Diagnosing fast ion redistribution due to sawtooth instabilities using fast ion deuterium- α spectroscopy in the mega amp spherical tokamak

Jackson, A. R.; Jacobsen, A. S.; McClements, K. G.; Michael, C. A.; Cecconello, M.

doi:10.1088/1741-4326/abb619

Title: Diagnosing fast ion redistribution due to sawtooth instabilities using fast ion deuterium- α spectroscopy in the mega amp spherical tokamak

Journal Article · Wed Oct 21 00:00:00 EDT 2020 · Nuclear Fusion

DOI:https://doi.org/10.1088/1741-4326/abb619· OSTI ID:1681072

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The efficient confinement of fast ions is crucial to the development of a fusion power plant. Sawtooth crashes are a manifestation of an internal kink instability, which can severely degrade this confinement. The effect of these on a fast ion population can be investigated using fast-ion deuterium-α (FIDA) spectroscopy, where charge exchange reactions between the fast ions and injected neutrals from a neutral beam injection (NBI) auxiliary heating system generate Doppler-shifted photons which can be observed and interpreted to provide information on the temporal evolution of the underlying fast-ion distribution function. Data obtained using this diagnostic can be interpreted using the codes TRANSP/NUBEAM and FIDASIM, which produces a synthetic spectrum for forward modelling of the experimental data. We have examined the effects of the sawtooth instability using these techniques on the Mega Amp Spherical Tokamak (MAST). We have extended this comparison between fast ion data and sawtooth models to FIDA measurements, and conclude that FIDA data also cannot be used to distinguish between the models. For FIDA channels that probe the sawtoothing region, at each sawtooth crash there is an overall drop in the emission of up to 60%. Data from passive FIDA channels (i.e. with emission resulting from neutralisation by thermal neutrals in the plasma periphery rather than beam neutrals) show a sudden increase in the emission following sawtooth crashes. The subsequent decay in the emission in these passive channels indicate that redistributed passing fast ions are rapidly lost from the edge region, probably as a result of charge-exchange reactions with edge neutrals.

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Research Organization:: Univ. of California, Los Angeles, CA (United States)

Sponsoring Organization:: USDOE Office of Science (SC), Fusion Energy Sciences (FES)

Grant/Contract Number:: SC0019007

OSTI ID:: 1681072

Alternate ID(s):: OSTI ID: 1760163

Journal Information:: Nuclear Fusion, Journal Name: Nuclear Fusion Vol. 60 Journal Issue: 12; ISSN 0029-5515

Publisher:: IOP ScienceCopyright Statement

Country of Publication:: IAEA

Language:: English

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