Magnetic Islands

Magnetic islands are chains of closed loops that break the smooth nesting of flux surfaces, short-circuiting confinement across them.

What

Closed loop structures that break flux-surface nesting

Cause

Magnetic reconnection at resonant surfaces

Effect

Flatten profiles; short-circuit transport across the island

Drives

Tearing and neoclassical-tearing modes

Ideally a tokamak's magnetic field forms smoothly nested flux surfaces. A magnetic island is a defect in that structure: at certain resonant surfaces the field can reconnect into a chain of closed loops — islands — that connect what were separate surfaces. Heat and particles then flow freely across the island, short-circuiting confinement in that region.

Islands are the underlying structure behind tearing and neoclassical tearing modes. Keeping them small is part of maintaining good confinement, and controlling them — by tailoring the current profile or driving localized current to heal a growing island — is a standard part of tokamak operation that the Kronos control stack and digital twin monitor.