Impurity Seeding Control
Injecting a small amount of a radiating impurity spreads exhaust power over the plasma volume and edge, protecting the divertor and first wall.
Radiative cooling
A controlled trace of an impurity such as nitrogen, neon, or argon radiates energy across the plasma edge and divertor. This radiation carries power away over a large surface rather than letting it concentrate on the divertor target. Seeding is a primary tool for keeping heat flux within material limits, working alongside detachment.
Choosing the species
Light impurities like nitrogen radiate mainly at the cool edge and divertor; heavier ones like argon radiate deeper and can build a core radiative mantle. The choice trades exhaust relief against the risk of core dilution and radiation, which lowers fusion power. Control selects the species and rate to relieve the divertor without cooling the reacting core.
Feedback
A seeding controller measures a radiation signal, such as the total radiated power fraction or divertor-specific radiation, plus target temperature, and adjusts the seeding-gas valve to hold the target. Because impurities accumulate and are pumped out on their own timescales, the loop accounts for this lag and avoids over-seeding, which can trigger a radiative collapse.
- Trace impurity radiates exhaust power away
- Light species cool the edge, heavy species radiate deeper
- Feedback on radiated power and target temperature
- Avoid over-seeding that dilutes or collapses the core
Coupling to other systems
Seeding, gas puffing, and detachment control all act on the divertor and share the same sensors, so they are coordinated as one exhaust-control problem. Vacuum pumping removes the injected impurities, linking seeding to particle balance. In the Kronos breeder design study, radiative exhaust management is part of protecting plasma-facing components in a high-power spherical tokamak; the machine is simulated, and control here is described in general terms.
A little of the right impurity, precisely controlled, is what lets the divertor survive full power.