A compact tokamak variant with a very tight aspect ratio — shaped like a cored apple rather than a doughnut — that achieves exceptionally high plasma beta for its size.
A spherical tokamak (ST) is a variant of the tokamak in which the aspect ratio — the ratio of the major radius to the minor radius of the plasma torus — is reduced to values near or below 1.5, compared with 2.5–4.0 in conventional tokamaks. The resulting plasma shape resembles a cored apple or sphere rather than the traditional doughnut, giving the device its name.[1]
The spherical tokamak concept was first explored in the 1980s and demonstrated experimentally on the Small Tight Aspect Ratio Tokamak (START) at Culham, UK, which achieved record plasma beta values — the ratio of plasma pressure to magnetic pressure — exceeding 40 percent.[2]
The tight aspect ratio of a spherical tokamak produces a magnetic field geometry that is naturally more stable to certain pressure-driven instabilities. The strong magnetic field curvature on the outboard side and the large fraction of trapped particles create favorable stability properties that allow the plasma to sustain higher pressures before reaching magnetohydrodynamic (MHD) stability limits.[1]
High beta is significant because it means a spherical tokamak can potentially achieve fusion-relevant plasma pressures with smaller, less expensive magnets — a potential route to more compact and economical fusion reactors.
The two largest spherical tokamaks operating today are the Mega Amp Spherical Tokamak Upgrade (MAST-U) at the UK Atomic Energy Authority in Culham, and the National Spherical Torus Experiment Upgrade (NSTX-U) at the Princeton Plasma Physics Laboratory in the United States. MAST-U has pioneered the Super-X divertor concept for exhaust heat management, while NSTX-U is designed to study high-performance ST plasmas at higher magnetic fields and plasma currents.[3]
In the private sector, Tokamak Energy (UK) is developing spherical tokamaks with high-temperature superconducting (HTS) magnets, aiming to combine the compact geometry of the ST with the high fields enabled by HTS technology.
The compact geometry that gives the spherical tokamak its advantages also presents engineering challenges. The narrow central column leaves limited space for a central solenoid, shielding, and neutron-resistant materials. Managing the intense neutron flux and heat loads on internal components at this tight geometry remains an active area of research and engineering development.[2]