Pedestal dynamics and turbulence in H-mode density ramp-up experiment on EAST

Abstract

Abstract Recent results of density ramp-up (<ne>/nGW = 0.5 - 0.75) experiment in type-I ELMy H-mode on experimental advanced superconducting tokamak (EAST) are presented, with a focus on pedestal dynamics and turbulence behavior. With the density increase, a general trend is that the pedestal electron pressure (peped) decreases while the ELM frequency (fELM) increases. Especially when <ne>/nGW arrives at ~ 0.68, a sudden decrease of peped and an increase of fELM are observed. A quasi-coherent mode (QCM) with a frequency of 200 – 300 kHz is observed in the pedestal region and appears in both density and magnetic fluctuations. The time evolution of peped during the inter-ELM phase is analyzed for different densities. The analysis suggests that the pedestal behavior for <ne>/nGW < 0.68 is mainly attributed to pedestal stability but not due to pedestal transport. It is shown that the resistivity effect on the peeling-ballooning mode [Zhang Y. et al 2017 Phys. Plasmas 24 062108] could be used to explain the present result. But the sudden decrease of peped and increase of fELM at <ne>/nGW > 0.68 cannot be attributed to the resistivity effect. It is found that for the plasma with <ne>/nGW > 0.68, the QCM disappears just before ELM and the density pedestal becomes steeper. This implies that the QCM can drive outward particle transport. The steeper density pedestal leads to a narrower pedestal width. A narrower pedestal width will lead to a lower pedestal pressure since peeling-ballooning mode limits the pressure gradient and could explain the observed sudden decrease of peped and increase of fELM.