Speaker
Hans Salzmann
(Retired from MPI f. Plasmaphysik, Garching)
Description
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.
Primary author
Hans Salzmann
(Retired from MPI f. Plasmaphysik, Garching)
Co-authors
Chris Gowers
(Retired from CCFE, Culham)
Per Nielsen
(Retired from Consorzio RFX, Padova)
