sawtooth oscillation is the periodic collapse and slow recovery of the core temperature of a tokamak plasma, named for the sawtooth waveform it traces on diagnostic signals.
The cycle is a relaxation oscillation in the heart of the 📝plasma. As the core heats, its conductivity rises and current concentrates there, pulling the central 📝safety factor (q) below 1 — the threshold at which the internal branch of the 📝kink instability becomes unstable. The mode grows, field lines inside the q = 1 surface reconnect, and in a fraction of a millisecond the hot core is flushed outward and flattened — the crash. Heating then rebuilds the peaked profile over tens of milliseconds until the next crash, tooth after tooth. On a soft X-ray trace the core shows a slow ramp and a sharp drop, while the region just outside the q = 1 surface shows the mirror image, an inverted sawtooth, as it receives the expelled heat.
Sawteeth are among the most routine of tokamak instabilities and are often tolerated, even welcomed — the periodic flush carries impurities and fusion-born helium ash out of the core. The danger is indirect: a large sawtooth crash can seed a 📝magnetic island at a nearby rational surface, which a 📝neoclassical tearing mode can then amplify into a serious confinement loss. Controlling sawtooth size and period is accordingly a standard piece of modern 📝Tokamak operation.