Utilization of main-chamber limiters for mitigation of steady-state first-wall heat and particle fluxes at reactor scale

Plasma contact at the first wall represents a key point of tension for the integrated design of a self-sufficient tokamak reactor, as the wall cladding must be very thin (< 5 mm) to allow for adequate tritium breeding, but must also withstand significant steady-state heat fluxes and sputter erosion. This issue is exacerbated by the experimental observation that main-chamber ion fluxes increase when operating the divertor at high density. Introducing main-chamber limiters may help alleviate this tension: these limiters, made of more robust materials than the rest of the first wall, would intercept ionic plasma fluxes in the far scrape-off layer before these fluxes can reach the fragile recessed breeding wall. However, careful limiter optimization is needed, as installing limiters over too large an area reduces the overall tritium breeding capability, and installing them too deep into the plasma leads to excessive limiter damage and plasma degradation. Heat flux modeling is presented for a variety of main-chamber limiter designs, using the ARIES-ACT2 reactor design as a starting point. It is found that toroidally-continuous main-chamber limiters are especially promising, as they protect the recessed wall without excessive heat flux peaking on the limiters themselves. Toroidal limiters have the additional benefit of reducing connection lengths in front of the breeding wall, which is theorized to reduce the cross-field propagation of plasma filaments and thus further reduce ion fluxes to recessed surfaces.