Study of interaction between energetic ions and MHD instabilities in a helical plasma

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Characteristics of energetic ion driven MHD modes and their effects on energetic ion transport were studied in neutral-beam-injection (NBI) heated plasmas of the Large Helical Device(LHD). The following new and important results were obtained.(1) Two types of toroidicity-induced Alfven eigenmodes (TAEs) were observed. One is a core-localized TAE(C-TAE) of which TAE gap location is near the plasma center with low magnetic shear. The other is a global TAE of which gap location is in the outer portion of the minor plasma radius. In a few shots, C-TAEs with even and odd parity were simultaneously excited.(2) When the rotational transform at the plasma center exceeds a certain value, the relevant C-TAE has been converted to two global Alfven eigenmodes (GAEs) because the TAE gap is removed. This mode transition from C-TAE to GAE is also observed in a reversed shear configuration of the JT-60U tokamak.(3) In high beta plasmas, high frequency Alfven eigenmodes of which frequency is by eight times higher than the TAE gap frequency were observed. Comparison of the modes with the calculated shear Alfven spectra suggests that the high frequency mode is a helicity-induced Alfven eigenmode (HAE).(4) About 30-50 % of total energetic ions generated by NBI is transiently lost by bursting TAEs with rapid frequency chirping. The bursting TAEs often trigger the formation of internal transport barrier.In the Compact Helical system (CHS) heliotron/torsatron, TAEs were excited by external magnetic perturbations generated by the inserted electrodes into a low density and low temperature plasma without energetic ions. This experiment was carried out as a basic study ofAlfven eigenmodes in a helical plasma.This research project has demonstrated that the Alfven eigenmode spectroscopy using energetic ions and externally applied magnetic perturbations is very useful to study MHD properties of a helical plasma.