How fusion reactors remove helium ash and impurities from the burning plasma and handle the extreme heat loads on the exhaust target — one of the most critical unsolved challenges for a fusion power plant.
ReviewedLast reviewed: 9 Aug 2026·Category: Glossary
The Exhaust Problem
A burning D–T plasma produces helium-4 (alpha particles) as fusion “ash.” This helium must be removed continuously; otherwise it dilutes the fuel and quenches the burn. The plasma also contains impurities (tungsten, beryllium, carbon) eroded from the walls. Both ash and impurities are exhausted through the scrape-off layer (SOL) to the divertor.[1]
Heat exhaust crisis: In ITER, approximately 100 MW of power flows through the SOL into a narrow channel (typically 1–5 mm wide at the midplane) and strikes the divertor targets. Without mitigation, the resulting heat flux (~GW/m²) would destroy any known material within seconds.
Divertor Detachment
The leading solution is “divertor detachment”: seeding the divertor plasma with impurities (typically nitrogen or neon) that radiate most of the exhaust power as light before the plasma reaches the target plates. In a detached regime, the target heat flux drops to manageable levels (~5–10 MW/m²).[2]
Advanced Concepts
Super-X and snowflake divertors expand the magnetic geometry to spread the heat over a larger area. Liquid metal divertors (flowing lithium or tin) could handle higher heat fluxes by continuously renewing the surface. These concepts are being tested on MAST Upgrade and other devices.[3]
Sources
Pitts, R.A. et al. "Physics basis for the first ITER tungsten divertor." Nuclear Materials and Energy, 20, 100696, 2019.
Krasheninnikov, S.I. and Kukushkin, A.S. "Physics of ultimate detachment of a tokamak divertor plasma." Journal of Plasma Physics, 83, 155830501, 2017.
Stangeby, P.C. The Plasma Boundary of Magnetic Fusion Devices. IOP Publishing, 2000.