Pedestal dynamics prior to type-III ELM onset on HL-2A tokamak

A dramatic increase of density gradient and a slight decrease of electron temperature gradient were observed in the pedestal just prior to each onset in a series of ELM bursts. An inward particle flux induced by a quasi-coherent mode was found to be responsible for such changes. The characteristics of the quasi-coherent mode, including frequency, wave vector, propagation direction and velocity were identified. The nonlinear three-wave coupling is a plausible mechanism for the generation of the mode. Decoupling of the mode induced density transport from energy transport was discovered. The gradient of density was observed always much higher than that of temperature for electrons at the starting point of the ELM burst and, therefore, the dominant role of density gradient over electron temperature gradient for ELM onset is demonstrated. Consistence and contrast of the results with confinement and transport in I-mode discharges are discussed.