safety factor (q) is the number of complete circuits a magnetic field line makes the long way around a tokamak for each circuit it makes the short way around — the central parameter of tokamak stability.
The name promises more than it means: q is not a safety rating but a winding number, so called because early stability theory showed that plasmas with higher q are harder to destabilize. In a 📝Tokamak, the confining field is a helix — external coils supply the toroidal component and the 📝plasma current supplies the poloidal one — and q measures the pitch of that helix on each nested flux surface. It typically runs from near 1 at the plasma center to 3 or more at the edge, and driving more plasma current through a given field lowers it.
The values that matter most are the rational ones. Where q equals a ratio of small whole numbers — 1, 3/2, 2 — a field line closes on itself after a few transits, and these resonant surfaces are where instabilities take root: the 📝sawtooth oscillation anchors at the q = 1 surface, and the 📝tearing mode grows at q = 3/2 and 2. Operating with too little margin above these values invites 📝disruption, which is why tokamak operators treat q as a hard boundary rather than a tuning knob. Raising the magnetic field allows more plasma current — and therefore better confinement — at the same safe q, a core argument for the high-field approach to fusion.