Soft H-L back transitions by applying resonant magnetic perturbations in the low q ₉₅ EAST plasmas

Abstract

Controlled ‘soft’ H-L transitions with a simultaneous ramp-down of plasma density and stored energy, by applying resonant magnetic perturbations (n = 2 RMPs), relevant to the control of ITER terminations, are achieved in the EAST tokamak. These H-mode plasmas are run at median electron densities and a low safety factor q95 (q95 = 3.5−4.0) under neutral beam injection (NBI) dominant heating. During a slow ramp in the RMP current the internal transport barrier for the electron density collapses at beginning and the edge transport barrier for the electron density collapses later. Before the controlled H-L back transitions the plasma stored energy W_MHD is linearly correlated with the core line-averaged density < n_e >. These controlled H-L back transitions ensue from the collapse of the edge transport barrier and at a similar energy confinement time τ_e^H-L ∼ 48 ms. These H-L transitions are not sensitive to the neutral beam injection power but are very sensitive to the fueling rate and ramp-up rate of the RMP current.