Temporal behaviour of the potential and fluctuations at the L-H transition on JFT-2M

Potential and density/temperature fluctuations at the L-H transition are measured by a heavy ion beam probe (HIBP) on JFT-2M. It has been observed that the time scale of 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 step by step, with sawteeth crashes accompanied by a decrease of potential. After a few sawteeth crashes, the potential drops rapidly to the level of the ELM-free H-mode. At that time, the density/temperature fluctuations are suppressed simultaneously. The observations of the temporal behaviour of the potential and fluctuations in such an L-H transition are reported.