Optimisation of the input polarisation angle on lines of sights of a polarimetry system for a fusion reactor

The study is dedicated to the evaluation of the effects of the input polarisation angle with respect to the toroidal magnetic field on the values of the Faraday Rotation, Cotton-Mouton phase shift angle (and then ellipticity). The need of this evaluation is connected to the optimisation of the polarimetry quantities in view of having a complete information related to magnetic fields, plasma density and electron temperature. A polarimeter with vertical as well as horizontal lines of sights at various angle with respect to the equatorial axis is considered. The equilibrium used is a circular analytic model which considers a tokamak in the slab approximation. The plasma parameters considered are B₀=3.1 T (toroidal magnetic field), I₀=3.5 MA (plasma current), N₀=2⋅ 10²⁰ m⁻³ (plasma density), R₀=3 m (major radius) and a=1 m (plasma radius), so R₀/a=3 (aspect ratio). The input polarisation angle is varied between −90^o and 90^o. It is found that the polarimetry signals of horizontal lines of sights are very sensible to the polarisation input angle leading to strengthening the capability of measuring the plasma density and electron temperature in these lines. Numerical calculations show that the largest change in the polarization state, leading to the reduction of the measurement errors, is for the beam with initial Stokes vector ₀=(0,± 1,0) (polarization angle ψ₀=± 45^o).