Relations among potential change, fluctuation change and transport barrier in the JFT-2M tokamak

Potential and density/temperature fluctuations at the L-H transition are measured by the heavy ion beam probe on JFT-2M. It has been observed that the timescale of the potential change is as fast as 10-100 μs 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 sawtooth crashes. After a few sawtooth crashes, the potential drops rapidly to the level of the ELM free H mode. From the gradual change of the potential, assuming that dE_r/dr is a key to forming and sustaining the transport barrier. the criterion of dE_r/dr is less than (1.2±0.4) × 10³ kV/m². At an ELM just before the H-L transition. the potential inside the separatrix also shows a rapid positive jump. The timescales of the positive jump and of the recovery of the potential to its negative value are about 40 and 150 μs, respectively. Before the H-L transition, the time between ELMs and/or dithering transition becomes shorter and the plasma finally goes back to the L mode.