Extending the Te range of the ITER core Thomson scattering system by detection of the unpolarized scattering radiation

It is well known that the Thomson scattering radiation by a high Te fusion plasma such as that of ITER is partially polarized and includes an unpolarized component. The intensity of this unpolarized component, that arises from relativistic effects, is an increasing function of the plasma Te, of the scattering angle and of the polarization state of the input laser beam. Previous work has shown that using this radiation it is possible to implement a polarimetric TS technique, in which the plasma Te and ne are determined by comparing the intensity of the polarized and unpolarized components of the TS radiation in a given, arbitrary, spectral interval, and in case of the core TS system of ITER this technique may be competitive with the conventional spectral analysis for Te > 25 keV. In this paper we discuss an alternative way to exploit this radiation and consider a scheme in which an additional detection channel, specifically designed to measure the unpolarized component of the scattering radiation (polarimetric channel), is included together with the conventional set of spectral channels in a typical TS polychromator. We first show that in the conditions of the ITER core TS system the intensity of the unpolarized component is maximized when the input laser beam has a circular polarization and calculate its spectrum and the dependence from Te for an input laser beam with both linear and circular polarization. Then we calculate the expected performances of the ITER core TS system and show that the inclusion in the detection channel set of such a polarimetric channel may considerably improve the measurement accuracy in the Te > 25 keV range.