impurity pumping is the removal of helium ash and injected impurity species from the 📝divertor region, so that what a 📝burning plasma produces and what operators deliberately inject both leave the machine instead of accumulating in the core.
The mechanism is vacuum engineering constrained by 📝plasma physics. Impurity ions flow along field lines into the divertor, neutralise on the target, and enter the sub-divertor volume as gas; cryopumps or turbomolecular pumps behind the target then remove them. Only neutrals can be pumped, so throughput scales with the neutral pressure the divertor sustains — which is set by how tightly the divertor is 📝baffled and by the recycling flux at the target. The figure of merit is enrichment: the concentration of a species in the divertor divided by its concentration in the core. Enrichment above one means the divertor concentrates the species and pumping is efficient; below one means the species prefers the core and must be pumped against its own gradient.
Different species behave differently, which is why the term matters. Seeded radiators such as argon and neon are relatively easy to enrich, so a machine can hold a strong radiating layer in the divertor while keeping the core clean. 📝helium ash is the hard case: helium is light, weakly bound to the divertor and poorly enriched, so ash removal usually sets the pumping requirement for a power plant rather than seeding does. Pumping is also in direct tension with 📝divertor detachment and 📝impurity seeding — the same neutral cushion that dissipates heat is the reservoir the pumps draw from, and removing too much of it can unseat the detachment front. For 📝ARC, Eich and colleagues (2026, DOI 10.1017/S002237782610155X) predict argon enrichment near six and helium enrichment near 0.4 at a divertor neutral pressure around 20 pascals, sufficient to hold core helium below two percent. Those are modelling results for a plant not yet built; the enrichment factors an operating machine actually achieves have only ever been measured on present-day devices, and never at reactor divertor conditions.