impurity seeding is the controlled injection of impurity gases into the edge of a 📝fusion plasma so that their radiation disperses exhaust power before it strikes material surfaces.
Seeding inverts the field's usual posture toward impurities. Most impurity physics is about keeping non-fuel ions out of the 📝plasma; seeding puts them in on purpose, exploiting 📝impurity radiation as a tool. Gases such as nitrogen, neon, and argon are puffed into the boundary, where they convert the exhaust stream's kinetic energy into photons that spread benignly over the whole wall rather than concentrating on the narrow 📝divertor targets. The choice of gas is a choice of where it radiates: lighter seeds like nitrogen radiate best in the cool divertor plasma, while heavier ones like argon radiate farther up the exhaust channel and into the edge.
The technique walks a knife edge. Too little seeding and the divertor targets take unmitigated heat flux; too much, and seed ions migrate into the core, diluting fuel and cooling the burn — the contamination measured by a plasma's effective charge. Reactor-scale machines are expected to depend on seeding as a matter of course, actively feedback-controlled, because their exhaust power cannot be handled by material armor alone; it is the standard route into the detached divertor regime.