measurement uncertainty in high performance ITER operating regimes*

The ITER Core Plasma Thomson Scattering (CPTS) system is the primary diagnostic for the electron temperature (T_e) and electron density (n_e) profiles.  For high performance ITER operating scenarios in which core T_e > 20 keV, the conventional spectral CPTS will not provide measurement accuracy sufficient for advanced machine control.  Addition of polarimetric measurement capability to the CPTS diagnostic is projected to increase measurement accuracy in high temperature / low density plasmas to required levels.  The need to develop Polarimetric TS is Issue A.17 in the 2020 ITER Technical Report on Required R&D.  To determine the feasibility of Polarimetric TS implementation on ITER, the polarization characteristics of the CPTS scattered light collection system have been modeled as part of a conceptual design study.  When linearly polarized light is propagated through the CPTS collections optics, polarization is rotated and ellipticized.  This characteristic of the ITER collection optical system complicates separation and analysis of the depolarized component of the Thomson scattered light that is produced at high T_e.  To address this challenge, a custom waveplate and polarizing beamsplitter should be mounted before the final lens in the CPTS collection optical system.  Additionally, polarization performance should be one of the criteria by which materials and coatings are selected for this optical system.