Neutral particle fluxes on the divertor during overload mimic scenarios in Wendelstein 7-X*

Neutral particle fluxes on the Wendelstein 7-X (W7-X) divertor are compared for the first two diverted operational phases (OP1.2a and OP1.2b). In OP1.2b, two test divertor unit scraper elements (TDU-SE) have been implemented in an attempt to prevent heat flux overload on divertor edges during long, high-beta discharges. While these conditions cannot be reached with current operating parameters, magnetic configurations have been designed to mimic the effect of high bootstrap current. Experiments are performed in each phase in order to compare divertor fluxes with and without the TDU-SE in these configurations, as well as in standard operational configurations. The SE have been predicted to have the side effect of reducing neutral pumping, which could have an adverse effect on density control. Results are presented from the H-α spectroscopic cameras, which measure flux of neutral hydrogen at the divertor in order to quantify the effect on pumping efficiency. This work also covers calibration of the H-α cameras to absolute neutral flux.