Rapid change of the neutral hydrogen energy distribution at theL/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 edge localized modes (ELMs) are studied with the time-of-flight neutral measurement on the JFT-2M tokamak. At the L to H transition, 200–400 μs prior to the start of the ${\mathrm{H}}_{\mathrm{\alpha }}$ drop, an increase of the high-energy outflux above an energy of 200 eV is observed. An energy of more than 200 eV for hydrogen corresponds to the collisionless condition ${\nu }_{\mathrm{*}\mathit{i}}$<1 just inside the separatrix. A change of the energy distribution preceding that of the ${\mathrm{H}}_{\mathrm{\alpha }}$ signal is also found for ELMs.