On MHD Oscillations in High Beta H-Mode Tokamak Plasma in JFT-2M

In the recent high beta H-mode experiments in JFT-2M tokamak, total beta value above ~2.5% is obtained. The high performance plasma is obtained in high density region (normalized density ~1) with high triangularity ~0.7 in the limiter configuration. Suppression of the low frequency (< 10kHz) MHD (magnetohydrodynamic) oscillations is very important to obtain such high beta plasma. We find a stabilization of low mode MHD oscillation by current profile control by ramp-up of plasma current, counter neutral beam injection and inward shift of the plasma column. In the study of MHD oscillations in the edge transport barrier, we observe precursor MHD oscillations in density before the intermittent ELMs (edge localized modes), or two kinds (low frequency and high frequency) of characteristic quasi-coherent MHD oscillations (~ 70kHz to ~ 310kHz) in density, or in magnetic field, or in potential. These quasicoherent modes are not found in the ELM-free H-mode phase but appear in the slightly degraded H-mode phase (H’mode) or in enhanced Dα phase. A comparison of these quasi-coherent MHD with background turbulent fluctuation level is presented.