Impact of B_t direction on the detachment threshold during H-mode discharge on EAST with SOLPS-ITER modeling

Abstract

Upper single-null H-mode detachment experiments on EAST were carried out with ITER-like tungsten divertor, and it is found that the detachment threshold of the upper outer divertor (UOD) is significantly lower under unfavorable toroidal magnetic field (Bt) than favorable Bt. In this work, SOLPS-ITER simulations are carried out to study the effects of Bt direction in the detachment threshold to explain the experiment observation. The simulation results are compared with the experiments, showing the reasonable agreement with the diagnostics, and the simulation setups and parameters are thus determined. Based on the simulation data, the two-point model formatting is applied to the flux tube where detachment occurs. A quantitative comparison highlights the critical roles of the upstream parallel heat flux and the downstream momentum and energy loss factors on the ion saturation current density at the target (jsat,t). Further analysis of the simulation results indicates that due to the influences of E×B drift on the convective heat flux, combined with the presence of heat transport associated with parallel current, the upstream heat flux is reduced under unfavorable Bt; while in the downstream region, the divergences of the E×B drift flux provide particle sources under unfavorable Bt and act as sinks under favorable Bt, leading to increased momentum and energy losses under unfavorable Bt. These factors collectively cause a lower detachment threshold under unfavorable Bt.