Surface power density on SPARC PFCs due to lost alphas

Alpha-particle simulations using the ASCOT and SPIRAL codes have been performed on candidate SPARC PFC CAD models that include misalignments of the vacuum vessel and limiter surfaces. The computed surface power density is large (~MW/m²), substantially larger than reported previously for SPARC with a simplified wall model (S. Scott et al., J. Plasma Phys 2020). Although the total ripple-induced alpha power loss is small (~100 kW), the loss is highly spatially concentrated on the outer x18 ~130mm wide poloidal rail limiters (peak/mean < 20). Tungsten-based PFCs allow localized fast-ion heat fluxes to be conducted away sufficiently fast to keep surface temperatures well below the melt limit, but at risk of exceeding the recrystallization limit. Banana-trapped alphas experience small radial excursions at the midplane due to ripple, yielding a mm-scale scrape off distance which may be a design driver in long-pulse reactors. The surface power density due to the lost alphas can be spread out by appropriate PFC shaping in the toroidal and poloidal directions, but these adjustments may not be compatible with the need to minimize heat loads from filaments and small ELMs. Additional loss of alpha particles due to 3/2 and 2/1 neoclassical tearing modes is found to increase the surface heating up to ~50%.