3D neutral distribution in C-2W determined from visible imaging and interpretive modeling

The C-2W device\footnote{M.~Binderbauer, et~al. AIP Conference Proceedings \textbf{1721}, 030003 (2016)} is equipped with eight 1.6 MW neutral beams which inject large-orbit fast ions to sustain a field-reversed configuration embedded within an axisymmetric magnetic mirror. Collisional energy transfer from fast ions is intended to be the primary source of heating of the bulk plasma; therefore, the spatial and velocity distribution of fast ions strongly affects the overall power balance and understanding fast ion loss is critical. At typical injected energies (15 keV) and external fields (0.7-1 kG) fast ions sample the core, scrape-off-layer, and diffuse outer-edge plasmas where they can be lost via charge-exchange with neutrals. Careful vacuum practices and titanium gettering successfully reduced neutral recycling from the vessel walls. As a result, warm neutrals generated from beam capture via charge-exchange with bulk plasma ions constitute a large fraction of the remaining neutrals. Balmer-$\alpha$ emission measured with a filtered high-speed camera is used with DEGAS2 neutral particle modeling to reconstruct the strongly non-axisymmetric neutral distribution. This distribution is then used with Monte Carlo modeling to estimate the charge-exchange loss rate of fast ions.