The ratio of a tokamak’s major radius to minor radius (A = R/a) — a fundamental geometric parameter that determines plasma stability, confinement, and engineering trade-offs.
The aspect ratio A of a toroidal fusion device is defined as the ratio of the major radius R (distance from the device’s axis of symmetry to the plasma centre) to the minor radius a (half-width of the plasma cross-section): A = R/a.[1]
Lower aspect ratio increases the fraction of trapped particles and raises the achievable plasma beta (ratio of plasma pressure to magnetic pressure), potentially allowing more compact and economical reactors. However, low aspect ratio also means a narrower centre column, complicating magnet and shielding design.[2]
Spherical tokamaks (A < 2) can achieve beta values 5–10 times higher than conventional tokamaks. This motivates devices like NSTX-U and MAST-U, and the UK’s STEP power plant concept. The trade-off is reduced space for a central solenoid and neutron shielding.[3]