Projected heat loads in SPARC from alpha losses

The SPARC tokamak is predicted to produce over 100 MW of DT fusion power in a high power H-mode plasma. Of this power, 20% will be carried by 3.5 MeV alpha particles, which, if poorly confined, can cause localized heating of the plasma-facing components (PFCs). This work will show results from the Monte Carlo orbit-following code ASCOT5 of alpha losses from SPARC’s Primary Reference Discharge, a Q ≈ 11 plasma, with realistic toroidal field (TF) coil sets including misalignments. Simulations are performed in full-orbit mode and follow hundreds of thousands of alphas, weighted by the local rate of DT fusion, from randomized starting positions within the plasma to the separatrix to determine losses. Alphas are then traced from the separatrix to a 3D wall adapted from CAD models of the SPARC PFCs to determine the spatial distribution and magnitudes of alpha power loads, particularly to the tungsten outer limiters and the ion cyclotron resonance heating (ICRH) antennas. Maximum power loads to these surfaces are determined for different magnitudes and combinations of TF and PFC misalignments, as well as for a variety of distances between the separatrix and limiters. It is projected that coil and PFC misalignments can combine to result in heat fluxes significantly higher than other sources of surface heating when the separatrix is close to the PFC surfaces, up to tens of MW/m². These results can be used to determine the safe separatrix-wall distance for operations, serving as a constraint to be balanced against ICRH coupling.