Disruption Mitigation
When a disruption is unavoidable, spreading the released energy safely and suppressing runaway electrons to protect the machine.
The last line
If avoidance fails, mitigation limits the damage. The goal is to convert a fast, localized energy dump into a slower, more uniform one, to keep forces on the structure within limits, and to suppress the runaway electrons that a disruption can generate. Mitigation is triggered by the disruption predictor with the small warning time it provides.
Mitigation methods
- Massive gas injection: flooding the plasma with impurity gas to radiate stored energy over a wider area
- Shattered pellet injection: firing a cryogenic pellet that shatters into fragments for fast, deep assimilation
- Radiative dispersal: spreading the thermal quench load across the wall rather than a single spot
- Runaway-electron control: raising density and applying fields to dissipate the runaway beam
What mitigation trades
Mitigation deliberately ends the discharge and injects impurities that must then be pumped out. It is destructive to the shot but protective of the machine. The timing is tight: injected material must penetrate and radiate before the thermal quench completes, which is why the trigger depends on reliable, early prediction.
Runaway electrons
During the current quench, the strong induced electric field can accelerate electrons to relativistic energies, forming a runaway beam that can bore into the wall. Mitigation aims to prevent this beam from forming or to dissipate it benignly - a distinct and difficult sub-problem, and an active research area.
Design posture
A well-designed stack treats mitigation as a system it hopes never to use in anger. It is tested, ready, and fast, but success is measured by how rarely it fires, because avoidance kept the plasma inside safe bounds. All of this, for Kronos, is design and simulation study rather than operation of built hardware.