Soft X-ray diagnostics

Diagnostic description:

The soft X-ray diagnostic is suitable for a monitoring of core plasma activity, e.g. sawtooth instability. Moreover, spatially resolved measurements give information on plasma position and its profile. In the modified setup, it can be also used for computation of electron temperature and for estimation of effective ion charge of plasma.

The main objective of the SXR detection on COMPASS is to monitor a distribution of radiated power in a soft X-ray spectral region and namely to observe fast radiating events connected to plasma instabilities. These measurements require a high temporal resolution, hence, the photodetectors, which provide time scales in range of microseconds, are used as SXR detectors.

Fig. 1: SXR detector array with the ceramic socket (left). Relative efficiency of SXR detection using thin beryllium foil (right).

The tokamak plasma, which is optically thin (negligible absorption) for VIS, UV and SXR radiation, is a volume radiator. Assuming a strong toroidal symmetry (along the torus), a complexity of the problem can be simplified to 2D observations in a poloidal cross-section of the tokamak plasma (often circular or D-shape). But these measurements do not provide information on local properties; they are integrated along the chord, i.e. originate from a whole solid angle of the detection. These data can be processed using special transformations to get local radiation. If radiation distribution is supposed to be radially symmetric, one observation array of detectors is sufficient and a simple procedure of Abel inversion is applied. Otherwise, several observation arrays are needed allowing a relatively complicated tomographic reconstruction.

Arrays of silicon photodiodes, which are mounted on a planar chip (LD 35-5T (JET) WINDOWLESS manufactured by Centronic Ltd. or AXUV20-ELM manufactured by International Radiation Detectors Inc.) are intended for measurements of soft X-ray radiation (Fig. 1). A thin beryllium foil of about 10 μm is used to select photons with energy above 0.5 keV.

Fig. 2: Field of views of installed AXUV detectors.

The SXR detector arrays have been successfully installed into the three diagnostic ports (Fig. 2) providing the spatial resolution about 2 cm at the plasma core.

Fig. 3: Port plug for SXR placement into the angular diagnostic port: scheme (left), vacuum part with ceramic socket for SXR arrays (middle), flange with multi-pin electrical feedthrough (right).

The detector in the angular upper port is placed into a special complex port plug (Fig. 3) combining the soft X-ray detector, AXUV diodes and an optical system for visible light detection.

References

[1] M.Imrisek, J.Havlicek, V.Weinzettl, J.Mlynar: "Observation of Sawtooth Oscillations in the COMPASS Tokamak", Proceedings of the 22nd Annual Conference of Doctoral Students - WDS 2013, pp. 67-72, Prague, 4th-7th June 2013, MATFYZPRESS, ISBN 978-80-7378-251-1

[2] V.Weinzettl, M.Imrisek, J.Havlicek, J.Mlynar, D.Naydenkova, P.Hacek, M.Hron, F.Janky, D.Sarychev, M.Berta, A.Bencze, T.Szabolics: "On Use of Semiconductor Detector Arrays on COMPASS Tokamak", World Academy of Science, Engineering and Technology 71 (2012), 628-634; published also in conference proceedings of ICPP 2012: International Conference on Plasma Physics, Venice, Italy, 14th-16th November 2012

[3] J.Mlynar, M.Imrisek, V.Weinzettl, M.Odstrcil, J.Havlicek, F.Janky, B.Alper, A.Murari, and JET-EFDA Contributors: "Introducing minimum Fisher regularisation tomography to AXUV and soft x-ray diagnostic systems of the COMPASS tokamak", Review of Scientific Instruments 83, 10E531 (2012)

[4] V.Weinzettl, D.I.Naydenkova, D.Sestak, J.Vlcek, J.Mlynar, R.Melich, D.Jares, J.Malot, D.Sarychev, V.Igochine: "Design of multi-range tomographic system for transport studies in tokamak plasmas", 1st International Conference on Frontiers in Diagnostic Technologies, Frascati, Italy, November 25-27 2009, Nuclear Instruments and Methods in Physics Research A 623 (2010) 806-808, doi:10.1016/j.nima.2010.04.010