On MHD Oscillation in high beta H-mode tokamak plasma in JFT-2M

K.Hoshino, K.Tsuzuki, H.Kawashima, H.Ogawa, M.Bakhtiari, K.Shinohara, Y.Nagashima 1) , K.Uehara, T.Ido 2) , M.Sato, K.Ohasa, S.Kasai and Y.Kusama

Japan Atomic Energy Research Institute, Tokai,Naka,Ibaraki 319-11,Japan
1) Department of Physics, The University of Tokyo, Tokyo 113-0033,Japan
2) National Institute for Fusion Science, Toki, Gifu 509-5292,Japan

In recent high beta H-mode experiments in JFT-2M tokamak, total beta value above ~2.5% is obtained. The high performance plasma is obtained at high density region (Greenwald normalized density up to ~1, line averaged density~0.7x1020m-3) with high triangularity up to ~0.7. The obtained normalized beta value (Toroyon factor)is up to ~3.2[1]. Suppression of the MHD oscillations is very important to attain the high beta plasma. First, the suppression of global low mode number MHD oscillations is important. To avoid m=1 sawteeth oscillation(~ hundred Hz), current profile flattening by plasma current ramp-up with utilizing counter neutral beam injection is effective. This small magnetic shear operation is accompanied by internal transport barrier (ITB) formation. The m=2 tearing mode (a few kHz) is observed to be often destabilized in high beta region and it disturbes the increase of the stored energy by affecting the ITB formation, though it has been difficult to control the mode practically. We find a stabilization of the m=2 mode by plasma inward shift. Wall stabilization is a possible candidate of this mechanism. In the edge transport barrier (ETB) region, we find a several kinds of characteristic MHD oscillations the frequency of which is ranged from a several 10kHz to a few 100kHz. The frequency of the MHD oscillation changes with the edge deuterium line radiation level and the oscillation accompanies the density oscillation and plasma potential oscillation. A characterization of these oscillations is made. But their quantitative influence on the transport is difficult to estimate as the co-existent turbulence the frequency of which is up to several 10kHz affects the transport, and it is difficult to discriminate the role of these coherent MHD oscillations.


References

[1] K.Hoshino et al., Meeting Abstracts of the Physical Society of Japan 58, Issue 1,part 2 (2003)233.