Observation of the Fast Potential Change at L-H Transition by a Heavy-Ion-Beam Probe on JFT-2M

The fast potential change near the separatrix is measured directly at the L-H transition by a heavy-ion-beam probe. The potential changes with two different time scales at the L-H transition triggered by a sawtooth crash: it drops at first with the time scale of $10–100\mu \mathrm{s}$ just after the arrival of the heat pulse due to the sawtooth crash. Then, it decreases again at a few $100\mu \mathrm{s}$ after the first drop at a time scale of about $200\mu \mathrm{s}$.