The thin boundary region of open magnetic field lines outside the last closed flux surface, where exhaust plasma streams toward material surfaces and governs the interface between plasma and machine.
The scrape-off layer (SOL) is the outermost region of the plasma in a tokamak or stellarator, lying between the last closed flux surface (LCFS, also called the separatrix in a divertor configuration) and the vessel wall. In the SOL, magnetic field lines are “open”—they intersect material surfaces rather than closing on themselves. Plasma particles and energy that cross the separatrix stream along these open field lines toward limiter or divertor target plates, making the SOL the primary channel for power and particle exhaust.1
Transport in the SOL is strongly anisotropic. Parallel (along-field-line) transport is fast, dominated by sonic or supersonic plasma flow to the targets, with characteristic transit times of microseconds. Perpendicular (cross-field) transport is much slower and governed by a combination of classical diffusion, turbulent convection, and intermittent coherent structures known as filaments or blobs—field-aligned plasma structures that propagate radially outward and can carry significant particle flux to the main chamber wall.3
The competition between parallel losses to the divertor and perpendicular spreading sets the effective width of the SOL. The Eich scaling, derived from multi-machine infrared thermography, finds λq ∝ Bpol−1.19, indicating that higher poloidal magnetic fields (and thus higher plasma currents) produce narrower SOLs—a concerning trend for high-current reactor designs.2
To protect divertor targets from the intense heat flux channeled through the SOL, reactor-grade tokamaks aim for a condition called divertor detachment. By seeding impurities (typically nitrogen or neon) into the SOL and divertor, plasma temperatures near the targets are reduced below ~5 eV, at which point volumetric recombination and radiation dissipate most of the power before it reaches the surface. Maintaining stable detachment while preserving core confinement is one of the foremost challenges for ITER and future power plants.4
The SOL determines the boundary conditions for core plasma performance: it controls fueling efficiency, impurity screening, helium-ash removal, and the heat loads that define component lifetime. Understanding and controlling SOL physics is essential for any viable fusion energy system.