Description
The dependence of plasma energy confinement time on the magnetic field was demonstrated to be much stronger in spherical tokamaks (STs), such as MAST, NSTX and GLOBUS-M2, than in conventional tokamaks with high aspect ratio A>2.5 [1]. The maximum toroidal field achieved in those STs was 0.9 T [2] and, therefore, to be applicable for future ST-based fusion devices with a foreseen toroidal field of 4-5 T, extrapolation of the ST confinement scaling law is required.
Experiments on the high-field spherical tokamak ST40 [3,4] with an NBI beam power of around 1 MW were carried out, with a toroidal magnetic field scan between 0.6 T and 1.7 T at the geometric axis, in L- and ELM-free H- modes plasmas with different plasma currents. Interpretation of these experiments took into consideration all available measurements including bolometry emission, spectroscopic data, neutron data, magnetic sensors and Thomson scattering data.
In our analysis we use the ASTRA transport code [5] together with the NUBEAM code [6] for calculation of the NBI heating power and fast particle fraction. As a result, it is found that the dependence of the confinement time on the magnetic field, which follows the STs scaling [1] at low B, is expected to saturate at B>1 T. Uncertainties in the ion temperature profile and fast particle losses are taken into account producing error bars on the calculated confinement time.
References
[1] G.S. Kurskiev et al 2022 Nucl. Fusion 62 016011
[2] G. S. Kurskiev, et al 2024 Phys. Plasmas 31, 062511
[3] S. McNamara et al., 2023 Nuclear Fusion 63 054002
[4] O. Asunta et al., 2026 submitted to Nuclear Fusion
[5] G.V. Pereverzev and P.N. Yushmanov, 2002 IPP Report 5/98
[6] A. Pankin et al., 2004 Comput. Phys. Commun. 159 157–84