To address the issues of mitigation and control of the heat loads on the divertor target, snowflake divertor (SFD) has been proposed on 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. These can be explained by more substantial Ar accumulation in the private flux region near the X-point in SD. Moreover, the simulation shows that D2 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×B drift on SFD are studied. It is found that with E×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 higher seeding rate is required to achieve detachment. Moreover, the E×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.

Mitigating tungsten (W) wall erosion and core accumulation is vitally important to the steady-state operation of tokamaks. It is well known that drifts have a great impact on the transport of charged particles in the edge region, which could affect W source and W impurity transport. In this work, SOLPS-ITER modeling is applied to study the W impurity behavior on EAST during neon seeding with the consideration of E×B drift. The objective is to find out the relationship between the eroded W flux, W transport and corresponding accumulation in core in different discharge regimes. The effect of drift on W sputtering at targets and W impurity distribution in the cases of different toroidal magnetic field (Bt) directions is assessed. The simulation results indicate that drift could influence W transport via W impurity retention and redistribution in divertor, and the leakage from divertor. In forward Bt (B×∇B points to the X-point), eroded W flux at outer target is increased remarkably, and most of the W ions transport from outer to inner divertor and escape to upstream region in high field side. While W ions mainly transport from inner to outer divertor and escape from divertor in low field side in reversed Bt due to the opposite drift flux. Ne puffing rate is scanned in forward Bt and without drift cases to further investigate the W erosion and W impurity transport in different divertor regimes. It is found that W source from targets is generally enhanced by drift compared to the without drift cases. The core accumulation as well as poloidal asymmetry is also influenced significantly by the drift. In attached regime, intense W source and strong drift flux lead to enhanced W accumulation in the core, and obvious poloidal asymmetry of W density distribution appears. The drift flux is reduced, and W erosion is suppressed after detachment. W concentration in the core and poloidal asymmetry declines consequently. Therefore, adequate Ne impurity seeding can be applied to control the W accumulation in the core.

The density shoulder at midplane may influence core plasma confinement during H-mode discharge, thus affecting long-pulse steady-state discharge. The drifts in the edge plasma plays remarkable role in the plasma transport and divertor operation regime, which determine the density shoulder formation (DSF). In this work, the SOLPS-ITER code package is used to evaluate the influence of full drifts on DSF in poloidal- and radial- coordinates. The open divertor of DIII-D like geometry with weak neutral compression is chosen for the modeling. Cases without drifts, with only E×B drifts in forward Bt, and with full drifts in both forward and reversed Bt are simulated for comparison. It is confirmed that the high upstream density promotes DSF when the drift is not considered, which has also been observed by various investigations. When the drifts are taken into account, the divertor in-out asymmetry (or upstream ionization source) is determined by the direction of Bt due to the variation of particle transport, thus the shoulder can be facilitated or suppressed. Two mechanisms of DSF with full drifts are elucidated: (1) the E×B and B×∇B drifts promote DSF at inner midplane (IMP) via raising ionization source (at IMP) in forward Bt; (2) the drifts contribute to DSF at outer midplane (OMP) by enhancing the radial transport in reversed Bt. In high recycling regime, the ionization is the dominant term for DSF, while in low recycling regime the enhanced radial transport by B×∇B drift plays more important role in DSF. Comprehensively understanding the DSF mechanisms is of great importance for the improvement of core-edge compatibility in the fusion reactor.