MHD Instabilities and Their Effects on Plasma Confinement in the Large Helical Device Plasmas

"Characteristic 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 plasma in the range of 2% averaged beta at low toroidal field (B_t leq 0.6T). The stored energy rapidly rises by the transition, but quickly saturates by 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 low beta plasmas, ELM like activities are sometimes induced in high performance plasmas with high edge pressure pedestal, and transiently reduce the stored energy by about 10%. Energetic ion driven MHD modes such as Alfven eigenmodes are studied in the very wide parameter range of the averaged beta of energetic ions up to 5% and the ratio of energetic ion velocity to the alfvben velocity V_(bparallel)/V_A up to 2.5. In addition to the observation of toroidicity indued Alfven eigenmodes (TAEs), coherent magnetic fluctuations of helicity induced 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 enhance energetic ion loss transiently, but lead to the transient improvement of bulk plasma confinement in the plasma central region.