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 (He) and neon (Ne) collisional-radiative models (CRM) enables measurement of n$_{\mathrm{e}}$ and T$_{\mathrm{e}}$ in front of the horizontal divertor target of the Wendelstein 7-X optimized stellarator. Spectral line emission from locally-injected helium and neon is channeled to multiple 20cm and 32cm Czerny-Turner spectrometers, allowing high spectral resolution observation of diagnostic helium and neon lines, as well as various visible impurity lines and Balmer series lines. In this work, T$_{\mathrm{e}}$ and n$_{\mathrm{e}}$ profiles across the divertor island are shown for a variety of experimental conditions, including impurity-seeded and detached plasmas. Profiles inferred from neon and helium are presented and compared. Also presented are first-time measurements of plasma parameters over changing island sizes. Systematic variations of the island size in front of the diagnostic have a marked impact on the profile shape and gradients. These first- time measurements provide evidence of for a local confinement in the islands of the divertor at W7-X.