plasma beta is the ratio of a plasma's thermal pressure to the pressure of the magnetic field confining it — the standard measure of how efficiently a confinement device uses its magnetic field.
A magnetic field exerts a pressure proportional to the square of its strength, and beta asks what fraction of that pressure the 📝plasma actually pushes back with. A beta of 1 would mean the plasma's pressure fully matches the field's; real machines run far lower, with conventional tokamaks typically at a few percent. Beta cannot rise without limit — above thresholds captured by results such as the Troyon limit, pressure-driven instabilities grow and confinement degrades — so every magnetic confinement concept lives under a beta ceiling set by its geometry.
Beta frames the field's central design fork. One route to higher fusion power is raising beta itself: the spherical tokamak's fat, low-aspect-ratio plasma tolerates beta several times the conventional value, extracting more performance per tesla. The other is holding modest beta but raising the field: because fusion power density scales roughly as beta squared times the fourth power of field strength, doubling the 📝toroidal field buys as much as an eightfold gain in beta headroom would. That second route is the premise of the high-field 📝Tokamak line running from Alcator C-Mod to 📝SPARC, where high-temperature superconductors supply the tesla that conventional magnets cannot.