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A metallic device, usually in the form of a plate or section of a ring, partially projecting from the inner wall of the tokamak chamber and defining the last closed magnetic surface in order to limit the contact of the plasma with the walls of the vacuum vessel. In the core of the plasma, the magnetic lines form nested closed magnetic surfaces. The magnetic surface that touches the limiter is referred to as the separatrix or last closed flux surface. Behind it is the scrape of layer, the region where the magnetic lines end on the limiter material or on the chamber walls. Heat flux and impurities are thus drawn towards the limiter. In its “shadow”, vacuum pumps can be installed to ensure the purity of the plasma. At present, a divertor is used in preference to a limiter.