The Behavior and Control of Injected Impurity in the Closed Divertor of HL-2A

Excessive heat load on the divertor target has been identified as a major challenge for present and future tokamaks. Divertor detachment achieved by injecting radiating impurity is an effective method to reduce heat load onto the divertor target surface. However, extremely serious effects on the core plasma could be given by the injected extrinsic impurity, such as fuel dilution and cooling due to energy radiation loss by the impurity in the core plasma region. Therefore, understanding the impurity behavior and then controlling the impurity content during divertor impurity injection are important issues of a tokamak. The closed divertor has the advantage of realization of divertor detachment and the Huan Liuqi-2A (HL-2A) tokamak has a very closed symmetrical divertor structure. In this work, experiments and SOLPS-ITER simulation gave the picture of the impurity behavior and showed that the friction force can play a more key role in screening and controlling radiated impurity, comparing with pressure/temperature gradient force during detachment in HL-2A with the closed divertor. Increasing the degree of divertor detachment (DoD), the screening ability of the divertor is strengthened, which is conducive to the control of impurity ions. It implies that the injected impurity can be confined in the closed divertor under detachment and, to some extent, the effect of DoD or impurity gas flux on main plasma can be attenuated for HL-2A with extrinsic impurity gas injection. During divertor detachment, the screening effect of N+ and Ar+ ions is stronger than that of Ne+ ions. As a result, the behavior and control of impurity with impurity injection in the closed divertor of HL-2A are presented with experimental and simulated results, which gives meaningful understanding and suggestion for heat load mitigation and controlling the effect of impurity in HL-2A and other tokamaks.