Diagnosing fast ion redistribution due to sawtooth instabilities using fast ion deuterium-alpha spectroscopy in the Mega Amp Spherical Tokamak

Diagnosing fast ion redistribution due to sawtooth instabilities using fast ion deuterium-alpha spectroscopy in the Mega Amp Spherical Tokamak

Diagnosing fast ion redistribution due to sawtooth instabilities using fast ion deuterium-alpha spectroscopy in the Mega Amp Spherical Tokamak 150 150 amit lakhani
UKAEA-CCFE-PR(20)88

Diagnosing fast ion redistribution due to sawtooth instabilities using fast ion deuterium-alpha spectroscopy in the Mega Amp Spherical Tokamak

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-alpha (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). Work reported by M. Cecconello et. al., (Plasma Physics and Controlled Fusion 60, 055008 (2018)) showed that neutron camera data could not be used to distinguish between three models used in TRANSP to simulate the effects of sawteeth on the fast ions. 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.

Collection:
Journals
Journal:
Plasma Physics and Controlled Fusion
Publisher:
IOP (Institute of Physics)
Want to keep updated?

For updates on latest research delivered straight to your inbox, just sign up to our newsletter.