Impact of E× B drift on tungsten impurity transport and analysis of its instantaneous response: a particle Monte Carlo simulation

Tungsten (W), as the primary plasma-facing material in tokamaks, is incompatible with the core plasma. Even extremely low concentrations of W impurities can contaminate the plasma. Therefore, the transport and core accumulation of W impurities are among the key challenges in the fusion community. In this study, the impurity transport code IMPEDGE is upgraded to include the effect of E× B drift, and then applied to investigate the impact of E× B drift on W impurity transport in single-null configuration on EAST. It is found that E× B drift not only affects the stagnation point location of W impurities but also exhibits an instantaneous response feature—i.e. it governs the impurity motion on a time scale shorter than the collisional relaxation and exceeding the gyro-period. The instantaneous response of E× B drift governs impurity behavior near the divertor targets and significantly affects the core W impurity density (). By comparing different E×B drift directions, it is found that the region of reversed E×B drift widens with increasing of neon (Ne) injection rate under forward Bt, leading to an increase in . The instantaneous response of E× B drift can induce a deposition phenomenon distinct from prompt redeposition. To evaluate this behavior, this work establishes a simplified theory for analysis. The poloidal drift deposition distance () is defined. When exceeds the W ionization mean free path (), impurities tend to return to the target due to the effect of E× B drift’s instantaneous response. This deposition behavior is more likely to occur under conditions of high electron temperature (Te) and high drift velocity (). Simulation results demonstrate that this theory can effectively identify W deposition induced by the instantaneous response of E× B drift. An equivalent force model was introduced in the simulation and compared with the instantaneous effect of E× B drift, further demonstrating the dominant role of the instantaneous effect. In addition, this study supplements stagnation point theory by revealing that the instantaneous response of E× B drift, together with the equivalent steady-state velocity of impurity (), jointly determines impurity penetration behavior. The findings suggest that adjusting the width of the reversed E× B drift region can optimize impurity screening efficiency, providing a potential strategy for active control of W impurity accumulation in the core.