# Inference of $\alpha$-particle density profiles from ITER collective Thomson scattering

## TL;DR

This paper demonstrates that the ITER collective Thomson scattering diagnostic can accurately infer alpha-particle density profiles within 10% despite spectral distortions caused by refraction and noise, meeting the required measurement precision.

## Contribution

The study develops a CTS model incorporating spatial refraction effects and shows that alpha-particle densities can be reliably recovered without accounting for these effects in the fitting process.

## Key findings

- Alpha-particle densities can be recovered within ~10% accuracy.
- The diagnostic achieves the required 20% accuracy at 100 ms resolution.
- Refraction effects, while distorting spectra, do not prevent accurate density inference.

## Abstract

The primary purpose of the collective Thomson scattering (CTS) diagnostic at ITER is to measure the properties of fast-ion populations, in particular those of fusion-born $\alpha$-particles. Based on the present design of the diagnostic, we compute and fit synthetic CTS spectra for the ITER baseline plasma scenario, including the effects of noise, refraction, multiple fast-ion populations, and uncertainties on nuisance parameters. As part of this, we developed a model for CTS that incorporates spatial effects of frequency-dependent refraction. While such effects will distort the measured ITER CTS spectra, we demonstrate that the true $\alpha$-particle densities can nevertheless be recovered to within ~10% from noisy synthetic spectra, using existing fitting methods that do not take these spatial effects into account. Under realistic operating conditions, we thus find the predicted performance of the ITER CTS system to be consistent with the ITER measurement requirements of a 20% accuracy on inferred $\alpha$-particle density profiles at 100 ms time resolution.