18th IAEA Fusion Energy Conference

Relation among Potential, Fluctuation Change and Transport Barrier in the JFT-2M Tokamak

Y. Miura¹⁾, T. Ido²⁾, K. Kamiya¹⁾, Y. Hamada²⁾ and JFT-2M GROUP¹⁾
1) Japan Atomic Energy Research Institute, Ibaraki, 311-0193, Japan
2) National Institute for Fusion Science, Gifu, 509-5292 Japan

Abstract.
Potential and density/temperature fluctuation at L-H transition are measured by the heavy ion beam probe (HIBP) on JFT-2M. It has been observed that the time scale of potential change is as fast as 10-100ms when the input power (P_in) is larger than the L-H threshold power (P_th). When P_in~P_th, the confinement is improved gradually step by step with sawteeth crashes accompanied with the decrease of potential. After a few sawteeth crashes, potential drops rapidly to the level of the ELM-free H-mode. From the gradual change of the potential with assuming that dE_r/dr is a key to form and to sustain the transport barrier, the criterion of the dE_r/dr is less than (1.2 /FONT> 0.4)x10³ kV/m². At an ELM just before H-L transition, the potential inside the separatrix also shows the rapid positive jump. The time scale of the positive jump and its recover to its negative value is about 40msec and 150msec, respectively. Before the H-L transition, the time between ELMs and/or dithering transition becomes shorter and the plasma finally goes back to L-mode.