国際発表 | 11th Toki Conference

Electric Field Bifurcation, Fluctuation Change and Edge Transport Barrier in the JFT-2M Tokamak

Yukitoshi Miura^a, Takeshi Ido^b, Kouji Shinohara^a, Kensaku Kamiya^a, Yasuji Hamada^b and JFT-2M Group^a
a) Japan Atomic Energy Research Institute, Ibaraki, 311-0193, Japan
b) National Institute for Fusion Science, Gifu, 509-5292 Japan

Abstract.
It is found that the potential at the edge of the plasma shows the very rapid change at L-H transition triggered by a sawtooth crash and there is a second drop of the potential a few millisecond after the first drop. The time scale of the former drop is less than 100msec and that latter one is more than 200msec. The low frequency (< 60kHz) fluctuation (density and/or temperature) decreases at the first drop of the potential. The high frequency (> 80kHz) fluctuation is not suppressed at the first drop of the potential, but is suppressed at the second drop of the potential. At an ELM, the potential inside the separatrix also shows the rapid positive jump with increasing the fluctuation. The time scale of the jump is about 40msec. The recover of the potential to its negative value is rapid and the time scale of the drop is about 150msec.
After an L/H transition, there is a confinement mode called H'-mode in the JFT-2M. It is characterized by the increase of D_a intensity and the coherent mode of density fluctuation (a few hundred kHz) with degrading confinement (t_E(H') ~ 0.7t_E(ELM-free)). In this phase, the edge pedestal width becomes narrow and steep that might have a strong relation with narrowing the width of large dE_r/dr.
In the case of P_NBI~P_th, the potential, SOL SX, and D_a intensities do not drop clearly just after a sawtooth crash and decrease gradually step by step with sawteeth crashes. In this case, with assuming that dE_r/dr is a key to form and to sustain the transport barrier, it is evaluated that the criterion of the dE_r/dr is less than (1.2/FONT>0.4)x10³kV/m².