The He/Ne beam diagnostic for line-ratio spectroscopy in the island divertor of Wendelstein 7-X

a versatile gas injection system and an absolute calibrated spectroscopic observation system with more than 200 lines-of-sight have been installed in two divertor locations in the Wendelstein 7-X stellarator. This powerful setup enables among other measurements to infer T-e and n(e) in the divertor region by means of line-ratio spectroscopy on thermal helium. This is a robust and routinely used diagnostic at W7-X and this work describes its main characteristics and shows some examples of T-e and n(e) profiles. In particular it is shown that the inclusion of high Rydberg states in the helium collisional-radiative model is necessary to obtain a realistic estimate of T-e. The applicability of the diagnostic is currently being extended towards the low T-e and high n(e) conditions occurring during detachment by means of line-ratio spectroscopy on neon. Initial T-e, n(e) profiles obtained with a neon-beam are here presented.