High-definition velocity-space tomography of fast-ion dynamics

• Published in: Nuclear fusion 2016-10, Vol.56 (10), p.106024
• Main Authors: Salewski, M., Geiger, B., Jacobsen, A.S., Hansen, P.C., Heidbrink, W.W., Korsholm, S.B., Leipold, F., Madsen, J., Moseev, D., Nielsen, S.K., Nocente, M., Odstr il, T., Rasmussen, J., Stagner, L., Stejner, M., Weiland, M.
• Format: Article
• Language: English
• Subjects: fast ions; fast-ion D-alpha spectroscopy; neutral beam injection; sawteeth; tokamaks; tomography
• ISSN: 0029-5515; 1741-4326
• DOI: 10.1088/0029-5515/56/10/106024

Velocity-space tomography of the fast-ion distribution function in a fusion plasma is usually a photon-starved tomography method due to limited optical access and signal-to-noise ratio of fast-ion Dα (FIDA) spectroscopy as well as the strive for high-resolution images. In high-definition tomography, prior information makes up for this lack of data. We restrict the target velocity space through the measured absence of FIDA light, impose phase-space densities to be non-negative, and encode the known geometry of neutral beam injection (NBI) sources. We further use a numerical simulation as prior information to reconstruct where in velocity space the measurements and the simulation disagree. This alternative approach is demonstrated for four-view as well as for two-view FIDA measurements. The high-definition tomography tools allow us to study fast ions in sawtoothing plasmas and the formation of NBI peaks at full, half and one-third energy by time-resolved tomographic movies.