LAUSR

To mitigate the issue of plasma exhaust in reactor scale tight aspect ratio tokamaks such as STEP (Spherical Tokamak for Energy Production), 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 δR on the power-sharing between the divertors in disconnected-double-null (DDN) configurations for the initial iteration of STEP design using the SOLPS-ITER code without drifts. The power fraction to the primary divertor increased with δR, reaching ~95 % at the highest δR which is representative of SN. The total power fraction to the inner divertors (upper + lower), however, did not show an increase with δR for δR/λ ≤ 2, where λ is the parallel heat flux decay length, and even at the highest δR it showed only ~ 30 % increase from connected-double-null (CDN), unlike the experimental results on current conventional aspect ratio machines. We found two underlying mechanisms that could explain this result - the total flux compression from the outer midplane to the primary inner divertor target and the parallel current in the primary SOL (between the two separatrices). This work implies that the benefit of DN over SN in power load onto the inner divertor in STEP may be less than found experimentally in conventional tokamak due to its tight aspect ratio. Further investigations by experiments especially on STs and simulations with additional physics such as drifts are the subject of a future investigation.