Polarimetric Thomson Scattering for high electron temperature measurements: perspectives, challenges, and preliminary assessment for its implementation on the DTT facility

Polarimetric Thomson Scattering (PTS) is a technique that allows for accurate measurements of electron temperature ( T e ) in hot plasma environments ( T e > 10 keV, expected to be regularly achieved in future machines like ITER), where classic Thomson Scattering (TS) systems suffer from spectra widening. In these conditions, in fact, the TS spectrum extends to low wavelengths, where the plasma background light significantly perturbs the measurements, and both the collection optics transmission and the available detector efficiency decrease. To overcome this issue, the PTS diagnostic, instead of performing a spectral analysis, measures the degree of depolarisation ( D ) of the TS light. Under certain conditions, in fact, that depend upon the scattering angle and the polarisation state of the probing laser, D increases monotonically with T e , allowing for a robust measurement of plasma electron temperature in hot plasmas. This work presents an overview of this technique, focusing on the analysis of the multiple dependencies of the degree of depolarisation and how these parameters can be tuned to significantly improve the PTS system performance. Perspectives for future diagnostic development are discussed, along with the challenges that must be faced when designing such complex systems. Examples from recent developments and a potential implementation of a PTS system on the DTT (Divertor Tokamak Test) facility are also discussed.