A region of broken magnetic topology inside a fusion plasma where field lines form isolated island chains instead of nested surfaces — degrading confinement by providing short-circuit paths for heat and particles.
In an ideal tokamak, magnetic field lines trace out nested toroidal surfaces. A magnetic island forms when a magnetic perturbation causes field lines to reconnect, creating an isolated chain of flux tubes with O-points (centres) and X-points (separatrices). Islands form at “rational surfaces” where the safety factor q = m/n is a ratio of small integers.[1]
In high-beta plasmas, the bootstrap current creates a self-reinforcing feedback: a small island reduces the local pressure gradient, reducing the bootstrap current inside the island, which makes the island grow further. These neoclassical tearing modes (NTMs) are the dominant performance-limiting instability in modern tokamaks.[2]
Magnetic islands can be stabilized or suppressed by driving current at the O-point using localized ECRH/ECCD. This technique has been successfully demonstrated on DIII-D, ASDEX Upgrade, and JT-60SA and is planned for ITER.[3]