Flux Surfaces & the Separatrix

Nested magnetic flux surfaces confine the plasma; the separatrix is the last closed one. Together they define where the plasma ends and exhaust begins.

Flux surface

A nested surface field lines wrap around

Separatrix / LCFS

The last closed flux surface

Beyond it

The open scrape-off layer → divertor

q95

Safety factor at the 95% flux surface

Inside a tokamak the magnetic field organizes itself into nested flux surfaces — like the layers of an onion — that the field lines wrap around without crossing. Plasma is confined because particles stay on their surface; pressure, temperature, and density are roughly constant on each one.

The outermost closed surface is the separatrix, or last closed flux surface (LCFS). Inside it the plasma is confined; just outside, field lines are open and lead to the divertor — this is the scrape-off layer. Many key metrics are quoted at a specific surface: the binding safety factor q95, for instance, is evaluated at the surface enclosing 95% of the plasma flux.