Pace making of edge localized modes with low-hybrid-wave power pulses in the EAST superconducting tokamak

Pace making of the edge-localized modes (ELMs) by low-hybrid-wave (LHW) power modulation has been observed in the EAST superconducting tokamak when the modulation frequency is sufficiently close to the natural-ELM frequency, i.e. within +/- 30%f(ELM). The highest pace-making frequency obtained so far is 120 Hz. The ELM triggering is typically delayed by 1-2 ms relative to the LHW power rising edge, during which the pedestal profiles evolve. The mechanism of the synchronized triggering of ELMs was found due to the LHW-induced pedestal density pumpout. A previous study indicates that LHW generates field-aligned helical current filaments in the scrape-off layer (SOL) (Liang et al 2013 Phys. Rev. Lett. 110 235002), which produces three- dimensional (3D) magnetic topology change at the plasma edge similar to the effect of the resonant magnetic perturbations. The observed pedestal density pumpout, local flattening of the density gradient near the separatrix and steepening near the pedestal top may be induced by the 3D effect. Pedestal linear stability analysis using ELITE code indicates that the pedestal is approaching the peeling-ballooning stability boundary as the profile is getting steep near the pedestal top until the ELM is triggered. The power threshold for the ELM pacing with 2.45 GHz LHW is similar to 1 MW.