Separatrix

A separatrix is a boundary between domains with distinct dynamical behaviour.

In the fusion context, the term 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 (such as divertors, limiters, or walls) - also known as the Scrape-Off Layer (SOL). This boundary is also called the Last Closed Flux Surface (LCFS).
• The boundary between the inside of a magnetic island to the surrounding region.