The challenge of keeping non-fuel atoms out of the plasma core — because even trace amounts of high-Z impurities radiate enormous power and dilute the fuel, quenching fusion reactions.
Any non-hydrogen atom that enters the fusion plasma is an “impurity.” Even small concentrations of high-Z (heavy) impurities like tungsten (Z=74) or iron (Z=26) cause catastrophic radiation losses because the radiated power scales approximately as Z2–Z4. A tungsten concentration of just 10−5 (10 parts per million) can radiate enough power to extinguish the fusion burn.[1]
Impurities enter the plasma from: sputtering of plasma-facing materials (tungsten from the divertor, beryllium from the first wall); chemical erosion (historically carbon); desorption from wall surfaces; and leaks (air, water). In ITER, the dominant impurity source will be tungsten sputtered from divertor targets.[2]
Divertor geometry: The X-point divertor directs most plasma-wall interaction to a small, engineered target. Wall conditioning: Boronisation, lithium coating, and baking reduce impurity release. Impurity screening: Tokamak edge physics naturally provides some screening of impurities. Seeded radiative cooling: Deliberately injecting light impurities (nitrogen, neon) to radiate heat uniformly, protecting the divertor without contaminating the core.[3]