Confinement studies in the ST40 High-Field Spherical Tokamak

The ST40 tokamak [1], built and operated by Tokamak Energy, is a high field spherical tokamak (ST), B=1.5 - 2.2T at R₀=0.4 – 0.5m, A=1.6 – 1.9 with a mission to extend the ST reactor physics basis [2]. Overall energy confinement in STs is largely determined by turbulent electron heat transport, with ion thermal conductivity close to neoclassical levels in higher collisionality H-modes. Electron confinement is found to increase with decreasing collisionality with evidence from Li wall conditioned plasmas in NSTX [3]. Experiments carried out in ST40, without Li and in future campaigns with Li, complement those of NSTX and allow to study electron confinement and turbulent heat transport scaling across a wide range of collisionalities accessible in the two devices. Dedicated parametric scans of both dimensional (B and Ip) and dimensionless (collisionality) parameters have been performed for both hot ion mode limited and diverted plasmas. The results of the scans have been used as the basis for gyrokinetic studies to understand the underlying microinstabilities driving transport, and for energetic particle transport studies. Similarity pulses with different impurity concentrations but the same line average electron density have been performed to provide information on the effect of impurities on energy confinement. Finally, a study to compare the ST40 operational regime with that of GLOBUS-M2 has been performed with the aim to extend the GLOBUS-M2 confinement scaling [4] to higher magnetic field and confirm the linear dependence of confinement on toroidal magnetic field which appears to characterize spherical tokamaks.





[1] M. Gryaznevich, O. Asunta (2017) Fusion Eng. Des. 123 177-180

[2] P F Buxton et al (2019) Plasma Phys. Control. Fusion 61 035006

[3] S.M. Kaye et al 2013 Nucl. Fusion 53 063005

[4] Kurskiev, G. S., et al. 2021 Nucl. Fus. 62.