Feasibility of Power and Particle Handling in an ST-FNSF and the Effects of Divertor Geometry

A spherical tokamak (ST) configuration is attractive as the basis for a fusion nuclear science facility (FNSF), due to its small size and relatively low cost. However, the compactness of the ST also exacerbates the power and particle handling problems anticipated in next-step devices, since local fluxes are higher and less space is available for optimizing plasma-facing components. On the other hand, novel divertor geometries that have recently been developed such as the snowflake and super-X divertors can be especially effective at reducing heat fluxes in an ST, helping to meet the exhaust challenge. Here, we present an analysis of the power and particle handling requirements of a candidate ST-FNSF, based on 0-D exhaust projections as well as 2-D edge plasma modeling using the SOLPS code. Both conventional and novel divertor geometries are considered. These show that, for reasonable assumptions on cross-field transport, operating points can be identified that are consistent with both core plasma operation and power and particle exhaust requirements, and these operating points are more easily accessible with novel divertors.