A periodic relaxation oscillation in the core of a tokamak plasma in which the central temperature slowly rises then suddenly crashes — named for the characteristic sawtooth shape on diagnostic traces.
The sawtooth instability is a periodic cycle observed in the core of most tokamak plasmas. The central electron temperature rises slowly (the “ramp” phase, lasting tens to hundreds of milliseconds), then drops suddenly (the “crash”, happening in less than 1 ms). This cycle repeats continuously and appears as a sawtooth waveform on temperature diagnostics.[1]
During the ramp phase, ohmic and auxiliary heating raise the core temperature and current, causing q(0) to drop below 1. The crash is driven by magnetic reconnection at the q = 1 surface, which flattens the core temperature and current profiles, raising q(0) back above 1. The cycle then repeats.[2]
Sawtooth crashes can trigger neoclassical tearing modes at outer rational surfaces (q = 3/2, 2) — one of the main pathways to disruption. ITER plans to use ICRH to control sawtooth period and amplitude, deliberately triggering frequent small sawteeth to prevent large crashes that could seed NTMs.[3]