separatrix is the last closed magnetic surface in a diverted plasma — the boundary dividing the confined core, where field lines close on themselves, from the exhaust region, where they run into material surfaces.
The word is mathematics before it is fusion: a separatrix is the curve that separates regions of qualitatively different behavior in a dynamical system, and the fusion usage is a direct application. Inside the separatrix, every 📝magnetic flux surface is closed and nested, and the 📝plasma riding those surfaces stays confined; outside it, field lines are open, ending on the 📝divertor targets. The surface is created deliberately, by shaping the 📝poloidal field so that it vanishes at one or two points — the 📝X-points — where the separatrix crosses itself.
Its near-neighbor is the 📝scrape-off layer, the thin halo of plasma immediately outside the separatrix: the separatrix is the boundary itself, while the scrape-off layer is the region beyond it through which escaping heat flows to the divertor. In a diverted 📝Tokamak the separatrix replaces the older 📝limiter — a material structure that once defined the plasma edge by touching it — with a purely magnetic boundary, keeping the hot edge away from any solid surface. Nearly every performance-defining quantity of the edge, from the 📝H-mode 📝pedestal to the exhaust heat-flux width, is measured relative to the separatrix. 📝Thomas Eich led the 2025 paper mapping the separatrix 📝operational space from 📝ASDEX Upgrade measurements onto 📝SPARC scenarios, and the 📝ARC exhaust design that followed from it in 📝The ARC Physics Basis (2026).