impurity radiation is the energy radiated away by non-fuel ions in a 📝fusion plasma — an unwanted loss channel in the core and a deliberately exploited cooling tool in the exhaust.
Any ion that is not fusion fuel — helium ash, atoms sputtered from the wall, gases injected on purpose — radiates energy out of the 📝plasma. Partially ionized impurities emit line radiation as their remaining bound electrons are excited and relax, and all ions enhance bremsstrahlung, the X-ray emission of electrons decelerating in their electric fields. The heavier the impurity, the worse the effect: a high-Z atom such as 📝tungsten keeps bound electrons even at core temperatures, so trace concentrations — parts in ten thousand — can radiate enough to cool the core below burning conditions, which is why tungsten-armored machines police their cores so carefully.
The same physics reverses into a virtue at the edge. Exhaust power that would otherwise arrive as a concentrated beam on the 📝divertor targets can be converted into radiation, which spreads over the whole surrounding wall harmlessly. Reactor designs deliberately introduce impurities at the boundary — the technique of impurity seeding — to radiate away much of the exhaust before it touches material. The art of the burning plasma is holding both truths at once: a core kept radiatively clean and an edge made radiatively opaque.