Steady-state spherical tokamak power reactor with high-temperature-superconductor

Abstract A spherical tokamak (ST) with an internal transport barrier (ITB) has the potential to be an ideal fusion power reactor because it not only offers high β performance and blanket replaceability, but also enables self-sustaining operation through a high bootstrap (BS) current. This paper investigates the downsizing of an ST using high-temperature superconductors (HTSs) and ITB. Plasma parameters are evaluated using a set of plasma burning equations, including the BS current equation and time-dependent 0D energy and particle balance equations, assuming that the energy confinement time is given by HH multiplied by the IPB98y2 scaling law. In reactors using Nb 3 Sn magnets, the minimum achievable size is determined by the β limit due to the lower maximum magnetic field. In contrast, HTS reactors can operate at higher magnetic fields, providing a greater margin for β , so the minimum size is determined by confinement conditions such as the HH factor, impurity concentration, and α power density. As a case study, we present a conceptual design for an HTS-based ST reactor named JUST-3 ( R = 2.9 m, A = 1.7, B t = 2.6 T, I BS = 17.6 MA, P f = 1 GW).