Time-dependent modelling of 104/137 GHz X2-mode ECRH on ST40 to aid future scenario development

Tokamak Energy Ltd, Milton, Abingdon, United Kingdom

^* Corresponding author: nicolas.lopez@tokamakenergy.com

Published online: 7 January 2026

Abstract

With a multi-Tesla magnetic field and the capability to achieve fusion-relevant ion temperatures, the ST40 experiment is uniquely positioned as a high-performance spherical tokamak on which to test reactor-relevant technologies. This will be further augmented by the addition of a 104/137 GHz dual-frequency gyrotron supplied by Kyoto Fusioneering Ltd capable of delivering 2s pulses of 1 MW EC power. Here we demonstrate the new capabilities afforded by the additional ECRH through time-dependent integrated modelling in TRANSP. Using two different reference shots, we find that, although accessibility constraints make it difficult to directly drive EC current, the 1MW 104 GHz X2-mode ECRH is nevertheless capable of reducing the loop voltage by increasing the bootstrap current via direct electron heating. The safety-factor profile is also strongly modified, attaining reverse shear in one scenario. More flexibility in the EC launcher geometry will be gained by increasing the magnetic field from 1.8 to 2.4 T; in this case, X2-mode ECRH at 137 GHz could be used instead, which will not suffer any accessibility constraints at typical ST40 densities.