The last closed flux surface in a diverted tokamak—the invisible boundary that separates confined, fusion-grade plasma from the open field lines of the scrape-off layer.
Every fusion plasma is bounded. In a limiter tokamak, that boundary is set by a physical object that intercepts the outermost flux surfaces. In a diverted tokamak—the configuration used by ITER and virtually every modern reactor concept—the boundary is defined magnetically. A set of poloidal-field coils creates one or more X-points where the poloidal field vanishes. The flux surface that threads through the X-point is the separatrix: the last closed flux surface (LCFS). Inside it, magnetic field lines close on themselves after many toroidal transits, confining particles and energy. Outside it, field lines are open and connect to material surfaces in the divertor.1
The separatrix is the most consequential surface in the plasma. Its shape, position, and the gradients across it determine almost everything about the plasma edge:
Confinement quality. In H-mode, a steep pressure pedestal forms just inside the separatrix. The height of that pedestal sets a floor for core confinement through profile stiffness, so the temperature and density at the separatrix directly influence the fusion power output.2
Power exhaust. All the heating power that is not radiated inside the separatrix crosses it into the scrape-off layer (SOL), where it must be safely conducted to the divertor targets. The width of the heat-flux channel in the SOL—the power decay length λq—is measured from the separatrix outward and is typically only a few millimetres in reactor-scale devices.3
No single diagnostic directly images the separatrix. Its position is inferred from magnetic measurements fitted to an equilibrium model (typically a Grad–Shafranov solution), supplemented by Thomson scattering profiles that show the sharp drop in electron temperature at the LCFS. Uncertainties of even a centimetre in the reconstructed separatrix position can significantly change the inferred pedestal width and the mapping of heat flux to the divertor.4
Active control of the separatrix shape—adjusting elongation, triangularity, and X-point position—is performed in real time by the plasma control system. Experiments with advanced separatrix geometries, including the snowflake and super-X divertors, aim to increase the volume of the SOL near the X-point, promoting radiative cooling before the exhaust reaches the target plates.