Rapid change of ion energy distribution and floating potential at L/H transition in the JFT-2M tokamak

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Abstract
Rapid changes of the main ion energy distribution at transitions from L-to-H, H-to-L and during ELMs are studied by time of flight neutral measurements in the JFT-2M tokamak. At the L to H transition, 200-400 mu s prior to the start of Halpha drop, an increase of the high energy neutral flux above an energy of 200 eV is observed. An energy of higher than 200 eV for hydrogen corresponds to the collisionless condition v*i<l just inside the separatrix. The change of the main ion energy distribution after sawtooth crash during an L-mode phase is also studied. The change of the ion energy distribution just after sawtooth crash is the same as that at L/H-transition. The floating potential measured in the SOL also shows the rapid jump to more positive just after the sawtooth crash (at the same time as the change of an ion energy distribution). This shows the increase of ion outflux in the SOL and might correspond to the change in the ion energy distribution. This may be the reason why most H-modes are triggered by a sawtooth. These observations support theories predicting an important role for ion loss for the L/H-transition.