Sawtooth Control

Sawteeth are periodic core relaxations; control tunes their period so they flush impurities without seeding larger instabilities.

The sawtooth cycle

A sawtooth is a repeating cycle in the plasma core: the central temperature slowly rises, then crashes as an internal instability reconnects the field inside the surface where the safety factor equals one. The temperature trace looks like the teeth of a saw. Each crash redistributes heat and particles from the core outward.

Why control the period

Sawteeth are not purely harmful. The crash helps flush helium ash and impurities from the core, which is useful in a burning plasma. But a long, infrequent sawtooth builds up a large crash that can seed a neoclassical tearing mode. Control therefore aims for a period long enough to be efficient but short enough to avoid seeding larger modes.

Actuators

The sawtooth period is set by conditions near the q=1 surface. Localized current drive or heating there changes the local magnetic shear, which either destabilizes sawteeth to make them more frequent and small, or stabilizes them to make them longer. Electron cyclotron systems are the usual tool because they can be aimed precisely.

Pacing

As with ELMs, sawteeth can be paced: a controller triggers a crash on demand by briefly changing the current drive near the q=1 surface, preventing the buildup that leads to a monster crash. This turns an uncontrolled event into a scheduled one.

In the Kronos program

In the Hyperion breeder, sawtooth control shares the electron cyclotron actuator with neoclassical tearing mode control, so the supervisor allocates that actuator between flushing the core and stabilizing rational surfaces according to the phase of the discharge. The strategy is developed in simulation against the machine's q-profile.