Abstract
Modification of a high energy particle distribution by a spectrum of low amplitude modes is investigated using a guiding center code. Only through resonance are modes effective in modifying the distribution. Diagnostics are used to illustrate the mode–particle interaction and to find which effects are relevant in producing significant resonance, including kinetic Poincaré plots and plots showing those orbits with time averaged mode–particle energy transfer. Effects of pitch angle scattering and drag are studied, as well as plasma rotation and time dependence of the equilibrium and mode frequencies. A specific example of changes observed in a DIII-D deuterium beam distribution in the presence of low amplitude experimentally validated Toroidal Alfvèn eigenmodes and reversed shear Alfvèn eigenmodes is examined in detail. Comparison with experimental data shows that multiple low amplitude modes can account for significant modification of high energy beam particle distributions. It is found that there is a stochastic threshold for beam profile modification, and that the experimental amplitudes are only slightly above this threshold.
Highlights
Energetic ion populations often drive Alfven waves unstable in toroidal magnetic confinement devices [1]
We have considered means by which low amplitude modes can produce changes in a high energy particle population
We have demonstrated that many toroidicity-induced Alfven eigenmodes (TAEs) and reversed shear Alfven eigenmodes (RSAEs) modes can significantly modify a beam particle distribution, even with amplitudes of the level of dB/B 2 × 10−4
Summary
Energetic ion populations often drive Alfven waves unstable in toroidal magnetic confinement devices [1]. To test the assumption that the Alfven modes cause the additional fast-ion transport, in previous works [7, 8], we inserted the magnetic part of the NOVA calculated eigenfunctions that were experimentally validated by ECE measurements into the guiding center code ORBIT [11] and calculated the expected fast-ion transport. Many small amplitude Alfven eigenmodes can cause fast-ion transport that approaches the experimentally observed levels, and simulations can reproduce this provided that all modes and all important effects are included in the simulation; that is, the guiding center equations must include many harmonics and all significant mode–particle coupling terms. The beam-ion transport possesses a stochastic threshold very near the experimental mode amplitude values, so the results are very sensitive to small effects.
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