Measurement of a 2D fast-ion velocity distribution function by tomographic inversion of fast-ion D-alpha spectra

• Published in: arXiv.org 2015-09
• Main Authors: Salewski, Mirko, Geiger, Benedikt, Asger Schou Jacobsen, Garcıa-Munoz, Manuel, Heidbrink, Bill, Soren Bang Korsholm, Leipold, Frank, Madsen, Jens, Moseev, Dmitry, Nielsen, Stefan Kragh, Rasmussen, Jesper, Stejner, Morten, Tardini, Giovanni, Weiland, Markus, the ASDEX Upgrade team
• Format: Article
• Language: English
• Subjects: Beam injection; Distribution functions; Ion velocity; Neutral beams; Simulation; Spectra; Velocity distribution
• ISSN: 2331-8422
• DOI: 10.48550/arxiv.1509.08482

We present the first measurement of a local fast-ion 2D velocity distribution function \(f(v_\parallel, v_\perp)\). To this end, we heated a plasma in ASDEX Upgrade by neutral beam injection and measured spectra of fast-ion D-alpha (FIDA) light from the plasma center in three views simultaneously. The measured spectra agree very well with synthetic spectra calculated from a TRANSP/NUBEAM simulation. Based on the measured FIDA spectra alone, we infer \(f(v_\parallel, v_\perp)\) by tomographic inversion. Salient features of our measurement of \(f(v_\parallel, v_\perp)\) agree reasonably well with the simulation: the measured as well as the simulated \(f(v_\parallel, v_\perp)\) are lopsided towards negative velocities parallel to the magnetic field, and they have similar shapes. Further, the peaks in the simulation of \(f(v_\parallel, v_\perp)\) at full and half injection energies of the neutral beam also appear in the measurement at similar velocity-space locations. We expect that we can measure spectra in up to seven views simultaneously in the next ASDEX Upgrade campaign which would further improve measurements of \(f(v_\parallel, v_\perp)\) by tomographic inversion.