Analytic optimization of far-SOL protection limiters for reactor-scale tokamaks

An analytic framework is presented for the design and optimization of main-chamber far-SOL protection limiters, necessary to ensure the survivability of fragile first wall breeding structures in the presence of potentially long far-SOL power decay lengths (5+ cm). Reactor-scale tokamak devices will face a unique challenge compared to current devices (and ITER), in that much of their first wall must be very thin (~ 5 mm) to allow tritium breeding, limiting the max heat flux density to around 1 MW/m2. This would easily be exceeded if reactor far-SOL power decay lengths broaden like electron density decay lengths at high density (the “density shoulder”). Protection limiters, made of thick heat-resilient materials, have been put forth as a potential mechanism to remove ionic heat fluxes before they strike the breeding surfaces (at the cost of some fraction of the first wall cross-sectional area). The framework is applied to determine the optimal depth, width, spacing, and shaping of protection limiters given a far-SOL power decay length, using the plasma-wall gap, cross-sectional wall area, and deposited heat fluxes as figures of merit. Robustness to uncertainties in the far-SOL power decay length is used as an additional optimization factor. It is shown that for FNSF-like plasma parameters, far-SOL limiters can reduce the needed plasma wall gap by 50% using less than 1% of the wall area.