Separatrix

A separatrix is a boundary between domains with distinct dynamical behaviour (phase curves) in a dynamical system.

In the fusion context, the term refers to the trajectories of the magnetic field lines and is used to describe:

• The boundary between closed and open field lines, separating the toroidally confined region from the region where field lines connect to material surfaces - in the case of a divertor configuration in tokamaks, or a similar situation in other devices.
• The boundary between the inside of a magnetic island and the surrounding region.

In the case of plasmas that are limited by material objects (limiters or walls), it is not strictly correct to speak of a separatrix to describe the boundary between the confined plasma and the external part of the plasma that connects to the wall (the Scrape-Off Layer, SOL). In this case, it is better to refer to this boundary as the Last Closed Magnetic Surface (LCMS) or Last Closed Flux Surface (LCFS).

See also

• Flux surface