Direct evidence of E x B flow changes at the onset of resonant magnetic perturbation-driven edge-localized mode crash suppression

Abstract

The bifurcation of perpendicular mode velocity near the pedestal top (nu(perpendicular to)) at the onset of RMP-driven edge-localized mode (ELM)-crash suppression has been directly measured by using an electron cyclotron emission imaging diagnostic on KSTAR. The ELM crashes are suppressed along with a sudden reduction of nu(perpendicular to), which synchronizes with the transition into and out of the ELM-crash suppression. The change of E x B flow appears mainly responsible for the change of nu(perpendicular to), whose magnitude is the smallest near the normalized flux surface Psi(N)similar to 0.95 during the ELM-crash suppression. The plasma response to the RMP field was most enhanced in the vicinity of Psi(N) similar to 0.95 during the ELM-crash suppression and showed a hysteresis behavior with respect to resonant field strength. With these changes, a strong nonlinear coupling between turbulent eddies was observed in the ELM-crash suppression, which is an important feature of the ELM-crash suppression.