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


We present the first measurement of a local fast-ion 2D velocity distribution function f(ν∥, ν⊥). 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(ν∥, ν⊥) by tomographic inversion. Salient features of our measurement of f(ν∥, ν⊥) agree reasonably well with the simulation: the measured as well as the simulated f(ν∥, ν⊥) are lopsided towards negative velocities parallel to the magnetic field, and they have similar shapes. Further, the peaks in the simulation of f(ν∥, ν⊥) 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(ν∥, ν⊥) by tomographic inversion. © 2014 IAEA, Vienna.

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