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

R. T. Osawa*, D. Moulton, S. L. Newton, S. S. Henderson, B. Lipschultz, A. Hudoba

*Corresponding author for this work

Research output: Contribution to journalArticlepeer-review


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.

Original languageEnglish
Article number076032
Number of pages17
JournalNuclear Fusion
Issue number7
Publication statusPublished - 15 Jun 2023

Bibliographical note

Funding Information:
This work has been funded by STEP, a UKAEA program to design and build a prototype fusion energy plant and a path to commercial fusion.

Publisher Copyright: © 2023 The Author(s)
1/22, where λ q is the parallel heat flux decay length, and even at the highest δRsep it showed only 1/430% increase from connected-double-null (CDN), unlike the experimental results for 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 tokamaks due to its tight aspect ratio. Further investigations through experiments, especially on STs and simulations with additional physics such as drifts, are the subject of a future investigation.


  • disconnected-double-null configuration
  • power-sharing
  • STEP

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