Inversion methods for fast-ion velocity-space tomography in fusion plasmas

Asger Schou Jacobsen, L. Stagner, Mirko Salewski, B. Geiger, W.W. Heidbrink, Søren Bang Korsholm, Frank Leipold, Stefan Kragh Nielsen, Jesper Rasmussen, Morten Stejner Pedersen, H. Thomsen, M. Weiland

Research output: Contribution to journalJournal articleResearchpeer-review

Abstract

Velocity-space tomography has been used to infer 2D fast-ion velocity distribution functions. Here we compare the performance of five different tomographic inversion methods: truncated singular value decomposition, maximum entropy, minimum Fisher information and zeroth and first-order Tikhonov regularization. The inversion methods are applied to fast-ion Dα measurements taken just before and just after a sawtooth crash in the ASDEX Upgrade tokamak as well as to synthetic measurements from different test distributions. We find that the methods regularizing by penalizing steep gradients or maximizing entropy perform best. We assess the uncertainty of the calculated inversions taking into account photon noise, uncertainties in the forward model as well as uncertainties introduced by the regularization which allows us to distinguish regions of high and low confidence in the tomographies. In high confidence regions, all methods agree that ions with pitch values close to zero, as well as ions with large pitch values, are ejected from the plasma center by the sawtooth crash, and that this ejection depletes the ion population with large pitch values more strongly.
Original languageEnglish
Article number045016
JournalPlasma Physics and Controlled Fusion
Volume58
Issue number4
Number of pages16
ISSN0741-3335
DOIs
Publication statusPublished - 2016

Keywords

  • Fast ions
  • Tomography
  • Sawtooth crash
  • Fast-ion D-alpha spectroscopy

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