Bolometric diagnostics

Diagnostic description:

Bolometer is a diagnostic designed for measurements of energy of incident electromagnetic radiation. Bolometers used in plasma physics typically monitor discharges in gases via total radiated power covering a wide spectral range.

The main objective of bolometry on COMPASS is to monitor a distribution of radiated power, and namely to observe fast radiating events connected to plasma instabilities like tearing modes with their mode locking. These measurements require a high temporal resolution, hence, the photodetectors, which provide time scales in range of microseconds, are used as bolometers.

Fig. 1: AXUV detector array with the ceramic socket (left). Typical responsivity of AXUV diode for photons, electrons and hydrogen ions (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 neccessary and in this case a relatively complicated tomographic reconstruction must be used.

Arrays of 20 silicon photodiodes, which are mounted on a planar chip (AXUV20-ELM manufactured by International Radiation Detectors Inc.), provide measurements of total radiation in the spectral range from ultraviolet (UV) up to soft X-ray radiation, i.e. photons with energy from several eV up to several keV (wavelengths from 0.2 nm to 180 nm) on the COMPASS tokamak (Fig. 1).

Currently, five AXUV arrays are installed in three diagnostic ports (Fig. 2) with the spatial resolution of about 2 cm at plasma centre.

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

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

Fig. 3: Complex port plug for AXUV placement: scheme (left), vacuum part with ceramic sockets for AXUV arrays (middle), flange with multi-pin electrical feedthroughs (right).

References

[1] V.Weinzettl, M.Imrisek, J.Havlicek, L.Kripner, J.Seidl, P.Bilkova, P.Bohm, M.Aftanas, K.Kovarik, J.Horacek, P.Vondracek, J.Adamek, R.Dejarnac, D.Cornelis, H.Janssens: "Experimental evidence of neoclassical tearing modes on COMPASS tokamak", 40th EPS Conference on Plasma Physics, 1st-5th July 2013, Espoo, Finland, ECA Vol.37D, P5.143

[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