Equilibrium studies of the SPARC X-Point Target divertor

An objective for the SPARC advanced divertor mission is to experimentally investigate the X-Point Target (XPT) divertor concept at reactor-relevant heat fluxes, This scenario may prove to be generally useful in fusion pilot plants for mitigating the damaging power and particle exhaust to the divertor, and is under consideration for ARC. The XPT features a radially extended divertor leg with a secondary x-point placed near the target plate, within one SOL heat flux width in the common flux. In this work we perform equilibrium design studies for identifying methods for entering the XPT configuration on SPARC. Lower null XPT and double null XPT scenarios are considered. Accounting for SPARC’s superconducting coil ramp rates and current limits, we present coil current trajectories and placement limits of the secondary x-point. We discuss the heat fluxes introduced on the divertor surfaces through the transition as the secondary x-point crosses the limiter into the main chamber. The flux separation between the primary and secondary x-points—which may be critical for power balance—is found to be sensitive to the coil currents, and control strategies are discussed.