Fast potential changes at H-mode transition in the JFT-2m tokamak

The fast potential change at H-mode transition was measured for the first time by a 500 keV heavy ion beam probe installed on the JFT-2M tokamak. It is found that in an ordinary sawtooth- triggered L/H-transition, the fast positive potential change is observed near the separatrix in coincidence with the fast Dα rise and the fast density change. The potential goes down at the fall of Dα. The structure of the radial electric field about 30 msec after L/H transition agrees with the previous results determined by the poloidal, toroidal rotation speed and the pressure gradient. (author)