Computational Assessment on Impurity Sourcing and Transport Using High-Temperature Graphite and Silicon Carbide Plasma-Facing Walls in a Tokamak Environment

Operating with hot tokamak plasma-facing components will be essential in fusion reactors to maximize the thermal efficiency of the blanket. The SOLPS-ITER edge plasma code package and the DIVIMP Monte Carlo impurity tracking code were used in tandem to simulate the effect of active wall heating on impurity sourcing and transport in a DIII-D–size tokamak. The SOLPS-ITER plasma background was generated based on a previous DIII-D discharge and includes the effect of particle drifts. DIVIMP simulations found that actively heating the lower divertor (versus the divertor shelf or the entire wall) was the most efficient way to minimize gross erosion and core impurity influx at temperatures above 1000 K. Replacing the graphite wall with a silicon carbide (SiC) wall yielded a 5 to 20× decrease in the estimated gross erosion rate of carbon, with a maximum decrease observed at a lower divertor temperature of 800 K. Gross erosion of Si from SiC was estimated to be almost 100× lower than that of C from SiC, due primarily to the low impact energy of incident D plasma on the divertor targets. The core impurity influx for SiC walls is predicted to be lower than that with graphite walls, but eroded Si ions appear to migrate preferentially (versus C) to the core due to a more peaked erosion profile closer to the strike points where the ion temperature gradient force drives particles upstream. These predictive simulations suggest that active heating of the plasma-facing wall may both lower wall erosion and improve core performance relative to the "warm" walls of current devices that are typically only heated via plasma contact. Relative reductions in gross erosion and upstream accumulation by using SiC instead of graphite as the wall material strengthen the argument for upgrades to current graphite-clad machines and continued development of SiC first-wall and blanket concepts.