Plasma performance improvement with favourable Bt relative to unfavourable Bt in RF-heated H-mode plasmas in EAST

Significant improvement of plasma performance in high-confinement mode (H-mode) discharges with favourable toroidal field Bt, i.e. the ion ∇B drift towards the primary X-point, has been widely observed in the EAST tokamak with pure radio-frequency (RF) heating in contrast to that with the unfavourable Bt. Statistical analysis indicates that plasma in the favourable Bt has higher core electron temperature, similar core ion temperature and relatively steeper pedestal density compared with that in the unfavourable Bt. The improvement in plasma performance is mainly contributed by the increase of core electron temperature in the favourable Bt. Further analysis indicates that the plasma with favourable Bt has much lower density and recycling in the scrape-off layer (SOL). Lower SOL density and recycling benefit the mitigation of parametric instability activity of lower hybrid wave (LHW), and thus facilitate the increase of core electron temperature in the favourable Bt. The performance improvement in the favourable Bt demonstrates to be more evident with high LHW power. Divertor local Er´B drift which can increase the backflow particles from the divertor region to the upstream region could be partly responsible for the much higher SOL plasma density in unfavourable Bt. These findings could facilitate the application of LHW power on future large fusion devices, such as the China Fusion Engineering Test Reactor (CFETR), to achieve high-performance steady-state operation.