Runaway Electron Control

Runaway electrons accelerated during a quench can damage the wall; control aims to prevent their formation or safely dissipate the beam.

How runaways form

When plasma current decays rapidly, as in a disruption, the changing field induces a strong toroidal electric field. If this field exceeds the drag that collisions exert on fast electrons, those electrons keep accelerating instead of slowing, becoming runaways. An avalanche process then multiplies them, forming a beam that can carry a large fraction of the plasma current at high energy.

Why they are dangerous

A runaway beam is narrow and energetic. If it terminates on a plasma-facing surface, it deposits its energy in a small volume and can melt or crack the material well below the surface. Preventing beam formation, or dissipating it harmlessly, is therefore a machine-protection priority.

Prevention

The primary prevention is to keep the density high during the quench so collisions damp the seed electrons before the avalanche builds. Massive material injection serves this purpose. Avoiding disruptions in the first place is the surest prevention, since no quench means no runaway drive.

Dissipation

If a beam forms anyway, control aims to dissipate it gently: injecting further material to collisionally slow the electrons, or applying perturbations that deconfine the beam so it spreads its energy over a wide area rather than a point. Both are active areas of research because no method yet fully solves the problem at reactor scale.

In the Kronos program

The Hyperion breeder addresses runaways chiefly through disruption avoidance, keeping the machine away from the quenches that generate them. The mitigation system's material injection provides the density needed to suppress an avalanche if a disruption does occur. These strategies are evaluated in simulation ahead of hardware operation.