The Sawtooth Instability

Sawteeth are periodic, benign relaxations of the plasma core that flatten the central profiles. They are a managed feature, not a threat, in tokamak operation.

What

Periodic core temperature crashes and rebuilds

Cause

An instability near the q = 1 surface

Effect

Flattens central profiles; expels core impurities

Severity

Generally benign and manageable

The sawtooth instability is a periodic, self-repeating relaxation of the plasma core: the central temperature and density slowly build up, then crash in a rapid internal reconnection event, then build again — tracing a sawtooth shape in time. It is driven by conditions near the surface where the safety factor equals 1.

Unlike ELMs or disruptions, sawteeth are generally benign and even useful: the periodic crash helps flush impurities and helium ash out of the core. They are a routine, managed feature of tokamak operation rather than a threat — one of the many plasma behaviours the Kronos control stack and digital twin track and accommodate as normal.