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

Abstract

In JT-60U ELMy H-mode discharges, the detailed behaviour of giant (type I) ELMs wasmeasured using a heterodyne reflectometer system in order to understandthe collapse mechanism of the pedestal structure and consequences on the core/edgeplasma. The phase signal of the reflectometer exhibits the movement of the cutofflayer (density layer) due to the collapse of the pedestal in a density profile byan ELM. An ELM event can be classified into a precursor phase, collapse phase,recovery phase and a relaxation phase. A certain density layer measured near theshoulder of the pedestal moves about 7 cm inside the plasma in the collapse phase. Theprecursor oscillation in a pedestal density and the relationship between the collapseof the pedestal structure and a Dα burst are also studied.