Modeling of the effects of impurity seeding on plasma detachment and impurity screening of snowflake divertor on HL-2M tokamak by SOLPS-ITER

To address the issues of mitigation and control of the heat loads on the divertor target, a snowflake divertor (SFD) has been proposed on the HL-2M tokamak. In this work, simulations have been performed by using SOLPS-ITER to demonstrate the advantages of SFD on HL-2M on plasma detachment and impurity screening during impurity seeding. Firstly, neon (Ne) and argon (Ar) seeding are chosen for comparison in SFD. It is found that Ar seeding significantly mitigates the in-out asymmetry compared with Ne seeding, mainly in high seeding rate cases. The impurity screening capabilities with Ar seeding are conspicuously better than that of Ne seeding. Subsequently, the SFD and standard divertor (SD) with Ar seeding are compared. The SFD achieves plasma detachment with a seeding rate of more than one order of magnitude lower and has better impurity screening capability than those of the SD. This can be explained by more substantial Ar accumulation in the private flux region near the X-point in SD. Moreover, the simulation shows that D-2 puffing near the OMP can drive more Ar ions to the divertor and promote the plasma detachment and impurity screening. Finally, the effects of E x B drift on SFD are studied. It is found that with E x B drift more Ar particles accumulate in the vicinity of both inner and outer targets, especially in the far-SOL region, thus raising the far-SOL power radiation. However, the peak heat flux is mainly located near the separatrix, therefore a higher seeding rate is required to achieve detachment. Moreover, the E x B drift drives more Ar particles away from the core region. In addition, the role of molecules on the plasma momentum loss during detachment is analyzed.