Runaway Electron Suppression

A disruption can accelerate a beam of relativistic electrons; the mitigation path is designed to keep that beam from forming or to dissipate it benignly.

The relativistic-beam hazard

During a current quench the strong toroidal electric field can accelerate electrons to relativistic energies, forming a runaway electron beam that carries a large fraction of the plasma current. Uncontrolled, such a beam can strike plasma-facing components in a small area. Suppression is therefore a named objective of the breeder mitigation strategy, alongside energy radiation and force control.

Two levers: avoid formation, or dissipate

• Suppress formation: raise the plasma density enough during mitigation that collisional drag exceeds the accelerating field.
• Benign dissipation: if a beam forms, control its final loss so its energy is spread rather than concentrated.
• Both levers depend on the same fast, uniform material injection used for energy radiation.

def runaway_suppressed(density_after_inject, critical_density):
    # above the critical density, collisional drag beats the E-field
    return density_after_inject >= critical_density

def avalanche_headroom(n_inject, n_crit, margin=1.5):
    # inject enough to clear the threshold with margin
    return n_inject >= margin * n_crit

The physics here is coupled to the injection actuators: reaching the critical density fast enough is the same problem as radiating the thermal energy fast enough, so the injection timing budget serves both goals. Getting one right without the other is not acceptable; the mitigation is designed as a single event that satisfies energy, force, and runaway limits together.

Because formation-suppression and energy-radiation are served by the same fast, uniform material injection, the mitigation is designed as one event that must satisfy energy, force, and runaway constraints together rather than as separate optimizations that could work against each other. Kronos treats runaway control as an open engineering problem and builds the reflex tier to invoke the best available mitigation while reporting honestly on residual risk, rather than claiming the relativistic-beam hazard is eliminated.

Runaway suppression is an active area of design-and-simulation work across the field and at Kronos; it is treated as an honest open engineering problem, and the reflex tier is built to invoke the best available mitigation, not to claim the hazard is solved. It closes out the breeder disruption triad with halo/force limits and the mitigation reflex itself.