Runaway Electron Codes
Runaway electron codes model the birth and avalanche of relativistic electrons during disruptions and the schemes that dissipate them safely.
Why electrons run away
In a strong electric field, the collisional drag on an electron falls as its velocity rises. Above a critical velocity, drag can no longer balance the accelerating field and the electron runs away to relativistic energy. During a disruption current quench, the induced electric field is large and can convert a substantial fraction of the plasma current into a beam of multi-mega-electron-volt runaway electrons capable of drilling into the wall.
Runaway codes predict how many runaways are generated, their energy, and where the beam terminates, quantifying a serious machine-protection risk.
Primary and avalanche generation
Two mechanisms operate. Primary (Dreicer) generation accelerates electrons from the thermal tail; the hot-tail mechanism adds runaways from a rapidly cooling distribution. Then avalanche multiplication, close collisions that knock additional electrons above the critical velocity, amplifies the seed exponentially. Codes solve a relativistic Fokker-Planck equation to capture both.
Mitigation physics
Suppression relies on raising the critical field by injecting heavy impurities, or on deconfining the runaways with applied magnetic perturbations. Modeling these requires coupling the kinetic runaway physics to impurity and MHD models, an active multiphysics problem.
Design relevance
For the Hyperion breeder, runaway-electron modeling bounds the potential runaway current in a disruption and informs the mitigation strategy needed to protect the first wall. These are simulation studies ahead of construction, part of the honest machine-protection accounting.
- Relativistic electrons born in the current quench
- Dreicer, hot-tail, and avalanche generation
- Relativistic Fokker-Planck kinetics
- Guides impurity and perturbation mitigation