Conceptual design of Thomson scattering system with high wavelength resolution in magnetically confined plasmas for electron phase-space measurements

TL;DR

The paper addresses measuring non-Maxwellian electron velocity distributions in magnetically confined plasmas using Thomson scattering with unprecedented wavelength resolution. It proposes a conceptual design for a spatially-resolved 2560-channel spectrometer with a triple-grating configuration and a high-energy, single-shot laser to maximize photon statistics, and validates parameter recovery and non-Maxwellian detectability through Monte Carlo simulations and Bayesian inference. Results show feasible per-channel photon counts, robust signal-to-noise after post-processing, and identifiable deviations from Maxwellian distributions (bi-Maxwellian and kappa) in synthetic spectra, demonstrating the method's potential to reveal kinetic features in CHD plasmas. This work outlines a practical path toward resolving electron distribution shapes in confinement devices and establishes design requirements that could enhance kinetic diagnostics beyond conventional Te–ne measurements.

Abstract

We discuss the conceptual design of a spatially-resolved spectroscopy system of Thomson scattering with high wavelength resolution capable of measuring the shape of electron velocity distribution functions in magnetically confined plasmas. We design a spatially-resolved spectrometer with 2560 wavelength channels. The estimated number of scattered photons in a single spectrometer channel is much larger than unity under the experimental setup and plasma parameters of the Compact Helical Device (CHD), indicating sufficient photon statistics for single-shot measurements. Simulations of the scattered spectra show that the signal-to-noise ratio exceeds 5 even under the most unfavorable conditions expected in CHD at full spectral resolution, and further improves with post-processing pixel binning. Bayesian inference applied to the simulated spectra demonstrates that the inferred plasma parameters agree with the input values within the estimated uncertainties. Comparisons between spectra generated from non-Maxwellian electron velocity distribution functions and their Maxwellian fits indicate that deviations from Maxwellian distributions can be identified using the proposed system.

Figures (11)

• Figure 1: Thomson scattering spectra in Maxwellian distribution functions at $T_e=100$, 1000, and 6000eV.
• Figure 2: Schematic illustration of (a) the Thomson scattering system and (b) the spatially-resolved spectrometer.
• Figure 3: The reciprocal linear dispersion in Eq. \ref{['eq:RLD']} as a function of $\theta_i$ and $f$ with $N_g=600grooves~mm^{-1}$, $m=1$, and $\lambda=527nm$.
• Figure 4: The detection efficiency as a function of wavelength assuming the setup in Fig. \ref{['fig:design']}(b).
• Figure 5: Estimated photon number of thermal bremsstrahlung normalized to Thomson scattering at the laser wavelength as a function of electron temperature and density. The inset shows the photon spectrum of Thomson scattering and thermal bremsstrahlung with $T_e=100eV$ and $n_e = 5e19m^{-3}$.