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 (mu)s when the input power (Pin) is larger than the L-H threshold power (Pth)- When Pin ~ Pth, 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 dEr/dr is a key to forming and sustaining the transport barrier, the criterion of dEr/dr is less than (1.2-+0.4) x 10^3 kV/m^2. At an ELM just before the HL 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 /(mu)s, 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.