Fast-ion deuterium alpha spectroscopic observations of the effects of fishbones in the Mega-Ampere Spherical Tokamak

Abstract

Abstract. Using the recently-installed Fast-Ion Deuterium Alpha (FIDA) spectrometer, the effects of low-frequency (20 − 50 kHz) chirping energetic particle modes with toroidal mode number n ≥ 1 on the NBI-driven fast-ion population in MAST plasmas are considered. Results from the FIDA diagnostic are presented and discussed in the light of the present theoretical understanding of these modes, known as fishbones, in plasmas with reversed shear. Measurements of the fast-ion population reveal strong redistribution of fast ions in both real and velocity space as a result of the fishbones. Time-resolved measurements throughout the evolution of a fishbone show radial redistribution of fast ions with energies up to 95% of the primary beam injection energy. Correlations between changes in the FIDA signal and the peak time derivative of the magnetic field perturbation are observed in a limited range of operating scenarios. The transient reduction in signal caused by a fishbone may in some cases reach 50% of the signal intensity before mode onset.

Measurements and modelling of fast-ion redistribution due to resonant MHD instabilities in MAST

“…The data are averaged over three channels with beam intersection radii 1.03 m, 1.07 m and 1.14 m. A different symbol is used for each shot, but the regression analysis takes into account the whole data set. As in the earlier study [28], the drops in FIDA signal are correlated much more strongly with the RMS Mirnov coil signal than with the fishbone amplitude. A similar linear scaling was found between the drops in global neutron rate and the peak Mirnov coil signal in a previous study of MAST data [29].…”

“…The results presented in this paper demonstrate the advantages of employing several complementary diagnostics to study the behaviour of the FI population. Extending previous studies on MAST [8,28,29,36], a systematic analysis of NC and FIDA data has allowed the deleterious effects of chirping TAE and fishbones on the confined FI population to be identified with confidence. The velocity-space sensitivity of the FIDA measurements moreover permits losses of FI to be identified unambiguously, and these losses are seen to be strongly correlated with the MHD activity.…”

“…In previous studies of MAST FIDA data [28], it was found that the relative change in core FIDA signal due to the fishbones correlated strongly with the maximum RMS amplitude of the Mirnov coil signal and only weakly with the amplitude of the perturbation. This analysis was repeated for the data from the recent set of discharges (#29207 et seq.).…”

Quantification of the impact of large and small-scale instabilities on the fast-ion confinement in ASDEX Upgrade

“…Already 20 years ago, a clear reduction of measured neutron rates was observed at the JET [5] and TFTR [6] tokamaks during sawtooth crashes, indicating a significant fast-ion transport. In recent years, measurements at DIII-D [7] and TEXTOR [8] showed that sawteeth redistribute passing fast ions more strongly than trapped ions (as predicted by theory [9] above a given critical fast-ion energy) and measurements at MAST [10] and JET [11] evidenced a reduction of the density of confined fast ions in the presence of fishbones.…”

Investigating fusion plasma instabilities in the Mega Amp Spherical Tokamak using mega electron volt proton emissions (invited)

“…1,2 Energy spectra were obtained for 3 MeV fusion protons (from Deuterium-Deuterium, or DD, fusion reactions) detected in the Princeton Large Torus and Axially Symmetric Divertor Experiment. [3][4][5] Spherical tokamak fast ion diagnostics such as the Mega Amp Spherical Tokamak (MAST) Neutron Camera 6 (NC) use 2.5 MeV DD fusion neutrons to infer emission profiles, while the MAST Fast Ion Deuterium Alpha (FIDA) spectrometer, 7 National Spherical Torus Experiment (NSTX) FIDA 8 spectrometer, and NSTX solid state neutral particle analyzer 9 observe charge exchange interactions, to study fast ion distributions during plasma MHD instabilities.…”