Speaker
Mr
Benjamin Vincent
(CNRS - ICARE)
Description
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.
Primary authors
Mr
Benjamin Vincent
(CNRS - ICARE)
Mr
Potrivitu George
(CNRS - ICARE)
Dr
Sedina TSIKATA
(CNRS - ICARE)
Dr
Stephane MAZOUFFRE
(CNRS - ICARE)
