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 the heliotron/torsatron devices Compact Helical System (CHS) and Large Helical Device (LHD) are presented. Alfvén eigenmodes (AEs) and fishbone-like burst modes (FBs) destabilized by energetic ions were observed in NBI heated plasmas of CHS. The AEs are toroidicity induced Alfvén eigenmodes (TAEs) and global Alfvén eigenmodes (GAEs), where the identified toroidal mode numbers are 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 density dependence related to Alfvén velocity. The modes have characteristic features of the energetic particle modes or the resonant TAEs excited by circulating energetic beam ions produced by NBI. Bursting amplitude modulation is observed in TAEs as well as in FBs. Rapid frequency chirping is observed in each burst, by a factor of 2-6 in FBs and about 25% in TAEs. In several shots, the power spectrum of the TAEs is split into multiple peaks having the same toroidal mode number through non-linear evolution of TAEs. A pulsed increase in energetic ion loss towards the wall is induced by m = 3/n = 2 FBs, but so far not by m = 2/n = 1 FBs, TAEs and GAEs, where m is the poloidal mode number. This research has been extended to LHD plasmas heated by neutral hydrogen beams with about 130 keV energy. Similar to CHS, TAEs and FBs were observed in relatively low density plasmas at low toroidal magnetic field (Bt = 1.5 T).

Design and initial operation of lost fast-ion probe based on thin Faraday films in CHS

“…This observation is consistent with that of the SLIP. 4,5 H␣ light emissivity from the edge chord at the outboard side of horizontally elongated cross section also increases periodically. This is due to enhanced transport of beam ions toward the outboard side of the torus.…”

“…4,5 Although the SLIP can provide the relative intensity of lost fast-ion flux at the probe position and information on their gyroradius centroid and pitch angle, an issue on their absolute flux still remains because of difficulty of absolute calibration of the SLIP. In order to enhance further fast-ion physics experiment in CHS, we have designed and constructed a new lost fast-ion probe based on thin Faraday films ͑FLIP͒, providing the electric current generated by lost fast ions.…”

Escaping Ion Measurement with High Time Resolution during Bursting Modes Induced by Neutral Beam Injection on CHS

“…Generally, it is considered that the mode structure of the EPM is sensitive to the energy of the resonating energetic ions, which drive the mode. In the previous study, the mode structure measurement for similar bursting modes was carried out by measuring a profile of a soft X ray emission [6], and the mode structures were considered to be a localized one. The energetic ions might be transported to the region, where escaping ions originate, by the localized mode.…”

“…Bursting modes during a NB injected plasma were observed in the compact helical system (CHS) as well as in tokamaks, and the enhanced energetic ion transport by these mode was also observed [6]. The mode-induced energetic ion transport was investigated using a scintillatorbased lost ion probe (LIP) [7,8].…”

Poloidal trapping of the high-frequency Alfvén continuum and eigenmodes in stellarators

“…Alfvén instabilities have been observed in many experiments on tokamaks and stellarators [1,2,3]. They are of interest since they can lead to enhanced loss of fast ions [1,2,3] and considerably affect the confinement of the thermal plasma [2,4].…”