Fast-ion energy loss during TAE avalanches in the National Spherical Torus Experiment

Abstract

Strong toroidal Alfvén eigenmode (TAE) avalanches on NSTX, the National Spherical Torus Experiment (Ono et al 2000 Nucl. Fusion 40 557) are typically correlated with drops in the neutron rate in the range 5–15%. In previous studies of avalanches in L-mode plasmas, these neutron drops were found to be consistent with modelled losses of fast ions. Here we expand the study to TAE avalanches in NSTX H-mode plasmas with improved analysis techniques. At the measured TAE mode amplitudes, simulations with the ORBIT code predict that fast ion losses are negligible. However, the simulations predict that the TAE scatter the fast ions in energy, resulting in a small (≈5–6%) drop in fast ion β. The net decrease in energy of the fast ions is sufficient to account for about 50% of the drop in neutron rate, redistribution for ≈40%, and fast ion losses account for only ≈10%. This loss of energy from the fast ion population is comparable to the estimated energy lost by damping from the Alfvén wave during the burst. The previously studied TAE avalanches in L-mode are re-evaluated using an improved calculation of the potential fluctuations in the ORBIT code near the separatrix.