The upgraded He/Ne beam diagnostic for active emission spectroscopy in the island divertor of Wendelstein 7-X

A significant enhancement of the line-ratio spectroscopy system on thermal helium and neon for the first divertor campaign of Wendelstein 7-X will enable measurement of electron density n_e and temperature T_e in front of the horizontal divertor target. The upgrade comprises 30 horizontal and 30 vertical lines of sight, which are channeled to a 19cm and a 32cm focal length spectrometers, allowing observation of the helium and neon lines as well as intrinsic impurities and Balmer series with high spectral resolution. Helium will be used as routine gas while neon will be tested in order to extend the applicability of the diagnostic to the detached divertor regime at very low T_e(<< 10eV). Gas injection is enabled by two boxes with 5 fast piezo valves each, mounted directly behind the divertor plates in one upper and one lower divertor module, which are magnetically connected in the 5/5 island configuration. Initial T_e/n_e measurements with helium beam will be shown together with results from neon injection and the status of the development of a dedicated collisional-radiative model for neon.

This work has been funded by the Department of Energy under Grant DE-SC0014210 and by EUROfusion under Grant No 633053.