgrade system. The 40 new spectrometers added to the system are designed to cover the full range of temperatures expected in TCV, able to measure electron temperatures ($T_e$) with high precision between (10 eV and 20 keV). The design of these compact spectrometers stems originally from the design of the MAST TS system, containing five sets of interference filters and detectors that facilitate the mentioned temperature coverage. Of these 40 spectrometers 20 are with two fibers and 20 with one fiber, with the overall layout designed to achieve a spatial resolution of $\sim 1$ % of the minor radius, i.e. $\sim 2.5$ mm, in the outer magnetic mid-plane in the H-mode pedestal region. These spectrometers greatly enhance the diagnostic spectral resolution, especially at the plasma edge, now with 52 spectrometers that measure to sub 10 eV electron temperatures, up from 12 spectrometers. These additional spectrometers allow for a much greater diagnostic flexibility, designed to allow for quality full Thomson profiles for $\sim 75$ % of TCV plasma configurations. In a similar timeline there was a second parallel upgrade taking place, in which old 4-channel spectrometers has the electronics refurbished, implementing a new gate integration system. The full upgrade and commissioning of the system was completed in May 2017 in time for the MST1 (Medium Sized Tokamak) campaign as part of EuroFusion. All the effort that went into this upgrade has resulted in a well performing system with enhanced capabilities. The work presented here will highlight the improved performance of the system as well as its impact on physics studies on TCV. *This work was supported in part by the Swiss National Science Foundation. This work has been carried out within the framework of the EUROfusion Consortium and has received funding from the Euratom research and training programme 2014-2018 under grant agreement No 633053. The views and opinions expressed herein do not necessarily reflect those of the European Commission.*

Speaker: Dr Joshua Hawke (Swiss Federal Institute of Technology (Swiss Plasma Center))

Poster LAPD_2017_poster_jhawke.pdf

Slides LAPD_Slide_Jhawke.pdf

17 Calibration of the Thomson scattering diagnostic system on VEST

A Nd:YAG laser Thomson scattering (TS) system has been developed on Versatile Experiment Spherical Torus (VEST) for measurement of the electron temperature and density of the core plasmas. To overcome the limitations of relatively low laser energy of 0.6 J/pulse for measuring low electron density of $\sim10^{18}$ m$^{-3}$, the system is delicately designed to improve a signal-to-noise ratio by using high transmission optical components. For effective collection of the scattered photons, the collecting lens have been designed for matching to detectors and the optical fiber of large etendue. Additional components for reducing the stray light and filtering H-alpha emission have been installed since the large background signal comparable to TS signals influence on reliable measurement. In addition, a synchronous circuit has been developed to reset the oscillation timing of the laser to synchronize the 10 Hz repetition rate with the VEST plasma whose pulse duration is $\sim 20$ms. An absolute calibration for the system was performed by Rayleigh scattering in various pressures of nitrogen. The electron temperature and the electron density are deduced from the TS signals using Bayesian statistics with assumption that the random noise follows the Gaussian distribution. The tendency of the measured electron properties are compared with the properties measured by triple Langmuir probes.

Speaker: Mr Young-Gi Kim (Seoul National University)

Slides Introductory_YGKIM.pdf

18 High resolution Thomson scattering on the COMPASS tokamak - extending edge plasma view and increasing repetition rate

The Thomson scattering (TS) diagnostic system on the COMPASS tokamak consists of separate collection optics for core and edge plasma. Till now, the adverse orientation of the viewport available for the edge TS limited the investigated plasma scenarios to those, where H-mode plasma pedestal was possible to be observed. The vacuum vessel port has been recently modified and directed more to the plasma edge. To bridge the time for designing and manufacturing a new lens, the so far used edge TS collection lens was tilted and the fibre bundles rearranged to comply with the new geometry. However, the full potential of the new port is utilized only by the newly designed and manufactured lens. Comparison of performance of the collection lenses before and after the viewport modification is presented. Apart from the edge collection system upgrade, the laser system was extended by two lasers with parameters similar to the original two, thus increasing the repetition rate of the TS system to 120 Hz or allowing to obtain four profiles with tight timing in order to measure fast transient events in the plasma.

Speaker: Petr Böhm

Poster [Bohm]_(2017)_LAPD.pdf

summary Bohm_LAPD2017_intro.pdf

19 Research of Fast DAQ system in KSTAR Thomson scattering diagnostic

Thomson scattering diagnostic is most important diagnostic system in fusion plasma research. This diagnostic system gives reliable electron temperature and density profiles in magnetically confined plasma. In KSTAR tokamak, Q-switched Nd:YAG Thomson system was installed several years ago to measure the electron temperature and density profiles. For the KSTAR Thomson scattering system, a Charged to Digital Conversion (QDC) type data acquisition system was used to measured pulse type Thomson signal. However recently some error was found during the Te, ne calculation, the reason was that the QDC system was integrate the pulsed Thomson signal which include a stray light like signals. To overcome this like errors, a Fast data acquisition (F-DAQ) system was introduced and tested. For this, CAEN V1742 5 GS/s was used which is VME type 12 bit Switched Capacitor Digitizer with 32 channels. In this experiment, the $T_e$ calculated results of Thomson scattering data measured simultaneously using QDC and F-DAQ were compared. For the F-DAQ system, the shape of the pulse was restored by fitting.

Speaker: Dr Jong-ha Lee (NFRI)

Poster 2017LAPD18_JHLEE_V1.pdf

Slides P_JHLee.pptx

20 LIDAR TS for ITER Core Plasma, Part 2: Simultaneous Two Wavelength LIDAR TS

We have shown recently, and in more detail at this conference (Salzmann et al) that the LIDAR approach to ITER core TS measurements requires only two mirrors in the inaccessible port plug area of the machine. This leads to simplified and robust alignment, lower risk of mirror damage by plasma contamination and a much simpler calibration procedure, compared with the awkward and vulnerable optical geometry of the conventional imaging TS approach, currently being adopted by ITER. In the present work we have extended the simulation code used previously to include the case of launching two laser pulses, of different wavelengths, simultaneously in LIDAR geometry. The aim of this approach is to broaden the choice of lasers available for the diagnostic. In the simulation code it is assumed that two short duration (300 ps) laser pulses of different wavelengths, from an Nd:YAG laser are launched together through the plasma. From the resulting combined scattered signals in the different spectral channels of the single spectrometer, the temperature and density profiles are deduced in the usual way. The spectral response and quantum efficiencies of the detectors used in the simulation are taken from catalogue data for commercially available Hamamatsu MCP-PMTs (types R3809U-61/-63/-64). The response times and gating properties of this type of photomultiplier have already been demonstrated in the JET LIDAR system. Here we present the new simulation results from the code. These results demonstrate that when the detectors are combined with the two laser wavelength approach, the full range of the specified ITER core plasma Te and ne can be measured with sufficient accuracy and spatial resolution. They also demonstrate that a suitable LIDAR system can be made with commercially available detectors combined with fundamental and 2$^{nd}$ harmonic outputs of the well developed Nd:YAG laser. In fact, with the addition of SBS compression, the laser could be very similar to that currently being used in the conventional ITER TS designs. The design of the conventional core TS system that ITER is currently pursuing is well suited to somewhat smaller plasma machines with good regular access. The LIDAR TS system on the other hand, was originally developed for JET, the largest and by comparison a rather inaccessible fusion machine. The JET system has been working successfully for 30 years and has also coped well with the increased neutron fluxes encountered in the Deuterium-Tritium campaigns. LIDAR is much better suited to the large machine environments to be encountered on ITER and DEMO. The versatility of the LIDAR system is further demonstrated in the paper by Nielsen et al at this conference.

Speakers: Mr Chris Gowers (Retired from JET, CCFE, Culham, Oxon, UK), Dr Hans Salzmann (Retired from MPI f. Plasmaphysik), Dr Per Nielsen (Retired: Consorzio RFX, Padova Italy)

Poster PN

Slides LIDAR_TS_slide_for_Part_2_.pptx

21 LIDAR TS for ITER Core Plasma, Part 3: Calibration and Higher Edge Resolution

Calibration, after initial installation, of the proposed LIDAR Thomson Scattering System requires no access to the front end and does not require a foreign gas fill for Raman scattering. As already described (Salzmann et al), the variation of solid angle of collection with scattering position is a simple geometrical variation over the unvignetted region. The additional loss over the vignetted region can easily be estimated and in the case of a small beam dump located between the Be tiles, it is within the specified accuracy of the density. The only calibration needed in the course of the ITER lifetime is the absolute spectral transmission of the front optics. With time, we expect the transmission of the two front mirrors to deteriorate. The reduction is likely to be worse towards the blue end of the scattered spectrum. By routinely comparing a calculated line integral from the measurements with the line integral measured by interferometry, we will see the first indications of such a drop in transmission. In low temperature plasmas, 3 – 5 keV, the fit will exhibit a variation with scattering position of the chi-square value. In this temperature range, the combined spectrum from the two-wavelength approach offers a means to determine the spectral variation of the transmission loss. At the outer plasma boundary, the standard resolution of the LIDAR system is not sufficient to determine the edge gradient in an H-mode plasma. However, because of the step like nature of the signal here, it is possible to carry out a deconvolution of the scattered signals, thereby achieving an effective resolution of $\sim 1$ cm in the outer 10 – 20 cm of the plasma. It is possible to get even better resolution at the edge by both shortening the laser pulse width and using faster detectors. Faster detectors exist with lower quantum efficiency. Combined with a shorter laser pulse width it is possible to have a spatial resolution of only $\sim 3$ cm. Deconvolution at the edge may still be applied in this case, resulting in a resolution of the edge gradient of $\sim 0.5$ cm. This approach will obviously decrease the signal to noise level slightly. In case this is a problem for the central plasma, one may bin the signals at the centre to maintain the required 7 cm resolution. Note please, that the LIDAR design allows changes of lasers, detectors and spectrometer as new technology develops.

Speakers: Mr Chris Gowers (Retired from CCFE, Culham Science Centre, Abingdon, Oxon, OX14 3DB, UK), Dr Hans Salzmann (Retired from MPI f. Plasmaphysik, Garching), Dr Per Nielsen (Retired: Conzorzio RFX, Padova Italy)

Poster PN

Slides PN

22 Additional Doppler shift in absorption spectrum obtained by laser absorption spectroscopy using optical vortex beams

We have been developing a new laser spectroscopy technique using a Laguerre-Gaussian (LG) beam, which is known as an optical vortex. One of the characteristics of optical vortices is helical equiphase surfaces associated with orbital angular momentum of light. Existing techniques using a plan-wave laser beam are insensitive to perpendicularly crossing motion of atoms, since they rely on the axial Doppler effect. However, this limitation is possible to overcome by using a LG beam, because its three-dimensional phase structure gives rise to three-dimensional Doppler effect [1]. This additional Doppler effect is proportional to the azimuthal velocity component of atoms and to the topological charge of the LG beam and inversely proportional to the distance from the beam axis in which the beam intensity banishes due to phase singularity. For example, the additional frequency shift is 10 MHz under the conditions that the topological charge is one, the distance from the singularity 100 $\mu$m and azimuthal velocity 100 m/s. Therefore, the additional frequency shift inhering in the absorption spectrum depends on positions in the beam cross-section. In order to detect this subtle frequency shift, we have performed laser absorption experiments for metastable argon atoms in an ECR discharge plasma using a LG beam produced by a spatial light modulator or by a q-plate. With scanning the laser frequency, images of the transmitted beam intensity were recorded by a beam profiler. Obtained frequency shift in absorption spectra qualitatively agreed with theoretical azimuthal Doppler shift in LG beams but not quantitatively. Pros and cons as well as the present status and problems of the absorption spectroscopy using an optical vortex beam will be reported. [1] L. Allen, M. Babiker, W. L. Power, Opt. Commun. 112, 141-144 (1994).

Speaker: Dr Shinji Yoshimura (National Institute for Fusion Science)

Poster LAPD2017_Poster.pdf

summary preposter.pdf

23 Spatially resolved absorption spectroscopy on microhollow cathode plasmas with noble gases using vertical-cavity surface-emitting lasers

Atmospheric-pressure plasmas have been attracting much interest because of their possibility for various applications, such as surface treatment, nano-particle formation and pollution gas processing [1]. One of the methods to generate high-pressure plasmas at moderate voltage is a microhollow cathode discharge. The gas temperature and electron density of microhollow cathode plasmas have been measured with emission spectroscopy [2]. In comparison with the emission spectroscopy, laser absorption spectroscopy is nearly free from spectral resolution. Since the spectral line width at atmospheric pressure broadens over several tens of GHz in full width at half maximum mainly due to the pressure broadening, we used a vertical-cavity surface-emitting Laser diode, which can scan the light frequency over several hundreds of GHz. We measured the absorption spectra of the helium 1s2p(^1P) -1s3d(^1D) transition with the spatial resolution of 0.030 mm in the Rayleigh criterion for the 0.3 mm diameter microhollow cathode helium plasmas [3]. The gas pressure ranged from 10 to 100 kPa. For the pressure below 20 kPa, the observed spectra near the electrode were asymmetric. By fitting the observed spectra with Voigt functions including the DC Stark effect, we evaluated the electric field strength, 1s2p(^1P) atom density, gas temperature and electron density, and then made two-dimensional maps of these quantities [3]. Since clear asymmetry was not detected in the observed spectra for the pressure over 20 kPa, we made two-dimensional maps of the 1s2p(^1P) atom density, gas temperature and electron density. We also performed such absorption spectroscopy of the argon 1s_5 – 2p_6 transition (Paschen notation) at for microhollow cathode argon plasmas. This work was partly supported by JSPS KAKENHI Grant-in-Aid for Challenging Exploratory Research (No.15K13607). [1] D. Mariotti, R. M. Sankaran, J. Phys. D: Appl. Phys. 44, (2011) 174023. [2] S. Namba, T. Yamasaki, Y. Hane, D. Fukuhara, K. Kozue, K. Takiyama, J. Appl. Phys. 110, (2011) 073307. [3] K. Torii, S. Yamawaki, K. Katayama, S. Namba, K. Fujii, T. Shikama, M. Hasuo, Plasma and Fusion Res. 10, (2015) 3406063.

Speaker: Prof. Masahiro Hasuo (Kyoto University)

summary LAPD18_Hasuo.pdf

24 154 GHz Collective Thomson Scattering in LHD

Collective thomson scattering (CTS) is a powerful diagnostic to measure the bulk and fast ion tempertures, densities and ion density ratios A 77 GHZ CTS system has been developed in LHD with a 1 MW gyrotron for electron heating and succesful results have been obtaned [1,2,3]. Use of the frequency of 77 GHz has an advantage of strong signal but suffers from refraction, which makes adjustment of the scattering volume difficult. In addition, multiple scattering occurs due to the wave reflected off the vaccume chamber wall. Since the multiple scattering makes interpretation of the signal difficult, it shoudl be elminated or mimized [4]. In order to solve these problems, a new CTS system with a 1 MW 154 GHz gyrotron for electron heating has been developed and installed in LHD. Both 77 and 154 GHz CTS systems are usualy operated with the electron cyclotron reasonance layer inside the plasma. Electron cyclotron emmision (ECE) from the resonance layer becomes a huge noise source. In particular, the ECE around 154 GHz becoems strong at 2.75 T-operation of LHD. ECE noise can be elminated by power modulation in principle, however, too strong ECE causes saturation of the detectors and amplifiers. To reduce ECE noise, LHD is operated at 1.375 T corresponding to 4th harmonic cyclotron resonance. Preliminary data at 1.375 T showed good signal to noise ratio and the scannable injection antenna was suscessfuly adjusted to obtaine the maximum scattering volume. Detailed spectrum analysis will be presented. 1. S. Kubo et al, Rev. Sci. Instrum. 81, (2010) pp. 10D535-1-10D535-6 2. M. Nishiura et al, Nucl. Fusion 54 (2014) 023006. 3. K. Tanaka et al, Journal of Instrumentation, 10 ,(2015),C12001 4. S. Kubo et al, Journal of Instrumentation, 11 ,(2016),C06005

Speaker: Prof. Kenji Tanaka (National Institute for Fusion Science)

25 Measurement of the depolarized Thomson scattering radiation in JET

The depolarisation of the Thomson scattering (TS) radiation, a relativistic effect noticeable only in very high $T_e$ plasmas and never before observed in a fusion machine, is measured for the first time in JET. This depolarisation effect is at the basis of polarimetric TS, an alternative method for the measurement of the plasma electron temperature, suitable for high $T_e$ plasmas such as in ITER. In our test, we employed unused fibres, at the fringe of the JET HRTS system, to detect both polarisation components of the TS radiation in high Te discharges, with measured $T_e$ up to 8 keV. Polarisers perpendicular to the laser light were placed in front of a fibre pair observing a region close to the plasma core, with another adjacent fibre pair used as a control with no polariser. The intensity ratio of the unpolarised component to the full signal is found to have an approximately linear relationship with the electron temperature, as expected from the theory. The measured intensity ratio is consistent with the theory, taking into account sensitivity coefficients of the two measurement channels determined with post-experiment recalibrations and Raman scattering. As polarimetric TS is not dependent on wavelength it can provide significant advantages as an alternative to the conventional TS for measurements at high $T_e$, and may be integrated into a standard TS system.

Speaker: Mr Oisin McCormack (RFX - University of Padova)

26 Thomson scattering upgrade in RFX-mod: Alternative designs for Injection and Collection Optics

RFX-mod RFP is now under shutdown and it is undergoing a deep modification in the plasma magnetic boundary, with changes in the vacuum vessel, first wall and stabilizing shell. A diagnostics upgrade is also foreseen; in particular, an improvement in the signal level of the main Thomson Scattering Diagnostics becomes crucial for the diagnostic operation in low density RFP and tokamak discharges. Launching and reflecting optics for a double passage of the laser beam in the equatorial plane: a retroreflector is needed, as well as a manipulator for retroreflector substitution and precise positioning. Analysis of the expected energy density on the optics is presented; stray light generation, positioning issues and optical damage risk are examined. Moreover, beam retro-reflection allows in principle re-injection in the edge Thomson Scattering system, lying in another toroidal position; this possibility is analyzed focusing on the reflected beam profile and pointing stability. Alone, laser beam recirculation can provide a doubling in the signal level, while a further 30% gain can be obtained maximizing the collection optics transmission: an alternative, simplified design featuring a 3 elements objective is presented and discussed, the major drawback being a non-planar image surface.

Speaker: Alessandro Fassina (Consorzio RFX-CNR)

Paper lapd_17_pap_unrev.pdf

Poster lapd_17_poster.pdf

Slides pres_35_fassina.pdf

27 Demonstration of dual-laser based Polarimeter-Interferometer scheme for simultaneous measurements of Faraday angle and electron density on HL-2A tokamak

The novel dual-laser based Polarimeter-Interferometer (PIer) scheme, in which the Faraday angle (αF) and electron density (ne) can be simultaneously measured by utilizing two laser sources, has been successfully demonstrated on HL-2A tokamak through upgrading the present Faraday-effect polarimeter. Comparing to the conventional three-wave PIer, dual-laser PIer generates only one intermediate frequency (IF), avoiding the cross-talk of frequency bands, so as to decrease the phase noise and increase the time resolution of the diagnostic. In the dual-laser PIer configuration, the two collinearly circularly polarized waves coming from HL-2A plasma are evenly divided into two components. One is received by a Schottky diode detector, serving for the Faraday angle measurement. The other orderly passes through a 1/4 wave plate and a wire-grid, and then mixes with the local oscillator (LO) beam, producing an IF associated with the density phase. In recent HL-2A experimental campaign, the dual-laser PIer is operational for the probing chord at Z=17.5 cm. Both Faraday angle and density phase have been simultaneously obtained, with a time resolution <1.0 $\mu$s, a phase resolution 0.1 and 1.0 degree, respectively. This work will be extremely valuable for the next-step far-infrared (FIR) laser PIer construction on HL-2M tokamak, and even the future PIer system on ITER.

Speaker: Dr Yonggao Li (Southwestern Institute of Physics)

summary LAPD-LiYonggao.pdf

28 Spatiotemporally resolved rotational Raman spectroscopy in a pulsed CO$_2$ glow discharge

Efficient reduction of CO$_2$ to CO is a key step in the process of storing renewable energy in the form of hydrocarbon fuels. This process is believed to be most efficient when selectively exciting the asymmetric-stretch vibration of CO$_2$. We study the temperature dynamics of CO$_2$ by *in situ* rotational Raman spectroscopy in a pulsed glow discharge. The cylindrical plasma reactor (23 cm length, 2 cm inner diameter) is operated under flowing conditions in the millibar range, with a 10–50 mA plasma current. The plasma is pulsed with a cycle of 5 ms on, 1 ms off, while the residence time of the gas is in the order of seconds. Raman scattered light is collected from the focal point of a pulsed Nd:YAG laser (532 nm, 100 mJ/pulse, 100 Hz), focused inside the reactor. Rayleigh scattered light is rejected using a volume Bragg grating, after which the remaining light enters a spectrometer and is detected by an ICCD camera. Raman spectra are taken at different time-points during the discharge cycle, as well as at different positions along the longitudinal axis of the reactor. The spectra are fitted to determine the rotational temperature and the molecular concentrations of CO$_2$, CO and O$_2$, exploiting the unique Raman ‘fingerprints’ of these molecular species. First analysis of a measurement at 6.66 mbar and 50 mA shows that temperatures range from 400 K at the start of the 5 ms plasma pulse to 900 K at the end. Variations along the longitudinal axis stay within ±50 K. Furthermore, the concentrations of CO$_2$, CO, and O$_2$ at the center of the reactor are determined to be 82%, 9%, and 9%, respectively, not varying strongly during the pulse cycle.

Speaker: Mr Bart Klarenaar (Department of Applied Physics, Eindhoven University of Technology, Eindhoven, The Netherlands)

Poster Poster_LAPD_2017_Bart_NOBLEED.pdf

summary Slide_-_pitch_BLM_Klarenaar_2x.pdf

29 Investigation of Spatiotemporal Velocity Distribution of Thruster Plasma Using a Laser Induced Fluorescence Diagnostics

A Hall plasma thruster is a type of electric propulsion that generates thrust by accelerating unmagnetized ions through an electric potential gradient in the axial direction established by azimuthal Hall drift (ExB) electrons within crossed (axial) electric and (radial) magnetic fields. In the Hall thruster plasma study, accurate measurement of the plasma and the ion beam parameters is a key to characterize the produced ion beam and to understand the underlying physics. In addition to electrical diagnostics such as Faraday probe, retarding potential analyzer, and ExB probe, a laser induced fluorescence (LIF) diagnostic is one of the most important diagnostics to provide spatio-temporal velocity distribution of the accelerated ions, which can provide information on the electric potential structure inside the acceleration zone. In this work, we present the design, the installation, and the preliminary result of the LIF system, using a 834.723 nm diode laser for xenon ion velocity distribution function. The LIF system is expected to also provide information such as the velocity map of ions, the plasma oscillations, etc [1]. [1] S. Mazouffre, Plasma Sources Sci. Technol. 22 (2013) 013001

Speaker: Prof. Wonho Choe (Korea Advanced Institute of Science and Technology (KAIST))

Poster 170920_LAPD_Poster_HallThruster.pdf

summary LAPD_PosterSummary_WonhoChoe.pdf

Poster Session #1 & Coffee Break

Tuesday, September 26

General

Convener: Dr Jean-Paul Booth (LPP-CNRS)

30 Multi-channel interferometer system for Keda Torus eXperiment using terahertz solid-state diode sources

A solid-state source based terahertz (0.65 THz) interferometer diagnostic system has been installed on a recently built Reversed Field Pinch device, Keda Torus eXperiment (KTX). The multichannel interferometer on KTX utilizes two independent solid-state diode sources based frequency multiplier (X48), which provid an Intermediate Frequency (IF) with wide range. It is proved that the solid-state sources have an inborn advantage of stable IF frequency. The IF frequency gradually reaches pre-described IF frequency within several minutes and keeps almost constant for several hours without any techniques of active frequency stabilization. Compared with the traditional FIR laser used in the multi-channel interferometer system, the solid-state sources provide much less output power, approximately 2 mW. The planar-diode mixers optimized for high sensitivity, ∼750 mV/mW, are used in the heterodyne detection system, which assists the realization of the seven channels interferometer on KTX with a low phase noise. A sensitivity of $n_{\mathrm{e}_\mathrm{min}} = 2 \times 10^{17}$ m$^{−2}$ and a temporal resolution of 0.2 μs have been successfully achieved. The evolution of density distribution in the small cross section of KTX has been determined. It should be addressed that the whole interferometer system is placed on a compact and lightweight support platform. To eliminate the electromagnetic disturbances in the harsh environment of a fusion device, nonmetallic epoxy resin has been selected as main material for the support platform and the commercially available metal optical mounts are replaced with non-metallic components. The phase shift caused by the vibration has been greatly reduced to the level of background noise. The techniques presented here provide a new solution for future interferometer system with turn-key operation, compact size and easy maintenance. The vibration-isolated stable support platform is useful not only for the interferometer system, but also for future Thomson scattering diagnostics on the KTX.

Speaker: Dr Jinlin Xie (University of Science and Technology of China)

Slides multi_channel_interferometer_2017LAPD_170926.pdf

Topical

31 Laser-ablation-based methods for in- and ex-situ material analysis of first wall components in fusion devices

Monitoring of material deposition on and fuel retention in the first wall plasma-facing components is a key diagnostic issue for the next step fusion devices in view of lifetime estimates and operational safety. Lifetime is determined by first wall erosion and in metallic components by the helium content (fusion ash), whereas operational safety is determined by tritium (fusion fuel) retention and dust production from thick layers. Laser-based diagnostics are suitable as in-situ as well as ex-situ diagnostics to access the critical quantities, and indeed, Laser-Induced Desorption (LID) is foreseen to measure in-situ the tritium content in ITER and monitor the inventory with respect to the safety limit and potential cleaning measures. Whereas with a microsecond laser a significant fraction of fuel in the first wall might not be released with LID [1], we present a setup for depth-resolved post mortem and possible in-situ analysis of hydrogen content combing picosecond laser ablation with quadrupole mass spectroscopy. Using a 35 ps laser pulse duration and a wavelength of $\lambda$ = 355 nm (Nd:VO$_4$ 3rd harmonic), the thermal penetration depth is in the same order of magnitude as the optical penetration depth (𝒪(100 nm)) and avoids any matrix and smearing effects, but provides complete fuel release within the laser spot. The ablation rate of the laser pulse is $\Delta h \approx$ (50 – 200) nm, allowing also to measure several µm thick deposition layers. The residual gas analysis enables in addition a clear separation of hydrogen isotopes including tritium and deuterium in fusion devices. A comparison of the complex material deposition and retention in graphite tiles installed in the stellarator W7-X in the first experimental campaign by Laser-Induced Breakdown Spectroscopy (LIBS) and Short Pulse Laser Ablation Gas analysis (SPLAG) will be presented. The quantitative information acquired by these techniques will be discussed in order to demonstrate the capabilities of these methods. Additionally, hydrogen retention analysis by Laser-Induced Ablation Spectroscopy (LIAS) measurements is presented and compared to LIBS using graphite samples from the EAST tokamak. [1]: G. De Temmerman et al., Nuclear Materials and Energy (2016),\\http://dx.doi.org/10.1016/j.nme.2016.10.016 *Corresponding author: tel.: +49 2461 613119, e-mail: *j.oelmann@fz-juelich.de*

Speaker: Jannis Oelmann (Forschungszentrum Jülich GmbH)

Slides LAPD18_Oelmann.pdf

Topical

32 Laser-induced plasmas and laser-induced breakdown spectroscopy

Laser-Induced Plasma (LIP) is produced by the interaction of high-energy laser pulses with matter in any state of aggregation. As a consequence of their high electron number density and excitation temperature, these systems are characterized by strong emission of radiation, whose study can allow determining the plasma parameters and composition. Therefore, a strong effort has been done by the scientific community to develop spectroscopic methodologies for the investigation of LIPS, mainly due to the wide range of applications in which they can be exploited. For instance, the optical emission spectrum, when the laser-produced plasma is in near-equilibrium conditions, enables the elemental analysis of the irradiated sample. This technique, which is usually referred to as Laser-Induced Breakdown Spectroscopy (LIBS), has been growing as an important tool in analytical chemistry thanks to its practical advantages in terms of speed of analysis and instrumental flexibility in real applications. Moreover this technique has been strongly improved by the use of NPs deposited on the sample allowing detection limit in the range of a few ppb. In the field of materials science different applications based on laser-matter interaction have been proposed in the last three decades, going from welding and cutting to the deposition of a large variety of thin films, with the Pulsed Laser Deposition (PLD) technique. Recently, the production of a wide range of nanomaterials has been accomplished by laser ablation of solid targets in liquid environment, including carbon nanotubes, graphene and metallic nanoparticles (NPs). In this work starting from the fundamental mechanisms involved in laser induced plasma and breakdown generation we will discuss the potentiality of laser induced plasma in recent analytical applications and the production of NPs with laser induced plasma in liquid environment.

Speaker: Prof. Alessandro De Giacomo (University of Bari)

Slides lapd_Praga.pdf

10:00 AM Coffee Break

General

33 Microwave comb reflectometry for micro turbulence measurements

As for studying the behavior of the turbulence affecting transport, the multi-scale interaction of turbulence between macro-, meso-, and micro-scale structures is receiving much attention at present. For this aim, higher spatial and temporal resolution diagnostics have been developed and applied in several devices [1]. In LHD, such the precise spatio-temporal behavior of turbulence flow velocity and intensity has been measured by the several channel microwave Doppler reflectometer system [2, 3]. Recently, we succeeded in increasing the radial observation points of this Doppler reflectometer system up to 20 (or especially up to 60). The high sampling rate of 40 GS/s is utilized for the digital signal processing. tthe fine structure measurements by. The detail of the system and some topical results will be presented and the application technique will be discussed. [1] T. Tokuzawa, Nuclear Fusion 57 (2017) 025001. [2] T. Tokuzawa et al., Plasma Fusion Res. 9 (2014) 1402149. [3] T. Tokuzawa et al., Phys. of Plasmas 21 (2014) 055904. The present study was supported in part by KAKENHI (Nos. 15H02335, 15H02336, 17H01368, 26630474 and 25289342), by a budgetary Grant-in-Aid from the NIFS LHD project under the auspices of the NIFS Collaboration Research Program, and by Collaborative Research Program of RIAM of Kyushu University. Additional support was provided by Japan/U.S. Cooperation in Fusion Research and Development.

Speaker: Dr Tokihiko Tokuzawa (National Institute for Fusion Science)

Slides LAPD2017_Tokuzawa_v6b.pdf

Topical

34 Diagnostic studies of technological plasmas using quantum cascade lasers

Molecular plasmas are increasingly used not only for basic research but also technologically for materials and surface processing, environmental challenges, and plasma medicine. Nowadays, they play a key role in branches of industry like semiconductor, automotive, mechanical engineering, light sources, and biomedical technology to name a few. Typical applications are thin film deposition, etching and structuring of semiconductor devices, and surface treatment, such as activation, passivation and cleaning as well as materials and waste treatment. The intense use of plasma technological processes calls for appropriate plasma diagnostic techniques for monitoring, controlling and optimization purposes in industrial environments. In particular, for reasons of efficiency of production processes, in situ diagnostic techniques with online capabilities are favorable. \\ Mid-infrared absorption spectroscopy between 3 and 20 µm using quantum cascade lasers (QCLs) has progressed considerably as a powerful diagnostic technique for in situ studies of the fundamental physics and chemistry of molecular plasmas [1]. QCL based absorption spectroscopy (QCLAS) provides a means of determining the absolute ground state concentration of stable and transient molecular species, which is of particular importance for the investigation of reaction kinetics. Since QCLs emit near room temperature, i.e., without the need of cryogenic cooling, very compact and robust spectroscopic instruments can be designed. This has stimulated the adaptation of infrared spectroscopic techniques to industrial requirements [2-10]. \\ The aim of the present contribution is to review recent achievements using QCLAS for plasma diagnostics and to emphasize the potential of QCLAS for plasma technological applications in industry.\\ \\ [1] J. Röpcke, P. B. Davies, S. Hamann, M. Hannemann, N. Lang, J. H. van Helden: Photonics 3 (2016), 45. \\ [2] G. D. Stancu, N. Lang, J. Röpcke, M. Reinicke, A. Steinbach, S. Wege: Chem. Vap. Deposition 13 (2007), 351. \\ [3] N. Lang, J. Röpcke, A. Steinbach, S. Wege: IEEE Trans. Plasma Sci. 37 (2009), 2335. \\ [4] N. Lang, J. Röpcke, A. Steinbach, S. Wege: Eur. Phys. J. Appl. Phys. 49 (2010), 13110. \\ [5] F. Hempel, N. Lang, H. Zimmermann, S. Strämke, J. Röpcke: Meas. Sci. -Technol. 21 (2010), 085703. \\ [6] N. Lang, F. Hempel, S. Strämke, J. Röpcke: Jpn. J. Appl. Phys. 50 (2011), 08JB04. \\ [7] S. Zimmermann, N. Ahner, F. Blaschta, M. Schaller, H. Zimmermann, H. Rülke, N. Lang, J. Röpcke, S. E. Schulz, T. Gessner: Microelectron. Eng. 88 (2011), 671. \\ [8] M. Hübner, N. Lang, S. Zimmermann, S. E. Schulz, W. Buchholtz, J. Röpcke, J. H. van\\ Helden: Appl. Phys. Lett. 106 (2015), 031102. \\ [9] N. Lang, S. Zimmermann, H. Zimmermann, U. Macherius, B. Uhlig, M. Schaller, S. E. Schulz, J. Röpcke: Appl. Phys. B 119 (2015), 219. \\ [10] N. Lang, U. Macherius, S. Glitsch, H. Zimmermann, J. Röpcke, J. H. van Helden: Contrib. Plasma Phys. 55 (2015), 758.

Speaker: Dr Norbert Lang (INP Greifswald)

summary LAPD-2017_talk_NLang.pdf

Topical

35 Electric field on liquid and dielectric surfaces exposed to atmospheric pressure plasma jets

Low temperature plasmas at atmospheric pressure are usable on materials sensitive to high temperatures, (bio)materials that are not resistant to vacuuming or even fully drying, (bio)targets that are sensitive to significant current transfer. A great number of scientific publications has followed this rise in interest for atmospheric pressure plasmas and they most commonly address the discharge dynamics, densities of heavy species, at times gas temperature measurements, imaging of flow fields and rarely electron densities and electric field but very few on electric field measurements. This paper will give an overview of the recent work in the electric field measurements in atmospheric pressure plasma jets that operate in the ’bullet mode’ when in contact with various surfaces. The results focus on the comparison between the jets driven by 30 kHz sine voltage and jets driven by short high voltage pulses. Two measurement methods have been used that allow for comparison between the electric field in the gas phase and on the treated targets, which vary from dielectrics to liquids. Acknowledgement: European Cooperation in Science and Technology Action COST TD1208 for financial support for a short-term scientific mission.

Speaker: Dr Ana Sobota (Eindhoven University of Technology)

Oral

36 Sensitivity, ambiguity, and resolution of electric field measurement by saturation spectroscopy in Balmer-alpha line of atomic hydrogen

The measurement of electric field by laser Stark spectroscopy aimed the detection of high Rydberg states for a long time to realize a high sensitivity. We changed our approach to the detection of the Stark effects of lower-lying states. The detection of a low-energy state is easier than that of a Rydberg state since it has a large transition probability. However, since the magnitude of the energy shift of the low-energy state by the Stark effect is much smaller than that of a Rydberg state, the energy shift is buried under the resolutions of conventional spectroscopic methods. In this work, we compensated this problem by employing saturation spectroscopy with a Doppler-free spectral resolution [1,2]. A planar electrode was inserted in an inductively coupled hydrogen plasma. The electrode was connected to a dc power supply. We injected pump and probe laser beams, which were obtained from a tunable diode laser system, into the sheath region in front of the electrode from the counter directions. The wavelength of the laser was scanned over the whole range of the Doppler-broadened Balmer-alpha line of atomic hydrogen. The saturation spectrum was obtained from the difference between the absorption spectra of the probe beam in the presence and the absence of the pump beam. We observed the change in the fine-structure spectrum of the Balmer-alpha line as a function of the distance from the electrode surface. The experimental spectra were compared with spectra obtained by the theoretical calculation, and we deduced the magnitude of the electric field. It was possible to detect the 10 MHz shift of the fine-structure line with the help of the Doppler-free resolution of saturation spectroscopy. The shift of 10 MHz corresponds to the detection limit of 10 V/cm in the evaluation of the electric field. The spatial resolution was essentially limited by the diameter of the laser beams, and was less than 0.2 mm. The measurement ambiguity of the electric field was evaluated to be 30 and 10 V/cm in strong and weak electric fields, respectively. Since the developed method utilizes an economical, maintenance-free diode laser system, it may be useful in various experiments which need the measurements of electric fields in plasmas. [1] S. Nishiyama, K. Katayama, H. Nakano, M. Goto, and K. Sasaki, Appl. Phys. Express 10, 036101 (2017). [2] S. Nishiyama, H. Nakano, M. Goto, and K. Sasaki, J. Phys. D: Appl. Phys. 50, 234003 (2017).

Speaker: Prof. Koichi Sasaki (Division of Quantum Science and Engineering, Hokkaido University, Japan)

Oral

37 Spectroscopic investigations of plasma nitriding processes: A comparative study using steel and carbon as active screen materials

Low-pressure pulsed DC H2-N2 plasmas were investigated in the laboratory active screen plasma nitriding monitoring reactor, PLANIMOR, to compare the usage of two different active screen electrodes, (i) a steel screen with the additional usage of CH4 as carbon containing precursor in the feeding gas and (ii) a carbon screen without the usage of any additional gaseous carbon precursor. Applying quantum cascade laser absorption spectroscopy (QCLAS) the evolution of the concentration of four stable molecular species, NH3, HCN, CH4, and C2H2 has been monitored. The concentrations were found to be in a range of 10$^{12}$…10$^{16}$ molecules cm$^{-3}$. By analyzing the development of the molecular concentrations at variations of the screen plasma power, a similar behavior of the monitored reaction products has been found for both screen materials, with NH3 and HCN as the main reaction products. When using the carbon screen, the concentration of HCN and C2H2 was 30 and 70 times higher, respectively, comparing to the usage of the steel screen with an admixture of 1% CH4. Considering the concentration of the three detected hydrocarbon reaction products, a combustion rate of the carbon screen of up to 69 mg$\cdot$h$^{-1}$ has been found. The applied optical emission spectroscopy (OES) enabled the determination of the rotational temperature of the N2+ ion which has been in a range of 650…900 K increasing with the power in a similar way in the plasma of both screens. Also with power the ionic component of nitrogen molecules, represented by the N2+ (0-0) band of the first negative system, as well as the CN (0-0) band of the violet system increase strongly in relation to the intensity of the neutral nitrogen component, i.e. the N2 (0-0) band of the second positive system. In addition, steel samples have been treated with both the steel and the carbon screen resulting in a formation of a compound layer with up to 10 wt-% nitrogen and 10 wt-% carbon, respectively, depending on the screen material [1]. [1] S Hamann, I Burlacov, H-J Spies, H Biermann, and J Röpcke, J. Appl. Phys. 121 (2017) 153301

Speaker: Prof. Juergen Roepcke (INP Greifswald)

12:30 PM Lunch

2:00 PM Excursion to IPP

4:00 PM Excursion to a brewery

Wednesday, September 27

General

Convener: Prof. Kenji Tanaka (National Institute for Fusion Science)

38 High-energy lasers at the Extreme Light Infrastructure

The Extreme Light Infrastructure – Beamlines facility in Prague, Czech Republic, is an EU funded research facility aiming to investigate new regimes of laser-matter interactions. One of three such laser facilities in Europe, it aims to provide users with state-of-the-art, high repetition rate petawatt-class lasers and laser-driven secondary sources to perform experiments in material science, molecular science, and particle acceleration. Three lasers are under construction which will drive the experimental stations. In order to drive a wide range of experiments, the lasers have very different parameters, with peak pulse powers ranging from 6 TW to 10 PW, and focus on different applications. These lasers will be used to produce high harmonic X-ray sources, accelerate protons through target normal sheath acceleration, and accelerate electrons via laser plasma wakefield acceleration. Many technical challenges must be confronted when producing such ultra-powerful laser sources and the development of these lasers pushes conventional laser technology to its limits. One of the primary topics of this talk is an overview of the technology used in the production of high energy, ultra-short laser pulses: specifically dispersion management, ultra-short pulse production, high average power lasers, and methods of broadband amplification (optical parametric chirped pulse amplification, and Ti:Sapphire amplification). These general topics will be supported by discussion of specific technologies used in the lasers developed in the ELI-Beamlines project. The other focus of this talk is on the applications of these lasers. While there are many experimental programs under development at ELI-Beamlines, the main topics discussed here will be related to particle acceleration. Laser plasma wakefield electron acceleration has been demonstrated to produce an accelerating force on the order of 100 GeV/m, and the use of these packets of relativistic electrons as a source of X-rays will be a major component of research at ELI. Laser driven proton acceleration, also a key research activity at ELI-Beamlines, has potential applications in radiation therapy and nuclear fusion.

Speaker: Jonathan Green (ELI-Beamlines)

General

39 Asymmetry of velocity distribution function associated with neutral depletion structure

A neutral depletion structure with strong inhomogeneity in the radial direction has been observed in an electron cyclotron resonance (ECR) plasma. We have measured the velocity distribution function of neutrals with a high resolution laser-induced fluorescence (LIF) system and examined the relationship between asymmetry of distribution function and flow induced by inhomogeneity. It is worth pointing out that skewness is a geometrical form factor to characterize asymmetric profile of distribution and that the relationship between the skewness and corresponding physical process has not been clarified yet. It has been revealed that the third order moment of distribution function, that is, skewness, is proportional to the inhomogeneity-induced flow, and a simple relation between the skewness and the normalized flow velocity has been obtained and confirmed in the experiment. The experiment was performed in the HYPER-I device at National Institute for Fusion Science, Japan, and the high density ECR plasmas were produced with a 2.45 GHz microwave. In order to measure the velocity distribution function, a high resolution LIF method was utilized. The laser beam was modulated by an electro-optical modulator and a polarization beam splitter at 100 kHz, and the LIF signal was lock-in detected to improve the signal-to-noise ratio. The saturated-absorption-spectroscopy (SAS) system was installed in the LIF system, and the Lamb dip was obtained on the SAS spectrum, the position of which was used as the frequency standard and also used as the velocity origin in the laboratory frame. We have precisely measured the velocity distribution function in an inhomogeneous neutral depletion structure. The experimental results indicate that the geometrical form factor of distribution function (skewness) is related to the flow generated by inhomogeneity. It has been found that the asymmetry of distribution function is generated in the direction parallel to the density gradient. Our experiment has clarified the physical significance of the third order moment of distribution function and related the geometrical form factor to the dynamical property (flow) in an inhomogeneous system for the first time. It is considered that the skewness is the fundamental quantity that describes inhomogeneity-induced flow.

Speaker: Dr Kenichiro Terasaka (Kyushu University)

Slides 18th_LAPD(Prague)_K.Terasaka_v04.pptx

Oral

40 Incoherent Thomson scattering diagnostic development for plasma propulsion investigations

Incoherent Thomson Scattering ($ITS$) involves the elastic scattering of electromagnetic radiation on randomly-distributed charged particles, and is used to recover information on electron properties in a wide range of plasmas. The scattered spectrum spectral shape gives access to both the Electron Velocity Distribution Function ($EVDF$) and the Electron Energy Distribution Function ($EEDF$), the overall spectral shift from the incident probe wavelength provides the global drift velocity of the electrons ($v_{e}$), while the integrated spectrum area over the wavelengths provides the electron density ($n_{e}$) from a calibrated diagnostic. For Maxwellian plasmas the spectral distribution of the scattered spectrum is Gaussian, its width provides access to the electron temperature ($T_{e}$). This diagnostic approach has a long history of implementation in magnetic fusion devices, dating back to the 1960s and up to the present day [1-3]. Over the past few decades, this technique has been also been increasingly used for the study of electron properties in a variety of low-temperature plasmas [4-6]. It is non-perturbative, non-invasive and provides direct access to the EEDF without the requirement of intervening oversimplications; these are all signicant advantages over conventional Langmuir probe diagnostics. In recent years, the application of this diagnostic has been further extended to propulsion plasmas. Yamamoto and colleagues implemented ITS on a miniature microwave plasma thruster [7], while Washeleski recently designed a system for preliminary Hall thruster measurements [8]. In this work, we describe development efforts and preliminary results using the newly-developed $THETIS$ (THomson scattering Experiments for low Temperature Ion Sources) platform. This platform is intended for measurements of electron properties in a diverse range of plasma environments for which such information has been lacking. This development effort is driven by certain questions and requirements: - The diagnostic is intended to provide measurements of electron properties in the Hall thruster exit plane region (and in future iterations, inside the thruster channel), where deviations from Maxwellian $EEDFs$ are expected. This will be important to our understanding of basic processes, such as microturbulence-induced transport in thrusters [9], and electron confinement in alternative architectures such as the wall-less Hall thruster [10]. Averaged and time-resolved measurements are envisaged. Coupled with the recently-developed coherent Thomson scattering diagnostic $PRAXIS$ [11], the new platform is expected to provide access to valuable information on electron dynamics. - The diagnostic has been designed for maximum compactness and flexibility of implementation on different sources (including magnetrons, electron cyclotron resonance ion sources and helicons). - The diagnostic is designed for increased sensitivity with respect to existing diagnostics of this type, facilitating measurements in low-density plasma environments. In this work, a hollow cathode is used as a test plasma source for the diagnostic optimization. Preliminary measurements in the vicinity of the cathode orifice provide evidence for electron temperatures on the order of $2 eV$ and below, and confirm an initial detection limit for the diagnostic approaching $10^{16} m^{-3} $. These results are compared to fluid simulation results [12] and Langmuir probe measurements obtained with the same cathode under similar conditions. [1] N. J. Peacock et al. Measurement of electron temperature by Thomson scattering in Tokamak-T3. Nature, 224:488, 1969. [2] S. Yu. Tolstyakov et al. Thomson Scattering Diagnostics in the Globus-M Tokamak. Tech. Phys., 51:846, 2006. [3] D. Eldon et al. Initial results of the high resolution edge Thomson scattering upgrade at DIII-D). Rev. Sci. Instrum., 83:10E343-1, 2012. [4] M. D. Bowden et al. Thomson scattering measurements of electron temperature and density in an electron cyclotron resonance plasma. J. Appl. Phys., 73:2372, 1993. [5] E. A. D. Carbone et al. The radial contraction of argon microwave plasmas studied by Thomson scattering. J. Phys. D: Appl. Phys., 45:345203, 2012. [6] H. Kempkens and J. Uhlenbusch. Scattering diagnostics of low-temperature plasmas (Rayleigh scattering, Thomson scattering, CARS). Plasma Sources Sci. Technol., 9:492, 2000. [7] N. Yamamoto et al. Measurement of xenon plasma properties in an ion thruster using laser Thomson scattering technique. Rev. Sci. Instrum., 83:073106, 2012. [8] R. L. Washeleski. Laser Thomson scattering measurements of electron temperature and density in a Hall-effect plasma. PhD thesis, Michigan Technological University, 2013. [9] J-C. Adam et al. Study of stationary plasma thrusters using two-dimensional fully kinetic simulations. Phys. Plasmas, 11:295, 2004. [10] S. Mazouffre et al. Development and experimental characterization of a wall-less Hall thruster. J. Appl. Phys., 116:243302, 2014. [11] S. Tsikata et al. Dispersion relations of electron density fluctuations in a Hall thruster plasma, observed by collective light scattering. Phys. Plasmas, 16:033506, 2009. [12] Sary, Gaetan. Modelisation d'une cathode creuse pour propulseur a plasma. PhD thesis, Universite de Toulouse, France, 2016.

Speaker: Mr Benjamin Vincent (CNRS - ICARE)

Transparents Incoherent Thomson Scattering (ITS) diagnostic development for plasma_propulsion investigations Presented by Benjamin Vincent

10:05 AM Coffee Break

General

41 Broadband and highly-precise spectroscopy with optical frequency combs

Absorption spectroscopy based on optical frequency comb (OFC) [1] allows measurements of absorption spectra in wide spectral range with high signal-to-noise ratios. It removes the limits of cw-laser spectroscopy caused by its step-like operation and usually limited tuning range. Fourier transform spectrometers (FTS) based on OFCs allow detection of broadband spectra within acquisition times orders of magnitude shorter than traditional FTIRs based on thermal sources [2]. Moreover, the resolution of OFC-based FTS is not limited by the maximum optical path difference (OPD) between the interferometer’s arms [3], shown reaching the levels of single kHz [4]. On the other hand, the setups based on dispersive elements and detector arrays [5, 6], have the advantage of fast acquisition, down to single microseconds, making it perfect for the detection of transient species [7]. In both cases the sensitivity can be increased by coupling the OFC light to the optical enhancement cavities [8]. These properties make the technique suitable for fast acquisition of tens of absorption lines of multiple species simultaneously. In the talk the technique of comb-based measurements with multiple approaches will be presented in the context of the measurement system for fast and broadband plasma diagnostic. References: 1. P. Masłowski, Cossel, K. C. , Foltynowicz, A. , and Ye, J. , Springer Series in Optical Sciences: Cavity-Enhanced Spectroscopy and Sensing: Cavity-Enhanced Direct Frequency Comb Spectroscopy, vol. 179. Springer, Berlin Heidelberg, 2014, pp. 271 2. M. Gołkowski et al. IEEE Trans.Plasma Sci. 40, 1984 (2012). 3. P. Maslowski et al., Phys. Rev. A, 93, 021802(R) (2016) 4. L. Rutkowski et al. in Conference on Lasers and Electro-Optics, OSA Technical Digest (online) (Optical Society of America, 2017), paper SW4J.6. 5. M. J. Thorpe, D. Balslev-Clausen, M. S. Kirchner, and J. Ye, Opt.Express 16, 2387 (2008). 6. G. Kowzan et al., Opt. Letters 41, 974 (2016) 7. A. J. Fleisher et al., J. Phys. Chem. Lett. 5, 2241 (2014) 8. A. Foltynowicz et al., Phys. Rev. Lett. 107, 233002 (2011)

Speaker: Dr Piotr Maslowski (Institute of Physics, Faculty of Physics, Astronomy and Informatics, Nicolaus Copernicus University, ul. Grudziadzka 5, 87-100 Torun, Poland)

General

42 Ultrashort-Pulse Spectroscopy and Imaging of Plasmas and Reacting Flows

Plasmas and reacting flows are essential in a wide array of defense, commercial, space, energy, medical, and consumer-product applications. Understanding the complex physical and chemical characteristics of such systems requires measurements of key parameters, including temperature, pressure, electric field, velocity, and number densities of chemical species. Time-resolved measurements of key chemical species and temperature are required to explore reaction kinetics and transient phenomena. Laser-based, noninvasive linear and nonlinear spectroscopic approaches have proven to be extremely valuable for providing key insights into the physicochemical processes governing plasmas and reacting flows as well as validating numerical models. The advent of kHz-rate amplified femtosecond lasers has enabled multidimensional imaging of key atomic species such as H, O, and N, providing unprecedented insights into preferential diffusion and production of these species through chemical reactions and electric-field-driven processes. These lasers not only provide two-dimensional imaging of chemical species but also have the ability to perform measurements free of various interferences. Moreover, these lasers allow one and two-dimensional temperature-field measurements which were unimaginable only a few years ago. The rapid growth of ultrashort-pulse spectroscopy and imaging has been fueled by the need to achieve 1) interference-free measurements (collisional broadening, Stark broadening, photolytic dissociation, etc.); 2) time-resolved single-shot measurements at kHz rates; 3) spatially resolved measurements; 4) increased dimensionality (point, linear, planar, volumetric); and 5) simultaneous measurements of multiple species and flow parameters. This lecture will address continuing efforts to achieve these goals through the application of emerging ultrashort-pulse laser technologies.

Speaker: Dr Sean Kearney (Spectral Energies LLC/Air Force Research Laboratory)

Oral

43 The effect of dielectric and conductor nanoparticles as impurities in Laser Induced Breakdown in distilled water in presence of applied electric field

In this paper, laser induced optical breakdown in colloidal nanoparticles and in the presence of an applied electric field is studied by using pump probe beam technique. Colloidal nanoparticles of Ag (as a good conductor), Al$_2$O$_3$ and TiO$_2$ (with good dielectric properties) were used in this investigation. We have shown recently that an external electric field can influence the generated nanoparticles by laser ablation in liquid atmosphere. In the experiments, the optical breakdown was induced by an Nd:YAG laser beam (operating at 1064 nm with pulse duration ∼30 ns). Applying an electric field (at the order of several 10 V) perpendicular to the laser beam direction can influence the breakdown plasma condition. Using an optical splitter, a small portion of the breakdown beam was taken and used as probe beam. The time varying transversely transmission of the probe beam through the plasma was measured during the breakdown process. According to the results, the nanoparticles characteristics, laser pumping beam intensity, and the applied electric field have significant influence in the breakdown process. Our results also show different dynamic behaviors for conductor and dielectric nanoparticles at different interaction conditions including the electric field effects. The results are useful for nanoparticle synthesis by laser ablation in distilled water in in the presence of external field. In such conditions the optical breakdown intensity threshold of ambient water can influenced by generated nanoparticles during the synthesis process when an electric filed is also applied.

Speaker: Prof. Mohammad Hossein Mahdieh (Department of Physics, Iran University of Science and Technology, Narmak, 16846-13114, Tehran, Iran)

Slides Mahdieh-Akbari_2017.docx

Oral

44 Optical monitoring systems for thermal spray processes: droplet behavior and substrate pre-treatments

Thermal spray is a technique to form molten droplets using either plasma- or combustion-heating, which impinge upon substrates to form coating layers for various purposes, such as anti-corrosion and anti-wear layers. Although it is an established technique having a history of more than a century, operations of spray guns together with preparing suitable substrate surfaces for obtaining good coating layers still rely on experienced technicians. Because of the necessity of meeting more and more stringent requirements for coating quality and cost from customers, there has been a strong need to try to monitor spray processes, so as to obtain best possible spray coating layers. Basic requirements for such monitoring systems are - reasonably cheap - easy operation for laypersons - easy access to targets to be investigated - an in-situ capability. The purpose of the present work is to provide suitable optical monitoring systems for (1) droplet behavior and (2) substrate pre-treatments. For the former (1), the first result was already presented at the last LAPD17 meeting in 2015 at Sapporo, and results of its subsequent applications into real spray environments are to be shown at this meeting in order to validate the proposal. On the other hand, the latter (2) has been added as a new topic in the research programs, and a preliminary experiment has been started. In addition, an overall strategy is being planned to fulfill the final objective of the optical monitoring of substrate pre-treatments. Details of these two programs (1) and (2) together with the present statuses are presented. *Presenting author: k-muraoka@plazwire.co.jp

Speaker: Prof. Katsunori Muraoka (Plazwire Co., Ltd, 2-3-54 Higashi-naka, Fukuoka 812-0892, Japan)

Paper jmwmb876@ybb.ne.jp

12:25 PM Lunch. ISC Meeting.

Oral

Convener: Prof. Uwe Czarnetzki (Institute for Plasma and Atomic Physics, Faculty of Physics and Astronomy, Ruhr-University Bochum)

45 The Conceptual design of high temporal resolution HCN interferometry for atmospheric pressure air plasmas

A heterodyne interferometer operating at the frequency f = 890 GHz has been designed for measuring electron densities of atmospheric pressure air plasmas its density range is from $10^9$ to $3\times 10^{13}$ cm$^{-3}$ and the pressure range is from 1 Pa to 20 kPa. The system is configured as a Mach-Zehnder type interferometer. The light source is hydrogen cyanide (HCN) laser with wavelength of 337 $\mu$m, which are suitable for the high density of atmospheric pressure air plasmas. The HCN laser power is up to 100 mW. The intermediate frequency, shifted by 500 kHz, is generated by the Doppler shift with a super high speed rotating grating, which is installed in a vaccum vessel. So the temporal resolution of the HCN interferometry is reach to 2 $\mu$s. Beam path analysis based on Gaussian optics is used in the design of the optical system. VDI planar-diode Integrated Conical Horn Fundamental Mixers optimized for high sensitivity, typical 750 V/W, will be used in the HCN interferometry. The detailed optical design and opto-mechanical design of the HCN interferometry are presented.

Speaker: Mr Jibo Zhang (Institute of Plasma Physics, Chinese Academy of Science)

Oral

46 Compact dispersion interferometer with a high efficiency nonlinear crystal for atmospheric pressure plasmas

When the electron density of an atmospheric pressure plasma is measured by a conventional interferometer, the phase shifts due to changes of the neutral gas density causes significant measurement errors. A dispersion interferometer, which measures the phase shift arises from dispersion of medium only, can suppress the measured phase shift due to the neutral gas. This is because the dispersion of neutral gases is much smaller than that of a plasma. Hence the dispersion interferometer is sensitive to the plasma only. In recent years, the CO2 laser dispersion interferometer has been applied to the atmospheric pressure plasmas and its feasibility has been demonstrated [1,2]. The probe beam of the dispersion interferometer is a mixed beam of the fundamental and second harmonic. Since the second harmonic beam is generated with a nonlinear crystal, relatively high power laser was necessary in order to generate the sufficient second harmonic power. This is one of reasons that the dispersion interferometer system is relatively big system. These days, a high efficiency nonlinear crystal OP-GaAs for the CO2 laser becomes commercially available. By using the high efficiency nonlinear crystal, a compact CO2 laser dispersion interferometer, which will be transportable, can be design. In this presentation, the design of such compact CO2 laser dispersion interferometer for atmospheric pressure plasmas will be shown. [1] K. Urabe, T. Akiyama, K. Terashima, Journal of Physics D: Applied Physics 47, 262001 (2014). [2] T. Akiyama, R. Yasuhara, K. Kawahata et. al., Journal of Instrumentation 10, P09022 (2015).

Speaker: Dr Tsuyoshi Akiyama (National Institute for Fusion Sceience)

Oral

47 Experimental study of the electric field in a hollow cathode discharge in hydrogen

Low-pressure plasmas generated in hollow-cathode discharges (HCD) devices have been widely used in industry and in research. Namely, discharges working in glow discharge regime have been used in material processing, etching, thin-film deposition, spectroscopy etc. All these applications require a deep knowledge of plasma behaviour, theoretically and experimentally. During many years the Laser Spectroscopy Laboratory (University of Valladolid) has developed and improved an experimental arrangement to perform high-quality electric field strength (E-field) measurements in the cathode fall region in a HCD in pure hydrogen working in abnormal glow-discharge regime. This is done by a non-invasive technique with high temporal and spatial resolution based on the Stark shifting and splitting of the 2S level of hydrogen and optogalvanic detection. The E-field is determined by the frequency difference of the 2P1/2 and 2P3/2 components. Pulsed UV-radiation (243 nm) used in these measurements is generated by an injection seeded Q-switched Nd:YAG (10 Hz) and a second laser based in non-linear crystals. This system provides single-longitudinal mode radiation, 300 MHz bandwidth, 5 mJ and 2.5 ns temporal duration. In the experimental arrangement, the UV radiation is divided in two counter-propagating beams circularly polarized in opposite directions (following the selection rules for two-photon absorption, $\Delta$L=0) and focused on the upper central part of the discharge, in a 120 μm focus. The discharge source is a cylindrical cathode placed between two cylindrical cone peaked anodes. All pieces have an axial perforation that allows end-on spectroscopic measurements at different distances from the cathode surface. The discharge can be handled in pressures from 400 Pa to 1350 Pa, and currents from 50 mA to 300 mA. This experimental arrangement has provided very good results in stainless steel cathodes with an inner diameter of 10 mm. In this work, a more exhaustive study of the discharge is performed, studying the influence of the diameter and the cathode material. Therefore, four different cathodes have been used: two stainless steel cathodes, with inner diameters of 10 and 15 mm; and two tungsten cathodes of the same diameters. To obtain the maximum information available from the discharge, several theoretical models are fitted to the experimental E-fields. From these studies important parameters can be obtained as the length of the cathode dark space, the voltage employed to maintain the discharge, etc. The early results analysis show very good agreement between theory and the experimental results.

Speaker: Mrs Veronica Gonzalez-Fernandez (University of Valladolid)

Diapositivas Gonzalez-Fernandez.pdf

Oral

48 Employing Doppler-free Saturation Spectroscopy to Measure Stark Broadening at Low Electron Densities

Passive spectroscopic measurements of Stark broadening have been reliably used to determine electron density for decades. However, a low-density limit ($\sim 10^{13}$ cm$^{-3}$) exists due to Doppler and instrument broadening of the spectral line profile. A synthetic diagnostic for measuring electron density capable of high temporal (ms) and spatial (mm) resolution is currently under development at Oak Ridge National Laboratory. The diagnostic is based on applying Doppler-free saturation spectroscopy (DFSS) to measure the Stark broadened, Doppler-free, spectral line profile of a Balmer series transition. The experimental data is fit to a quantum mechanical model to extract relevant parameters. The quasi-static approximation with the Holtsmark distribution is used to model the Stark broadening. DFSS uses two counter-propagating laser beams to excite atoms with a velocity vector perpendicular to the beams, resulting in a Doppler-free measurement. The extreme reduction in line broadening allows access to the Stark broadening regimes not previously available through passive spectroscopy. This technique has been successfully employed to measure spectral data in an electron cyclotron resonance (ECR) source for an electron density range of 10$^{11}$ - 10$^{12}$ cm$^{-3}$. Theoretical modeling continues to improve as crossover peaks, an artifact of the diagnostic, are better understood and captured in the simulations. Details of diagnostic implementation and agreement between experimental data and theoretical results is discussed.

Speaker: Mr Abdullah Zafar (North Carolina State University)

Oral

49 Plasma diagnostics using THz waves generated by laser-plasma interactions

There are some diagnostic methods for fusion plasma, but we still need to develop new and better diagnostic methods. In this research, we developed a new way for plasma density measurement, where strong THz (tera-hertz) waves were produced by focusing femto-second laser pulses in gas and they were used for plasma density diagnostics. The strong THz pulses were sent through an ICP (inductively-coupled plasma) source, where the 13.56 MHz RF can produce fusion plasma densities of the order of 10$^{13}$ cm$^{-3}$, and analysis of the transmitted THz waves can give the information for the plasma density. In this way, we successfully measured the plasma density experimentally and developed a simple model to explain the plasma. In this presentation, the new THz-based plasma diagnostic method is introduced and some recent results will be shown.

Speaker: Prof. Hyyong Suk (GIST)

Poster Session #2 Introduction

50 Two-photon absorption laser induced fluorescence of atomic hydrogen in a dielectric barrier discharge

Concentration of atomic hydrogen radicals in atmospheric-pressure dielectric barrier discharge (DBD) was measured by means of two-photon absorption laser-induced fluorescence (TALIF) and calculated by means of combination of gas flow model with zero-dimensional chemistry model. The volume DBD was ignited in a so-called atomizer (device used in analytical chemistry for detection of hydride-forming elements by absorption spectrometry) in a mixture of argon, hydrogen and oxygen. Both the TALIF measurements and simulation revealed that two sharply separated regions arose in the discharge: A region with a low concentration of atomic hydrogen was formed at the inlet of gases where atomic hydrogen was consumed namely by a reaction with molecular oxygen. The second region with a high concentration of atomic hydrogen was formed farther from the inlet of gases, where most of oxygen was converted to water vapour. Acknowledgement: This research has been supported by the Czech Science Foundation under contracts GA13-24635S, 17-04329S and P206/14-23532S, by the projects CZ.1.05/2.1.00/03.0086 funded by European Regional Development Fund and LO1411 (NPU I) funded by Ministry of Education, Youth and Sports of Czech Republic, by Institute of Analytical Chemistry of the CAS, v. v. i., Institutional Research Plan no. RVO: 68081715 and by the Masaryk University (F. of Science) project 1102/030.

Speaker: Dr Pavel Dvořák (Masaryk University)

Poster poster_Dvorak.pdf

Slides slide_Dvorak.pdf

51 Optimization of POINT Diagnostic on EAST for Realtime Feedback Control

Polarimetric and interferometric measurement with high temporal, spatial resolution on EAST provides essential tools to study more than 100 s high beta H-mode operation, optimization of High-Performance Advanced Tokamak and ITER-like scenarios. Optimization optical layout and hardware of POINT system is essential for implementing plasma density, current and vertical displacement real-time feedback control. The Intermediate Frequency (IF) and Digital Phase Demodulator (DPD) real-time output module system are necessary condition to obtain the density and Faraday rotation angle signal simultaneously. Plasma density signal is consistent with the EFIT data well, which can clearly demonstrate that the POINT system can provide real-time density feedback control. Faraday rotation phase can be used to real time control plasma current in that plasma current profile can be changed by Lower hybrid wave antenna phase variation and power modulation. Faraday rotation angle with spatial symmetry is sensitive to plasma vertical displacement via analysis lower and upper signal-null discharge, which can be used to feedback control for long pulse discharge. Based on the optimized Opto-mechanical design, the POINT system resolution for Faraday rotation and line integral electron density measurement are 0.1$^\circ$ and 1×10$^{16}$ m$^{−2}$, respectively. Thus, POINT system can provide real-time feedback control for long-pulse discharge.

Speaker: Mr Zhiyong Zou (Institute of Plasma Physics, Chinese Academy of Sciences)

Poster LAPD18_zou.pptx

52 Correction of time varying offset in a heterodyne reflectometer for fluctuation measurement on GAMMA 10

Heterodyne reflectometers are used for fluctuation measurements of a high-performance mirror plasma on GAMMA 10. It is important that the offset in IQ signals is minimized to evaluate density fluctuations quantitatively when the phase and amplitude modulation components are separated. A horn antenna pair for transmitting and receiving microwaves is installed inside the vacuum chamber in the anchor cell. When the microwave power reflected from the cutoff layer inside the plasma directly is low, the finite level of the offset will become a big issue. The offset will arise from unintended reflections inside the vacuum chamber. If such reflections have variable path length in relation to the displacement of the plasma column and the change in diffraction, the offset will be varied in time. In addition to efforts to minimize unintended reflections, we use an RF power detector (PD) to check such a time-dependent offset component. By dividing the received microwave, two amplitude components are obtained; one is from PD, which is the actual reflected power received with the antenna, and the other from calculation of IQ outputs. The offset can be evaluated by comparing these two amplitudes and then eliminated even if unintended reflections change in time. The amplitude component that was calculated with the offset eliminated by this method nicely recreated the PD signal. The phase component with the offset correction showed clear peaks of fluctuations. Financial support was received in part from Grants-in-Aid for Young Scientists (B) (Grant No. 15K17797) and from Bidirectional Collaborative Research Program of NIFS (Grant No. NIFS17KUGM132).

Speaker: Mr Junpei Itagaki (Plasma Research Center, University of Tsukuba)

Poster LAPD18_poster_itagaki_v4.pdf

summary LAPD18_introduction_itagaki.pdf

53 Design of dispersion interferometer basing on ratio of modulation amplitudes on EAST

Interferometer for plasma density measurement is sensitive to path length change and mechanical vibration introduced by optics, temperature, airflow and even sound waves. It is a particular problem for a short wavelength interferometer which will be used for burning plasmas like ITER since the phase change due to vibrations is inversely proportional to wave length. To minimize these effects on interferometer, traditionally two-color interferometer is utilized to compensate path length change and mechanical vibration. The alignment between two-color interferometer has to be carefully done to eliminate vibration effects. Dispersion interferometer is a another good choice to solve this problem since it provides auto-compensation for path length change and mechanical vibration This paper describes a conceptual design of a CO2 dispersion interferometer based on a modulation amplitude technique for Experimental Advanced Superconducting Tokamak (EAST). This dispersive CO2 interferometer will enhance the reliability of the electron density measurement and reduces maintenance of interferometer for routine operation. The optical design, nonlinear crystal properties and harmonics laser profile as well as bench test of interferometer will be presented.

Speaker: Mr Weiming Li (University of Science and Technology of China)

summary PPT_li.pdf

54 Vibration Signals from Interferometers in KSTAR

Signals of vibrations are measured in different interferometers of Korea Superconducting Tokamak Advanced Research, KSTAR. The KSTAR device is equipped with three different interferometers. Millimeterwave (MMWI), far-infrared (FIRI) and two-color (TCI) whose viewing chords are all different : Horizontal, vertical and tangential way respectively. The mountings of each retro-reflector are also different. Center-stack concave mirror was installed for MMWI. And a reflector for FIRI was originally fixed under the 2.5m-long chimney inserted under a top-port (Gt), and later moved to outside vacuum vessel with specially designed compact-vibration-isolator (CVI). A reflector for TCI is installed on top of the vacuum vessel surface. The main vibration sources are major disruptions. It induces several seconds of large vibration signals in every interferometer including even MMWI. In addition, during the plasma discharge, the primary source of vibration is in-vessel-control-coil (IVCC). The IVCC takes load after 120ms to activate iso-flux control. This transition and influence of IVCC vibration is not observed in MMWI but clearly seen in both FIRI and TCI although the impact in FIRI has been reduced about 1/10 after installation of CVI and the most of vibrations are canceled with the operation of TCI. The detail of the vibration level in relationship of IVCC and disruptions will be presented.

Speaker: Dr June-Woo Juhn (National Fusion Research Institute)

Poster Juhn_Poster_Vibrations_Interferometer.pdf

summary Brief_Summary_Juhn.pdf

55 Application of a heterodyne dispersion interferometer and density fluctuation measurements on DIII-D

Interferometers which measure plasma induced phase shifts are a common approach for probing line-integrated electron density. However, since changes in the beam path length caused by mechanical vibrations can often cause comparable or larger phase shifts, one of the main challenges of this measurement is reduction of the vibration contribution either through an optical solution or simply a stabilizing structure. One solution, a dispersion interferometer (DI), where the probe beam is a mixture of fundamental and second harmonic components, measures the phase shift which arises from dispersion alone. The conventional DI is a homodyne interferometer, hence variations of the detected intensity lead to measurement errors. In order to resolve this problem, phase modulation was introduced and its feasibility was demonstrated on LHD [1] and is planned on ITER [2]. While the phase-modulated DI approach can achieve high density resolution, sufficient for density control, the temporal resolution (< 50 kHz) is not adequate for density fluctuation measurements. By implementing an acousto-optic cell based heterodyne technique, developed as part of a US-Japan collaboration [3], the DI bandwidth can be expanded into the MHz range and eliminate unwanted sensitivity to intensity variations. Following successful proof-of-principle bench tests, a CO2 laser based heterodyne DI was installed on DIII-D. The round trip path length from the laser room to DIII-D is approximately 100 m and requires active feedback alignment [4]. The return fundamental beam power after the 100 m round trip is reduced to 0.3 W from 9 W injection due to transmission losses (primarily absorption in the BaF2 vacuum windows). Because the DIII-D DI is equipped with high efficiency nonlinear OP-GaAs crystals, the 0.3 W return power generates sufficient 2nd harmonic power and beat signal amplitude (400 mVpp) for phase shift extraction. The DIII-D heterodyne DI is capable of measurements of the electron density even in a disruption phase and shows good agreement with the density measured by the existing two-color laser interferometer [5]. The line-integrated density resolution, which is likely determined by offset drifts due to interaction with water vapor, is $\sim 3$ ×10$^{18}$ m$^{-2}$ for 1 s. This density resolution is achieved despite the presence of centimeter level motion during a discharge, the perpendicular component of which is cancelled at the nonlinear crystal by feedback alignment and the parallel component induced phase shifts by the dispersion interferometer scheme itself. Additionally, the 40MHz heterodyne beat frequency, determined by the acousto-optic cell drive allows low-noise measurements of density fluctuations into the MHz range. This work was supported by US-Japan Fusion Collaboration Program FP5-3 (2015) and FP5-8 (2016), as well as U.S. DOE under DE-FC02-04ER54698 and DE-AC02-09CH11466. [1] T. Akiyama et. al., Rev. Sci. Instrum. 85, 11D301 (2014). [2] T. Akiyama et. al., Rev. Sci. Instrum. 87, 11E133 (2016). [3] T. Akiyama et. al., Rev. Sci. Instrum. 87, 123502 (2016). [4] M.A. Van Zeeland et. al., PPCF, in submission (2017) [5] T.N. Carlstrom et. al., Rev. Sci. Instrum. 59 1063 (1988).

Speaker: Dr Tsuyoshi Akiyama (National Institute for Fusion Sceience)

summary Preposter_P2-6_rev4.pdf

56 Negative hydrogen ion density behavior in an inductively coupled hydrogen plasma

Hydrogen plasmas are widely used in various applications. Particularly, in the fields of nuclear fusion and accelerators, a sufficient amount of hydrogen negative ions are extracted from the plasma and accelerated to form high current negative ion beams. In order to develop plasma sources producing high density negative ions, negative ion density measurement as well as plasma characterization is the first crucial step. Measurement of negative hydrogen ion densities using laser photo-detachment was carried out in an RF inductively coupled hydrogen plasma. Nd:YAG laser at 1064 nm was used to detach electrons from hydrogen negative ions and the detached electrons were collected by a positively biased electrostatic probe. The electrostatic probe, equipped with RF-filters compensating the plasma RF fluctuations, was also used to obtain electron energy distribution functions (EEDFs). Based on electron densities and temperatures calculated from measured EEDFs, negative ion densities were calculated using a set of particle balance equations considering all processes related to negative ion generation and loss. Both negative ion density behaviors obtained by laser photo-detachment and the calculation show quite good agreement.

Speaker: Dr Min Park (Korea Atomic Energy Research Institute)

summary MinPark_introduction.pdf

57 Iodine atom detection in a plasma thruster by infrared diode laser absorption

Electric propulsion technology (plasma thrusters) for satellite positioning and orbit change is a rapidly expanding field. Whereas most existing devices (principally Hall Effect Thrusters) use xenon gas, due to its high mass and low ionisation potential, there is growing interest in the use of Iodine[1], due to its ease of storage (solid state) and low cost, in addition to a mass and ionisation potential comparable to xenon. However, the physical processes occurring in an iodine plasma are more complex, including electron-impact dissociation, negative ion formation by dissociative attachment, and surface-catalysed recombination. Models of iodine plasmas are being developed, but much fundamental data is absent, and there is an urgent need for experimental validation. We have used infra-red diode laser absorption spectroscopy to detect iodine atoms in a pure I2 inductively-coupled discharge, using the spin-orbit transition occurring at 1315 nm [2]. Since the transition strength is weak, the beam is passed 4 times through the 10 cm-wide reactor, operating at pressures of 10-100 mTorr and with radiofrequency power up to 250 W. From the absorption intensity and Doppler width the absolute I atom density and translational temperature was determined as a function of gas pressure and RF power. In future work we intend to determine the I atom recombination probability at the reactor surface from the I density decay rate in the afterglow in pulsed discharges. [1] P. Grondein, T. Lafleur, P. Chabert, and A. Aanesland, Physics of Plasmas, 23, (2016) [2] T.K. Ha, Y. He, J. Pochert, M. Quack, R. Ranz, G. Seyfang, and I. Thanopoulos, Berichte Der Bunsen-Gesellschaft-Physical Chemistry Chemical Physics, 99, (1995) 384

Speaker: Dr Jean-Paul Booth (LPP-CNRS)

Poster LAPD_2017_Iodine.pdf

Slides LAPD_2017_Booth_3_min.pdf

58 Overtone spectroscopy of N2H+ molecular ions – application of cavity ring-down spectroscopy

A stationary afterglow apparatus in conjunction with a laser absorption cavity ring-down spectrometer has been employed to observe absorption lines in the P- and R-branches of the $2v_1$ vibrational band of the $\mathrm{N_2H^+}$ molecular ion as a part of an ongoing experimental campaign whose goal is to study the electron-ion recombination of $\mathrm{N_2H^+}$ in afterglow plasmas. The probed absorption lines lie in the near-infrared spectral region around 1580 nm. The observed transition wavenumbers were fitted to experimental accuracy and improved molecular constants of the upper vibrational state were obtained. The experimental apparatus can been used to investigate translational and rotational temperatures of the ions in a wide range of experimental conditions. The employed cavity ring-down spectroscopy is also able to determine absolute ion number densities that are needed for recombination measurements. Acknowledgement: This work is partly supported by Czech Science Foundation projects GACR 17-08803S, GACR 15-15077S, GACR 17-18067S, and by Charles University Grant Agency project GAUK 1583517.

Speaker: Mr Ábel Kálosi (Charles University, Faculty of Mathematics and Physics, Department of Surface and Plasma Science)

Poster kalosi_poster_LAPD.pdf

summary kalosi_summary_LAPD.pdf

59 Development of a Multi-chord Interferometer System on Versatile Experiment Spherical Torus

A single channel of 94 GHz heterodyne interferometer system for measurement of the electron density [1] was successfully developed for Versatile Experiment Spherical Torus (VEST) at Seoul National University. The interferometer system has been upgraded to a multi-chord heterodyne interferometer system for the profile of electron density. The Gaussian probe beam from a horn antenna is expanded into a uniformly distributed beam by using a pair of Powell lenses [2]. The expanded beam is guided through 9 holes and collimating lenses, located vertically with the same interval of 75 mm, generating 9 channels of Gaussian beams. Each collimated beam is splitted and focused to generate minimum beam waist near the center of plasma utilizing the plano-convex lens. The beam is reflected at a concave mirror mounted on the center stack of VEST. After passing through the plasma, the reflected beam at the beam splitter is focused onto the multi-chord detector array. The probe beam and 97.2 GHz LO is mixed to produce Intermediate Frequency (IF) signal at 3.2 GHz, and the IF signal is further down-converted to 80 MHz through a frequency down-conversion circuit. Optimization of the lengths between the optical components and beam waists along the beam path is achieved by the Gaussian optics and the optical design code, CODE V [3]. Experimentally optimized lengths between the optical components of this system will be discussed with the simulated ones. The first results from the upgraded interferometer system will be presented as well. [1] D. H. Choi *et al.*, *Design of interferometer system on Versatile Experiment Spherical Torus (VEST) at Seoul National University*, 2012 *JINST* 7 C01107. [2] Y. B Nam *et al.*, *Compact ECEI system with in-vessel reflective optics for WEST*, *Rev. Sci. Instrum.* 87 (2016) 11E130. [3] CODE V is an optical design software of Synopsys Inc., https://optics.synopsys.com/codev

Speaker: Mr Jongin Wang (NUPLEX, Seoul National University)

60 Forward scattering measurements by terahertz solid state source interferometer on KTX reversed field pinch

A one chord microwave interferometer has been applied to measure forward scattering from electron density fluctuations on KTX reversed field pinch. Two terahertz microwave souces are used to form a heterodyne interferometer system. Frequency of the diagnostic beam is about 650 GHz and the power of each source is about 2 mw. The diagnostic beam goes vertically through the center of the plasma, and the scattering beam are also collected by the sensitive Schottky planar diode mixer. The forward scattering measurement provide more sensitive fluctuation diagnostic than interferometer signal. The spectrum of electron density fluctuation in different operation mode on KTX has been compared and analyzed using this kind of method.

Speaker: Dr Wenzhe Mao (University of Science and Technology of China)

Poster 17LAPD_forward_scattering_poster.pdf

summary Poster_Intorduction_Forward_Scattering_experiment.pdf

61 High speed FPGA-based Phasemeter for the far-infrared laser interferometers on EAST

The far-infrared laser-based HCN interferometer and POlarimeter/INTerferometer (POINT) system are important diagnostics for plasma density measurement on EAST tokamak. Both HCN and POINT provide high spatial and temporal resolution of electron density measurement and used for plasma density feedback control. The density is calculated by measuring the real-time phase difference between the reference beams and the probe beams. For long-pulse operations on EAST, the calculation of density has to meet the requirement of Real-Time and high precision. In this paper, a Phasemeter for far-infrared laser-based interferometers will be introduced. The FPGA-based Phasemeter leverages fast ADCs to obtain the three-frequency signal from VDI planar-diode Mixers, and realizes digital filters and an FFT algorithm in FPGA to provide real-time, high precision electron density output. The implementation of the Phasemeter will be helpful for the future plasma real-time feedback control in long-pulse discharge.

Speaker: Dr Yuan Yao (Institute of Plasma Physics, Chinese Academy of Sciences)

summary summary_yao.pdf

62 Numerical study of collisional effects in saturated absorption spectrum of Argon arcjet plasma

The narrow Lorentzian spectrum is obtained by using saturated absorption spectroscopy in the atomic ensemble without strong collisions. In the plasma with strong collisions, on the other hand, the saturated absorption spectrum is partially or fully broadened to the Gaussian shape whose width is the same order as the Doppler broadening. In the argon arcjet plasma, as a plasma which exhibits intermediate pressure region, the variety of saturated absorption spectra can be observed by tuning the pressure in the discharge section. In particular, spectrum with inverted pedestal which has not been previously reported was obtained. We performed a numerical analysis to validate the shape of the spectrum observed by numerically solving the two-state rate equation system under an assumption of strong velocity-changing collisions. The shape of the spectrum with inverted pedestal was reproduced by assuming the collisional frequency is significantly different for each electronic state. We also discuss the perspective of the saturated absorption spectroscopy as a tool for measuring the spatially-resolved collision parameters in the plasma.

Speaker: Dr Leo Matsuoka (Hiroshima university)

Poster poster.pdf

summary LAPD2017_intro.pdf

63 A Lagrangian model for laser-induced fluorescence measurements of plasma ion temperature and electrostatic waves

Extensive information can be obtained on wave-particle interactions and wave fields by direct measurement of perturbed ion distribution functions using laser-induced fluorescence (LIF). For practical purposes, LIF is frequently performed on metastable states that are produced from neutral gas particles and ions in other electronic states. If the laser intensity is increased to obtain a better LIF signal, then optical pumping can produce systematic effects depending on the collision rates which control metastable population and lifetime. We numerically simulate the ion velocity distribution measurement and wave-detection process using a Lagrangian model for the LIF signal for the case where metastables are produced directly from neutrals. This case requires more strict precautions and is important for discharges with energetic primary electrons and a high density of neutrals. Some of the results also apply to metastables produced from pre-existing ions. The simulations show that optical pumping broadening affects the ion velocity distribution function (IVDF) $f_0(v)$ and its first-order perturbation $f_1(v,t)$ when laser intensity is increased above a certain level. The results also suggest that ion temperature measurements are only accurate when the metastable ions can live longer than the ion-ion collision mean free time. For the purposes of wave detection, the wave period has to be significantly shorter than the lifetime of metastable ions for a direct interpretation. It is more generally true that metastable ions may be viewed as test-particles. As long as an appropriate model is available, LIF can be extended to a range of enviroments.

Speaker: Mr Feng Chu (University of Iowa)

Poster Feng.pdf

Slides LAPD.pdf

64 The electronic quenching of the N$_2$(B$^3\Pi_g$, $\nu$) revealed by the LIF technique

The presented study reports on the technique of extraction of the collisional quenching coefficients of the N2(B$^3\Pi_g$) vibrational levels by nitrogen molecule. The quenching was examined by the laser-induced fluorescence technique in the monofillamentary volume DBD streamer discharge ignited in pure nitrogen at low pressures [1, 2]. The discharge was powered by periodic high-voltage bursts (superimposing two sine-waves, $f_{AC} = 1$ kHz, with a positive pulse of a short duration of 100 ns, all applied at a fixed repetition frequency of 10 Hz). Superposition of the HV pulse during second positive AC half-cycle results in locking the onset of the streamer with respect to the HV pulse and allows fixing arbitrary delay between the streamer and laser pulse. The quenching of individual vibronic levels was tracked by the LIF scheme described in detail in [2]. Vibrational states of the N$_2$(B$^3\Pi_g$, $\nu = 3-12$) were individually excited by the laser pulse, the fluorescence originated in two different vibronic transitions was then tracked by two different fast photomultipliers and sampled simultaneously by the oscilloscope and multichannel photon-counter. Combination of various approaches enabled recording the long-time development of the fluorescence signal, including the beginning of the pulse, where the photon-counting signal is saturated, and the late fluorescence decay tail, where the signal-to-noise ratio of the oscilloscope is insufficient. The analysis of obtained LIF waveforms reveals the two-term exponential decay, we will discuss the origin of both components. The rate coefficients for electronic quenching of the vibrational levels $\nu = 3-12$ by N$_2$ will be presented. We will also discuss the best practices in the implementation of the LIF technique based on a broadband nanoseconds OPO laser under streamer discharge conditions, as well its advantages and limitations. **Acknowledgements:** Work supported by the Czech Science Foundation (GAČR no. GA15-04023S). **References:** [1] M Šimek, P F Ambrico and V Prukner, J. Phys. D: Appl. Phys. 46 (2013) 485205, [2] M Šimek, P F Ambrico and V Prukner, J. Phys. D: Appl. Phys. 48 (2015) 265202

Speaker: Martina Mrkvickova (Department of Physical Electronics, Masaryk University, Brno, Czech republic)

Poster posterMrk_ver4.pdf

summary mrk_intro_v2.pdf

65 Improvement of the q profile by the polarimeter/interferometer system on EAST tokamak

A novel method has been developed to improve the accuracy of safety factor (q) profile by combining the polarimeter/interferometer (POINT) measurements with the external magnetic measurements on the EAST tokamak. The POINT system, measuring the accurate line-integrated electron density and Faraday rotation angle, provides the magnetic field information inside the plasma. By adding these data to the equilibrium confinement, the results from POINT measurements show a difference with the original equilibrium and the difference becomes larger from boundary to core of the plasma. This correction process makes up for the deficiency of magnetic measurements, the details of the correction process are specified, which bypass the equilibrium fit (EFIT) code. Results with and without these corrections are presented, comparisons of the corrected results and experimental results are also shown and they are found agree well with each other. The feasibility and reliability of the correction process are also discussed in this paper.

Speaker: Dr Xiang Zhu (Institute of Plasma Physics, Chinese Academic of Sciences)

summary Summary-xzhu.pdf

66 Measurement of ICRF wave propagation using a microwave reflectometer with fast antenna switching on GAMMA 10

Slow Alfvén waves in ion cyclotron range of frequencies (ICRF) are a powerful tool to heat ions confined in a mirror field. In spite of its efficient heating effect that has been attained in the central cell of GAMMA 10, there are still unknown characteristics concerning boundary condition, transient variation of heating effect, exact picture of cyclotron damping, and so on. What we need to study these characteristics in detail is a multi-point measurement of the waves inside the hot plasma. We have been developing such a measurement system by using a microwave reflectometer. In addition to the radial profile that is easy to measure with a reflectometer, axial measurements have been achieved by arraying transmitting and receiving horn antennas in the axial direction, which are repeatedly switched in time during a discharge with PIN diode switches [1]. Another transmitting and receiving horn antenna pair was newly added to the system and probing at five cross sections was achieved in a single discharge with time resolution of about 1 ms at each antenna pair position. The time resolution is not determined by electrical characteristic of the system but by the time window required for fluctuation analysis. With the upgraded antenna array, axial variation of wave-induced fluctuation intensity was clearly observed, offering valuable data on wave physics in a hot mirror plasma. Financial support was received in part from Grants-in-Aid for Young Scientists (B) (Grant No. 15K17797) and from Bidirectional Collaborative Research Program of NIFS (Grant No. NIFS17KUGM132). [1] R. Ikezoe et al., Rev. Sci. Instrum. 88, 033504 (2017).

Speaker: Dr Ryuya Ikezoe (Plasma Research Center, University of Tsukuba)

Poster LAPD18_poster_Ikezoe.pdf

summary Summary.pdf

67 Velocity-space cross-correlation for electrostatic mode identification

We show how cross-corrlelation of the fluctuations in LIF between two laser beams exciting optically isolated metastable lines can be used to identify and separate the power spectra of plasma electrostatic modes while using only a single measurement location. A matrix of measurements of: C(v,v',t')= {df(x,v,t)df(x,v',t-t')}t, Fourier transformed over the time delay t', produces a Hermitian matrix when the two laser systems are tuned to the same set of Doppler-selected velocities. We consider the effects of electrostatic waves on the LIF cross-correlation matrix. Broad-band electrostatic waves are excited using a capacitive antenna driven with electrical white noise in a singly-ionized weakly-collisional ArII plasma column produced by a low power inductively coupled plasma source. Modes can be identified by a generalized wave admittance which comes from the eigenvectors of the cross-correlation matrix. We demonstrate the connection between this generalized impedance and the usual definition. Other extensions of the technique will be considered.

Speaker: Prof. Fred Skiff (University of Iowa, Dept. Physics and Astronomy)

summary skiff_2m.pdf

68 Streamer-induced vibrational kinetics of N$_2$(A$^3\Sigma_u^+$, $\nu$) revealed by LIF technique in nitrogen-oxygen mixtures

We report on the N$_2$(A$^3\Sigma_u^+$, $\nu$) vibrational kinetics in nitrogen-oxygen mixtures revealed by LIF technique under DBD streamer discharge conditions at low pressures. Triggered single streamer events were produced in pure N$_2$ and N$_2$-O$_2$ mixtures employing volume DBD geometry at a pressure of 50 Torr. The discharge concept is based on a pair of metallic electrodes embedded in dielectric disks [1,2]. The discharge was powered by periodic high-voltage bursts (superimposing two sine-waves, f$_A$$_C$ = 1 kHz, with a positive pulse of a short duration of 100 ns, all applied at a fixed repetition frequency of 10 Hz). The evolution of individual vibronic levels was tracked by a modified excitation-detection LIF scheme described in detail in [2]. The LIF signal originating from the N$_2$(A$^3\Sigma_u^+$, $\nu$=0–10) vibronic levels was acquired for various delays after the streamer onset. In pure nitrogen, the observed evolution of the LIF signal during the decaying streamer channel period evidences fast initial relaxation of high vibrational levels towards the v = 2 and 3 levels, followed by a delayed increase of terminal v = 0 and 1 levels [2]. In nitrogen-oxygen mixtures, however, the efficient quenching of higher metastable levels by oxygen significantly inhibits vibrational relaxation towards the lower and terminal levels, causing much lower populations of the v = 0–3 levels. This is already clearly visible in the N2 + 0.8% O2 mixture with vibrational kinetics limited to the first 10 microseconds of the streamer decay. In the synthetic air, the kinetics is limited only to a few microseconds in the post-discharge. We evidenced a very fast relaxation of all vibrational levels while terminal level v=0 and 1 were below the detection threshold. Furthermore, much more effective quenching of fluorescence makes the measurements extremely challenging. Acknowledgements: Work supported by the Czech Science Foundation (GAČR no. GA15-04023S). References: [1] M Šimek, P F Ambrico and V Prukner, J. Phys. D: Appl. Phys. 46 (2013) 485205 [2] M Šimek, P F Ambrico and V Prukner, J. Phys. D: Appl. Phys. 48 (2015) 265202

Speaker: Dr Milan Simek (Institute of Plasma Physics of the CAS, Czech Republic)

summary misimek61

69 More-than-50-$\%$ detachment of a H${^-}$ beam by CW 1064 nm laser illumination

Neutral beam injection, for plasma heating, will supposedly be achieved, in ITER, by collisional detachment of a pre-accelerated D${^-}$ beam. Collisional detachment however makes the presence of a D2-filled neutralisation chamber necessary, on the beam line. This has severe drawbacks, due to collisions with the injected gas up- and downstream from the collision chamber and the impossibility of setting the chamber at high voltages, which makes it necessary to set the D${^-}$ ion source itself at -1 MV. Photodetachment, in comparison, has many advantages as a neutralisation method: there is no further need of gas injection, the photodetachment zone can be set at a high (positive) voltage, which makes the ion source operation much easier. Photodetachment efficiencies can get infinitely close to 100%. The only difficulty of the photodetachment scheme, as has been known for decades, is that it requires a high laser flux. Fortunately continuous-wave lasers are now available commercially with higher and higher output powers and a monochromaticity good enough to make efficient injection into a high-finesse optical cavity possible. The present work has consisted in implementing such an optical cavity, with a finesse greater than 1000, on a 1.2 keV H${^-}$ beam (which is only 20 times slower than the D${^-}$ ion beams prepared for ITER). About 26 kW of continuous wave illumination could be obtained with 24 W of laser input, at the wavelength 1064 nm. Photodetachment efficiency has reached a more-than-50% level in a preliminary version of the experiment, which demonstrates that development of real-scale photodetachment-based neutral beam injectors has become realistic. This experiment also provides the opportunity of a measurement of the photodetachment cross-section, which was seldom determined in actual experiments [1]. [1] Vandevraye, M., Babilotte, P., Drag, C., & Blondel, C., "Laser measurement of the photodetachment cross section of H${^-}$ at the wavelength 1064 nm", *Phys. Rev. A* **90**, 013411 (2014)

Speaker: Dr Cyril Drag (Laboratoire Aimé-Cotton)

summary Second, corrected version

Poster Session #2 & Coffee Break

6:20 PM Transport to a cruise

7:00 PM Banquet

9:30 PM Karaoke bar

Thursday, September 28

General

Convener: Dr Milan Simek (Institute of Plasma Physics v.v.i.)

70 Absolute measurements of reactive atomic species in atmospheric pressure plasmas using picosecond two-photon absorption laser induced fluorescence spectroscopy

Atmospheric pressure plasmas are versatile and efficient sources for reactive species production at ambient room temperature. The non-equilibrium chemical kinetics is initiated and determined by the electron dynamics. Due to the strongly collisional environment and associated short electron energy relaxation times the electron dynamics can be tailored using multi-frequency power coupling techniques, enabling separate control of key parameters like electron density and electron mean energy. Reactive atomic species play key roles in the chemical kinetics and details strongly depend on the feed-gas composition. Measurements and predictive simulations of key reactive species are equally challenging due to the strongly collisional environment and their multi-scale nature in space and time. The most promising approach is the exploitation of complementary advantages in direct measurements combined with specifically designed numerical simulations. Picosecond two-photon absorption laser induced fluorescence (TALIF) spectroscopy allows us to measure absolute densities of atomic oxygen (O), nitrogen (N) and hydrogen (H), even in chemical environments with complex reaction kinetics and associated collisional quenching processes, through directly resolving the effective lifetime with sub-nanosecond resolution. This is particularly important in realistic situations for technological applications with plasma operation and species penetration into ambient air. The picosecond TALIF measurements are compared with direct VUV synchrotron absorption spectroscopy under well-defined gas compositions showing very good agreement. Further insight into the chemical kinetics is obtained through additional UV & IR absorption spectroscopy (OH, O$_3$, CO$_2$, CO) measurements and synergistic combination with multi-scale numerical simulations of the chemical kinetics. The presentation will focus on examples of He-O$_2$-N$_2$-H$_2$0 mixtures for bio-medical applications and He/Ar-CO$_2$ mixtures for CO$_2$ conversion into value-added chemicals.

Speaker: Prof. Timo Gans (University of York)

Topical

71 Measurement of electric fields in high-pressure discharges by CARS-based four-wave mixing

The electric field strength is a basic parameter of non-thermal plasmas since it determines the energy that electrons gain. On timescales which are of interest in high pressure plasmas, the ions are at rest and the discharge dynamic is determined by the electrons. If the electric field strength and its temporal develeopment are known and additional diagnostics are applied, many other quantities of interest e.g. displacement and conduction current, dissipated power, and electron density can be inferred. Two laser beams (pump and Stokes) are focussed into the plasma and together with the quasi-static electric field a fourth IR signal wave is generated. The signal intensity is proportional to the square of the local electric field. A high temporal (5 ns) and spatial (0.1 mm) resolution can be achieved, which is ideal for the investigation of atmospheric pressure plasmas. The method can be applied in pure hydrogen or nitrogen discharges or by an admixture of about 100 mbar partial pressure of one of these as a tracer gas. Since the signal strength depends of the molecular gas density, the sensitivity is determined by the partial pressure and Raman cross section of the applied gas. Typical values are 100-1000 V/mm. The method is used to investigate several kinds of atmospheric pressure discharges: ns-pulsed discharges between two parallel electrodes (jet-like geometry) [1], single streamer dielectric barrier discharges (DBD) [2], plasma bullets [3], and inside a fast ionization wave discharge [4]. In all cases the temporal development of the elctric field is measured, mostly spatially resolved. In case of the ionization-wave discharge even the direction of the electric vector has been determined. The field measurements are complemented by streak camera measurements in which spectral lines or the total emission is determined with the same temporal resolution like the laser method. [1] S. Müller et. al., J. Phys. D.: Appl. Phys. 44 (2011) 165202 [2] P. Böhm, et. al., Plasma Sources Sci. Technol. 25 (2016) 054002 [3] M. Van der Schans, et. al., submitted to Plasma Sources Sci. Technol. [4] B. Goldberg, et. al., Plasma Sources Sci. 24 (2015) 055017

Speaker: Dirk Luggenhölscher (Ruhr-University Bochum)

Topical

72 On radical densities in atmospheric pressure plasmas

Atmospheric pressure plasma jets require the highest sensitivity and specificity from diagnostic techniques in order to properly quantify the radical chemistry taking place. In this presentation I will describe several laser based techniques with the necessary capabilities for probing small levels of transient species, and demonstrate how they have been applied to plasma systems. These include optical feedback cavity enhanced absorption spectroscopy and Faraday rotation spectroscopy. Example data on the detection of peroxy radical (RO2) species are presented: HO2 in particular has been highlighted as an important intermediate, implicated in the production of reactive oxygen species in cold atmospheric plasma sources, and is integral to the complex chemical network which generates hydrogen peroxide as one of the by-products. Using convenient technology at near-IR wavelengths, baseline absorption sensitivities have been achieved of the order of a few parts in 10(11), which corresponds to parts-per-billion concentrations within the plasma plume.

Speaker: Prof. Grant Ritchie (University of Oxford)

10:00 AM Coffee Break

Topical

73 Polydiagnostic studies on ${CO}_2$ plasmas at elevated pressures

The conversion of ${CO}_2$ into value-added chemicals or ${CO}_2$-neutral fuels are widely regarded as partial solution to integrate renewable electricity into process chains and the existing energy infrastructure. Non-equilibrium plasmas have been shown to enhance the efficiency of the ${CO}_2$-to-$CO$ dissociation process under certain conditions, though fundamental understanding of plasma-chemical processes is still lacking. Dielectric barrier discharges in different configurations have been characterised by (i) infrared spectroscopy as well as phase- and time-resolved laser spectroscopy, (ii) spatio-temporally resolved optical emission spectroscopy, and (iii) Raman scattering in order to quantify e.g. the conversion degree, byproducts, gas and surface temperatures. The conversion efficiency remained low (< 5%). Apart from unfavourably high reduced electrical fields, the recycling of (atomic) oxygen seemed to be hindered, as e.g. indicated by the ozone concentrations. The CO production was found to be mainly driven by electron-impact dissociation and could be directly linked with the total number of transferred charges during the residence time of ${CO}_2$ in the active plasma zone. Recent modelling efforts confirmed electron-impact dissociation as the dominant production channel next to dissociative electron attachment and recombination. Vibrationally stimulated dissociation seemed to play a negligible role. By contrast, microwave plasmas have received widespread attention given high conversion efficiencies that have been reported. More recently, two operating regimes -- a diffuse and constricted mode -- have been identified as function of pressure. In constricted mode gas temperatures in excess of 3000 K are determined suggesting a significant contribution of thermal dissociation. Moreover, these temperatures may hamper the desired vibrationally stimulated dissociation. To further disentangle individual dissociation processes a microwave phase shift technique has been developed to retrieve electron densities. Values between ${10}^{18}$ and ${10}^{19}$ m$^{-3}$ were measured in the diffuse and constricted mode.

Speaker: Stefan Welzel (DIFFER)

Oral

74 High resolution Two-photon Absorption Laser-Induced Fluorescence (TALIF) with a single-mode nanosecond Ti:Sapphire laser

Parameters of laboratory discharges in molecular gases, as the gas translational temperature, the collisions rates and the density, are difficult to determine. We measure, with excellent spatial and temporal resolution, the velocity distribution of oxygen ground-state atoms in plasmas from the Doppler broadening of their laser excitation spectra. The method is based on the well-known Two-Photon Absorption Laser-induced Fluorescence (TALIF) technique. We have developed a pulsed tunable ultraviolet laser with very narrow bandwidth which allows the Doppler profiles to be measured with high precision. This laser consists of a pulsed Nd:YAG-pumped Ti:Sapphire ring cavity that is injection-seeded by a singlemode CW Diode laser. The single-mode infrared output pulses are frequency quadrupled by two non-linear crystals to reach the necessary UV wavelength (226 nm, 0.2 mJ) for TALIF excitation. Measuring the Doppler profile in different experimental conditions, we have determined the temperature of oxygen atoms with an uncertainty of only +/- 10K in a simple DC glow discharge [1]. By reflecting the laser beam upon itself with a flat mirror, we could make Doppler-free TALIF spectra, allowing observation of the pressure-broadening effects [2]. Accurate measurement of the injection-seeding wavelength provides new data on the excitation energy of the fine structure of the O 3p 3P level. We have also observed the isotope shift of the 2p4 3P2 → 3p 3P2 transition making the experiment on 16O and 18O isotopes [3]. We will discuss the possibility of the determination of the quenching rates by the analysis of the TALIF signal. Nevertheless, we will show how the Amplified Spontaneous Emission (ASE) can modify the decay rate of the fluorescence. Finally, we will discuss different possibilities to measure the density of Oxygen atoms in the plasma. [1] J-P. Booth, D. Marinov, M. Foucher, O. Guaitella, D. Bresteau, L. Cabaret and C. Drag, “Gas temperature measurements in oxygen plasmas by high-resolution Two-Photon Absorption Laser-induced Fluorescence”, *J. of Instrum.* **10**, C11003 (2015) [2] D. Marinov, C. Drag, C. Blondel, O. Guaitella, J. Golda, B. Klarenaar, R. Engeln, V. Schulz-von der Gathen and J.-P. Booth, “Pressure broadening of atomic oxygen two-photon absorption laser induced fluorescence”, *Plasma Sources Sci. Technol.* **25**, 06LT03 (2016) [3] D. Marinov, J.-P. Booth, C. Drag and C. Blondel, “Measurement of the isotope shift of the 2p4 3P2 -> 2p33p 3P2 two-photon transition of O I and a revision of the triplet energy levels of atomic Oxygen”, *J. Phys. B: At. Mol. Opt. Phys.* **50**, 065003 (2017)

Speaker: Dr Cyril Drag (Laboratoire Aimé-Cotton)

Closing