A polarization-based Thomson scattering technique for burning plasmas

Abstract

The traditional Thomson scattering diagnostic is based on measurement of the wavelength spectrum of scattered light, where electron temperature measurements are inferred from thermal broadening of the scattered laser light. At sufficiently high temperatures, especially those predicted for ITER and other burning plasmas, relativistic effects cause a change in the polarization state of the scattered photons. The resulting depolarization of the scattered light is temperature dependent and has been proposed elsewhere as a potential alternative to the traditional spectral decomposition technique. Following similar work, we analytically calculate the degree of polarization for incoherent Thomson scattering. For the first time, we obtain exact results valid for the full range of incident laser polarization states and electron temperatures. While previous work focused only on linear polarization, we show that circularly polarized incident light optimizes the degree of depolarization for a wide range of temperatures relevant to burning plasmas. We discuss the feasibility of a polarization based Thomson scattering diagnostic for ITER-like plasmas with both linearly and circularly polarized light and compare to the traditional technique.

Conceptual design of a polarimetric Thomson scattering diagnostic in ITER

“…The problem of the optimization of the polarization state of the input laser beam for polarimetric TS in the conditions of ITER has already been investigated by calculating the depolarization of TS radiation integrated over the entire wavelength spectrum [1,3,4]. These calculations have shown that for the above backward-like geometry the depolarization of the scattering radiation is maximized by using an input laser beam with circular polarization.…”

“…In order to obtain a quantitative estimate it is necessary to calculate the expected measurement error on a set of suitably chosen 2016 JINST 11 C01071 polarimetric signals and propagate them to the final values of T e and n e . To this purpose it is important to consider that the polarization state of the TS radiation is frequency dependent and therefore when the spectral range of the detected radiation is limited as in our case, the signal in a polarimetric channel must be calculated using the spectral TS Mueller matrix M(ω, θ, T e ) whose elements are functions of the frequency ω of the scattered radiation [2][3][4].…”

“…To this purpose we have implemented the integration method in the velocity space described in [2]. The code has been extensively validated by showing that numerical integration of the elements of the spectral Mueller matrix over the entire frequency spectrum consistently reproduce the frequency integrated Mueller matrix elements obtained by analytical integration [3]. In addition we showed that the power spectrum of the TS signal in the typical conditions of spectral TS, calculated using the spectral Mueller matrix from our code is coincident with the relativistic TS spectrum given by Naito [12].…”

“…It must be considered that these values of the scattering angle are more favorable to both the spectral and the polarimetric TS. In fact for scattering at about right-angle, spectral TS benefits from the reduced values of the spectrum width and of the relativistic blue shift that limit the effects of the short wavelength detection cut-off, and polarimetric TS benefits from the fact that for a given T e the intensity of the unpolarized component of the scattering radiation available over the entire spectrum, is higher than in the backward-like conditions above [2][3][4]. To compare the two measurements we assumed in both cases a scattering angle θ = 75 • , a collection optic solid angle ∆Ω = 1.78 × 10 −3 sr (F/21) and a scattering volume of length L = 23 cm, all the other parameters being the same as for θ = 145 • .…”

“…Polarimetric Thomson scattering (TS) is a novel diagnostic method in which the plasma electron temperature T e is determined by the depolarization of the Thomson scattering radiation, a relativistic effect that can be exploited only in a very hot (T e > 10 keV) plasma. The method, that so far has never been employed for diagnostic purposes, was proposed by Orsitto and Tartoni [1] and was subsequently studied by Segre and Zanza [2] and by Mirnov et al [3,4]. Recently the problem of the calibration of a polarimetric TS diagnostic has also been investigated [5,6].…”