Direct fast-ion density measurements with FIDA spectroscopy in Wendelstein 7-X

Good fast-ion confinement is one of the main scientific premises of the Wendelstein 7-X (W7-X) stellarator. In order to gain insight into their distribution, a multi-view spectroscopy system is utilized that measures the Doppler-shifted Balmer-alpha line radiation of charge-exchange neutralized fast-ions (FIDA). The interpretation of the measured spectra is performed using the FIDASIM code. The fast-ions originate from three neutral hydrogen beam injectors (NBI) located in two beam boxes, each capable of delivering 1.7 MW of power with an injection energy of 55 keV. The donor neutrals for the charge exchange reactions consist of both recycling neutrals and those provided by the NBIs. In this study, we present the results of fast-ion experiments and the corresponding observations obtained during the most recent operation campaign of the W7-X.

Through the investigation of the interaction between recycling neutrals and fast-ions, we identified and characterized edge-localized prompt losses, which were subsequently quantified using forward modeling techniques. Our findings revealed that a significant proportion, up to 10% of the injected power, could be lost in the edge region of the plasma.

Additionally, dedicated discharges involving NBI modulation allowed us to isolate active FIDA signals for the first time, which enabled the reconstruction of the density of confined fast-ions, providing valuable insight into their behavior. Moreover, the obtained data will serve as a crucial validation resource in the future for fast-ion slowing down codes.