Collapse of density pedestal by giant ELM on JT-60U

In JT-60U ELMy H-mode discharges, the detailed behaviour of giant (type I) ELMs was measured using a heterodyne reflectometer system in order to under stand the collapse mechanism of the pedestal structure and consequences on the core/edge plasma. The phase signal of the reflectometer exhibits the mo vement of the cutoff layer (density layer) due to the collapse of the pedes tal in a density profile by an ELM. An ELM event can be classified into a p recursor phase, collapse phase, recovery phase and a relaxation phase. A ce rtain density layer measured near the shoulder of the pedestal moves about 7 cm inside the plasma in the collapse phase. The precursor oscillation in a pedestal density and the relationship between the collapse of the pedesta l structure and a D-alpha burst are also studied.