Evaluation of the reliability of Rayleigh scattering as calibration method for the Thomson scattering diagnostic at W7-X

Abstract Rayleigh scattering is used in an in-situ approach to determine the relative and absolute calibration factors of the Thomson scattering diagnostic at Wendelstein 7-X (W7-X). Using light pulses from an optical parametric oscillator (OPO), it is shown for the first time that the method is allowing for the calibration of all spatial channels (scattering volumes) at once. That has been realized by reducing the high amount of stray light observed in prior attempts. The improved setup is presented and compared to the literature ones. The Rayleigh scattering evaluation is containing the vacuum stray light measurement, which is yielding a rough estimate of the relative calibration without the need of any gas and used for background subtraction and a Rayleigh measurement on argon inside the vessel. The pressure dependency of the Rayleigh contribution in argon is evaluated for two wavelengths in each spectral channel. At the pressure of 377 mbar, the full spectral measurement used for the calibration is recorded. Under the assumption of the typical Rayleigh scattering cross-section from literature, the relative and absolute calibration factors are estimated. While the in-situ Rayleigh relative calibration and the conventional relative calibration mostly agree within the error bars, the absolute calibration is too high by a factor of roughly 3. Variations between different scattering volumes need further investigation and probably make additional improvements of the setup necessary. However, the general suitability of the method as absolute calibration is confirmed.