Quantification of systematic errors in the electron density and temperature measured with Thomson scattering at W7-X

TL;DR

This paper investigates the sources of systematic errors in Thomson scattering measurements of electron density and temperature at W7-X, highlighting laser position fluctuations as a significant factor affecting data accuracy.

Contribution

It introduces a Bayesian analysis approach to identify laser position fluctuations as a key error source, offering a method to optimize laser alignment.

Findings

Spectral calibration errors do not explain unphysical profile features.

Laser position fluctuations significantly impact measurement accuracy.

A method to optimize laser alignment based on profile sensitivity.

Abstract

The electron density and temperature profiles measured with Thomson scattering at the stellarator Wendelstein 7-X show features which seem to be unphysical, but so far could not be associated with any source of error considered in the data processing. A detailed Bayesian analysis reveals that errors in the spectral calibration cannot explain the features observed in the profiles. Rather, it seems that small fluctuations in the laser position are sufficient to affect the profile substantially. The impact of these fluctuations depends on the laser position itself, which, in turn, provides a method to find the optimum laser alignment in the future.