Conceptual study of a polarimetric Thomson scattering diagnostic in ITER

Polarimetric Thomson scattering (TS) is a novel diagnostic technique proposed as an alternative to the conventional (spectral) TS, for the measurement of the electron temperature Te in very hot fusion plasmas. Contrary to spectral TS that is based on the reconstruction of the Doppler broadened frequency spectrum, in polarimetric TS Te is determined from the depolarization of scattering radiation. The technique is suitable for ITER, where it is expected to be competitive with the conventional spectral TS for measurements in the highest Te range. In this paper we consider a hypothetical polarimetric TS diagnostic for ITER supplementing the conventional core TS system. By using a tangential laser beam a plasma volume of ~ 23cm length in the central region of the ITER plasma can be observed under a scattering angle ? ~ 75? with a F/21 collection optics system. with this geometry we first determine the polarization state of the input laser beam that maximize the unpolarized component of the scattering radiation and then we calculate the expected error on the measured Te obtained with a simple, two-channel polarimeter and with a more complete, four-channel, full Stokes polarimeter. In both cases the expected performances are compared with those of the conventional spectral core TS diagnostic to determine the plasma conditions in which the polarimetric technique is more convenient. A possible test of polarimetric TS using the HRTS TS system of JET is also discussed.