scrape-off layer is the thin layer of plasma just outside the separatrix, where magnetic field lines no longer close on themselves but run into material surfaces — the channel through which a tokamak's exhaust flows.
The name is literal: this is the region where the 📝plasma is scraped off the confined core. Its near-neighbor is the 📝separatrix, and the distinction is between a boundary and a region — the separatrix is the last closed surface, while the scrape-off layer is everything immediately beyond it, where any particle or parcel of heat that crosses the boundary is carried along open field lines to the 📝divertor targets. Transport along the field is vastly faster than across it, so the layer is startlingly thin: in high-field machines the heat-flux width can be measured in millimeters. What crosses the field does so largely in bursts — coherent 📝filament (blob)s of dense plasma that detach from the edge and carry particles and heat outward toward the wall.
That thinness is the crux of the exhaust problem. Hundreds of megawatts of heat leaving a power-plant plasma are funneled through a ribbon of plasma a few millimeters deep, and the resulting flux density at the divertor can exceed what any solid material tolerates. Scrape-off layer physics — how the layer's width scales with field and machine size, how 📝impurity seeding and 📝divertor detachment radiate power away before it strikes the targets — is therefore one of the defining research fronts for machines like 📝SPARC and every power plant that follows.