spherical tokamak is a tokamak with a very low aspect ratio — its plasma shaped less like a doughnut than a cored apple — trading engineering room in the machine's center for markedly higher 📝plasma pressure per unit of magnetic field.
The concept is a limiting case of the standard 📝Tokamak, not a different confinement principle: shrink the 📝aspect ratio from a conventional 3 toward 1.5 or below and the central hole nearly closes, the plasma hugging a slender central column. The reward is 📝plasma beta several times what conventional machines sustain — the fat plasma uses its field more efficiently and is more stable at high pressure — a promise demonstrated by the START experiment at Culham in the 1990s and pursued since on MAST-U in the UK and NSTX-U at Princeton.
The price is paid in the center column. The slim core must carry the inboard legs of every 📝toroidal field coil plus a 📝central solenoid, leaving almost no room for neutron 📝shielding — a manageable constraint in experiments but a defining challenge for power-plant designs, which must protect their magnets from years of bombardment. The approach anchors the UK's STEP program and the private company Tokamak Energy, making it the leading alternative geometry to the conventional high-field machine within the tokamak family.