Energetic ion driven MHD instabilities observed in the heliotron/torsatron devices Compact Helical System and Large Helical Device

Abstract

Recent results of energetic ion driven MHD instabilities observed in theheliotron/torsatron devices Compact Helical System (CHS) and Large HelicalDevice (LHD) are presented. Alfvén eigenmodes (AEs)and fishbone-like burst modes (FBs) destabilized by energetic ions were observed inNBI heated plasmas of CHS. The AEs are toroidicity induced Alfvén eigenmodes (TAEs)and global Alfvén eigenmodes (GAEs), where the identified toroidal mode numbersare n = 1 and 2 for TAEs and n = 0 for GAEs. The frequencies of the FBs are less than,at most, half of the minimum TAE gap frequency and do not exhibit the obvious densitydependence related to Alfvén velocity. The modes have characteristic features of theenergetic particle modes or the resonant TAEs excited by circulating energetic beamions produced by NBI. Bursting amplitude modulation is observed in TAEs as well as inFBs. Rapid frequency chirping is observed in each burst, by a factor of 2-6 in FBsand about 25% in TAEs. In several shots, the power spectrum of the TAEs is split intomultiple peaks having the same toroidal mode number through non-linear evolution ofTAEs. A pulsed increase in energetic ion loss towards the wall is induced by m = 3/n = 2FBs, but so far not by m = 2/n = 1 FBs, TAEs and GAEs, where m is the poloidal modenumber. This research has been extended to LHD plasmas heated by neutral hydrogen beamswith about 130 keV energy. Similar to CHS, TAEs and FBs were observed in relatively lowdensity plasmas at low toroidal magnetic field (Bt = 1.5 T).