SOLPS-ITER analysis of a proposed STEP double null geometry : Impact of the degree of disconnection on power-sharing

To mitigate the issue of plasma exhaust in reactor scale tight aspect ratio tokamaks such as Spherical Tokamak for Energy Production (STEP), a double-null (DN) configuration is thought to be advantageous over a single-null (SN) configuration. However, practical control of the plasma vertical stability will likely lead to an oscillation around the symmetry point, which may lead to transient loading of the divertors. In this work we investigated the impact of disconnection of the two separatrices δRsep on the power-sharing between the divertors in disconnected-double-null configurations for the initial iteration of STEP design using the SOLPS-ITER code without drifts. The power fraction to the primary divertor increased with δRsep, reaching 1/495% at the highest δRsep which is representative of SN. The total power fraction to the inner divertors (upper + lower), however, did not show an increase with δRsep for δRsep/λq.