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

• Published in: Nuclear fusion Vol. 54; no. 2; pp. 23005 - 7
• Main Authors: Salewski, M., Geiger, B., Jacobsen, A.S., García-Muñoz, M., Heidbrink, W.W., Korsholm, S.B., Leipold, F., Madsen, J., Moseev, D., Nielsen, S.K., Rasmussen, J., Stejner, M., Tardini, G., Weiland, M.
• Format: Journal Article
• Language: English
• Published: IOP Publishing 01.02.2014
• Subjects: fast ions in tokamaks; fast-ion velocity distribution function; inverse problems; Inversions; Magnetic fields; Mathematical analysis; Neutral beams; Simulation; Spectra; tomography; Two dimensional; Velocity distribution
• ISSN: 0029-5515; 1741-4326
• DOI: 10.1088/0029-5515/54/2/023005

We present the first measurement of a local fast-ion 2D velocity distribution function f(v|, v⊥). To this end, we heated a plasma in ASDEX Upgrade by neutral beam injection and measured spectra of fast-ion Dα (FIDA) light from the plasma centre 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|, v⊥) by tomographic inversion. Salient features of our measurement of f(v|, v⊥) agree reasonably well with the simulation: the measured as well as the simulated f(v|, v⊥) are lopsided towards negative velocities parallel to the magnetic field, and they have similar shapes. Further, the peaks in the simulation of f(v|, v⊥) 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|, v⊥) by tomographic inversion.