Separatrix

The boundary between confined, closed flux surfaces and open scrape-off field lines.

The separatrix is the last closed magnetic surface; inside it the plasma is confined, outside it field lines lead to the divertor. Its shape, including the X-point, is central to divertor design.

The separatrix is the boundary between the confined plasma and the open field lines that lead to the divertor, and its shape — especially the X-point geometry — is a primary design lever. Its exact position is reconstructed in real time from magnetic measurements so the control system can hold the plasma steady.