Divertor heat flux challenge and mitigation in SPARC

The SPARC experiment, because of its high magnetic field, high power, and compact size, will operate with divertor conditions at or above those expected in reactor-class tokamaks. Power exhaust at this scale remains one of the key challenges for practical fusion energy. Based on empirical scalings, the peak unmitigated divertor parallel heat flux is projected to be greater than 10 GW/m2. This is nearly an order of magnitude higher than has been demonstrated to date. Furthermore, the divertor parallel ELM energy fluence projections (~11-34 MJ/m2) are comparable to those for ITER. However, the relatively short pulse length (~25 second pulse, with a ~10 second flat top) provides the opportunity to consider mitigation schemes unsuited for long pulse devices including ITER and reactors. The baseline scenario for SPARC employs a ~1 Hz strike point sweep to spread the heat flux over a large divertor target surface area to keep tile surface temperatures within tolerable levels without the use of active divertor cooling systems. In addition, SPARC operation presents a unique opportunity to study divertor heat exhaust mitigation at reactor level plasma densities and power fluxes. Not only will SPARC test the limits of current experimental scalings and serve for benchmarking theoretical models in reactor regimes, it is also being designed to enable the assessment of long-legged and X-point target advanced divertor magnetic configurations. Experimental results from SPARC will be crucial towards retiring risk for a fusion pilot plant divertor design.