Studies on neutral beam ion confinement and excitation of beta-induced Alfvén acoustic eigenmodes in the HL-2A tokamak

Abstract

Experiments with high-energy deuterium neutral beam (NB) injection were performed on the HL-2A tokamak. To obtain information on NB deposition and the slowing down of beam ions in HL-2A plasmas, very short-pulse deuterium NB injection, or the so-called ‘blip’ injection, was applied to MHD-quiescent ohmic deuterium plasmas. Analysis of neutron decay following the NB ‘blip’ injection indicates that tangentially injected beam ions are well confined, slowing down classically in the HL-2A tokamak. In contrast to the MHD-quiescent plasma, anomalous losses of beam ions were observed when a core-localized mode with a frequency up-chirping from 15 to 40 kHz appeared in the plasma. The core-localized mode was identified as a beta-induced Alfvén acoustic (BAAE) mode by its frequency sweeping behaviour and numerical calculation. Such a high energetic particle driven mode led to fast-ion loss, showing the strong influence of the core-localized fast-ion-driven BAAE mode on the fast-ion transport. Furthermore, a clear frequency splitting was first observed on the Alfvén-acoustic-type mode, and is found to be strongly linked to the effect of resonant wave–particle interaction, providing further insights into how frequency splitting structures are generated in the plasma.