J. Plasma Fusion Res. SERIES, Vol.3 (2000) ll3-116 Pressure Driven MHD Instabilities Observed in Neutral Beam Heated Plasmas of the Large Helical Device

Two types of low frequency (< l0 kHz) magnetohydrodynamic (MHD) instabilities are observed with the magnetic probes in neutral beam heated plasmas of the Large Helical Device (LHD). One has m = 3 | n = 3 mode structure and the frequency increases with the increase in the bulk plasma beta and tendstosaturate.Thismodegeneratesthesecondandthirdharmonics.Theotherism=2/n=lmode and seems to be excited when the plasma beta value exceed the threshold (- 0.3 Vo) and the plasma current reaches a certain level. The radial structure of soft X-ray fluctuations suggests that the former mode is excited near the edge. The former mode is considered to be the resistive interchange mode excited in the plasma edge region around the l/q = I surface and the latter is the ideal interchange mode excited in the plasma core region around the l/q = 0.5 surface. Keywords: MHD-activity, magnetic fluctuation, Soft X-ray fluctuation, rotational transform, rational surface, magnetic shea