A resistive interchange mode of the

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).

have started this year after a successful eight-year construction and test period of the fully superconducting facility. LHD investigates a variety of physics issues on large scale heliotron plasmas ͑Rϭ3.9 m, aϭ0.6 m͒, which stimulates efforts to explore currentless and disruption-free steady plasmas under an optimized configuration. A magnetic field mapping has demonstrated the nested and healthy structure of magnetic surfaces, which indicates the successful completion of the physical design and the effectiveness of engineering quality control during the fabrication. Heating by 3 MW of neutral beam injection ͑NBI͒ has produced plasmas with a fusion triple product of 8ϫ10 18 keV m Ϫ3 s at a magnetic field of 1.5 T. An electron temperature of 1.5 keV and an ion temperature of 1.4 keV have been achieved. The maximum stored energy has reached 0.22 MJ, which corresponds to ͗␤͘ϭ0.7%, with neither unexpected confinement deterioration nor visible magnetohydrodynamics ͑MHD͒ instabilities. Energy confinement times, reaching 0.17 s at the maximum, have shown a trend similar to the present scaling law derived from the existing medium sized helical devices, but enhanced by 50%. The knowledge on transport, MHD, divertor, and long pulse operation, etc., are now rapidly increasing, which implies the successful progress of physics experiments on helical currentless-toroidal plasmas.

Characteristics of MHD instabilities and their impacts on plasma confinement are studied in current free plasmas of the Large Helical Device(LHD). Spontaneous L-H transition is often observed in high beta plasmas in the range of 2% averaged beta at low toroidal field (B t ≤ 0.6T). The stored energy rapidly rises by the transition, but quickly saturates due to the growth of m=2/n=3 and m=2/n=2 modes (m and n: poloidal and toroidal mode numbers) excited in the plasma edge region. Even in low beta plasmas, ELM like activities are sometimes induced in high performance plasmas with steep edge pressure gradient, and transiently reduce the stored energy by about 10%. Energetic ion driven MHD modes such as Alfven eigenmodes are studied in the very wide range of characteristic parameters: the averaged beta of energetic ions <β b// > up to 5% and the ratio of energetic ion velocity to the Alfven velocity V b// /V A up to 2.5. In addition to the observation of toroidicity induced Alfven eigenmodes (TAEs), coherent magnetic fluctuations of helicity induced Alfven eigenmodes (HAEs) have been observed for the first time in NBI heated plasmas. Transition of TAE to global Alfven eigenmode(GAE) is also observed in a discharge with temporal evolution of the rotational transform profile, having a similarity to the phenomenon in a reversed shear tokamak. At the low magnetic field, bursting TAEs transiently induce a significant loss of energetic ions, but lead to the transient improvement of bulk plasma confinement in the plasma central region.