RAPID CHANGE OF ION ENERGY-DISTRIBUTION AND FLOATING POTENTIAL AT L HTRANSITION IN THE JFT-2M TOKAMAK/

Rapid changes of the main ion energy distribution at transitions from L-to-H, H-to-L and during ELMs are studied with the time of flight neu tral measurement in the JFT-2M tokamak. At the L to H transition, 200- 400 mus prior to the start of H(alpha) drop, an increase of the high e nergy neutral flux above an energy of 200eV is observed. An energy of higher than 200eV for hydrogen corresponds to the collisionless condit ion v(i)<1 just inside the separatrix. The change of the main ion ene rgy distribution after sawtooth crash during an L-mode phase is also s tudied. The change of the ion energy distribution just after sawtooth crash is the same as that at L/H-transition. The floating potential me asured in the SOL also shows the rapid jump to more positive just afte r the sawtooth crash (at the same time of the change of an ion energy distribution). This shows the increase of ion outflux in the SOL and m ight correspond to the change of the ion energy distribution. This may be the reason why most of H-modes are triggered by a sawtooth. These observations support theories predict an important role of ion loss fo r the L/H-transition.