Recent Advances on The He/Ne beam diagnostic for line ratio spectroscopy in the Island Divertor of Wendelstein 7-X

            A line-ratio spectroscopy system based on thermal helium and neon collisional radiative models (CRM) has been implemented and successfully shown to enable measurement of n_e and T_e above two magnetically connected divertor targets in the standard divertor configuration of the Wendelstein 7-X optimized stellarator. Spectral line emission from gas injection is channeled to multiple Czerny-Turner spectrometers, allowing high spectral resolution measurements of diagnostic helium lines. Neon has also been implemented in select discharges to investigate the expansion of the measurement envelope of the diagnostic into the low T_e and high n_e regime found in detached discharges.

            In this work, in-situ measurements of gas cloud atomic density distribution from a gas injector identical to those installed at W7-X are presented. Also presented are forward modeling results of the diagnostic based on these gas puff measurements. These profiles facilitate the forward modeling of the diagnostic for future improvements to e.g. collection optics. Also shown are comparisons between helium beam data and multipurpose manipulator probe data using a novel flux coordinate system, an improvement over previous field-tracing methods for diagnostic comparison within the 5/5 island chain. This comparison provides evidence supporting the observation of hollow temperature profiles in the device’s 5/5 island chain.