When a rotating MHD island grinds to a halt against the vessel wall, disruption often follows within milliseconds
A locked mode is a magnetohydrodynamic (MHD) perturbation—typically a magnetic island generated by a tearing mode—that has lost its natural rotation in the plasma frame and become stationary relative to the vacuum vessel wall. In a healthy tokamak discharge, MHD modes rotate with the plasma at kilohertz frequencies. When electromagnetic braking torques from eddy currents in the wall, or error fields from imperfect coil alignment, decelerate the mode below a critical threshold, it "locks" to the external structure. Locked modes are among the most reliable precursors to major disruptions.1
The physics of mode locking involves a torque balance. A rotating magnetic island induces eddy currents in the resistive wall, which exert a drag torque that scales with the island width squared. Simultaneously, static error fields (from coil misalignment, welding asymmetries, or port structures) exert a resonant electromagnetic torque that peaks when the mode rotation matches zero. Once the mode slows below a critical velocity, the error-field torque dominates and pulls the mode to a standstill. The transition is often abrupt—a bifurcation—because slowing the mode increases the wall torque, creating positive feedback.2
Once a mode locks, the stationary magnetic island flattens the temperature profile locally, driving radiative collapse as impurities accumulate in the cool island O-point. The resulting thermal quench can deposit the full plasma thermal energy on the wall in less than a millisecond. In large devices, this energy deposition can damage plasma-facing components.
Mitigation strategies include: (1) error-field correction coils that minimize the static resonant field components before the discharge, reducing the locking torque; (2) active rotation drive via neutral beam injection torque; (3) electron cyclotron current drive (ECCD) targeted at the island O-point to suppress the tearing mode before it grows large enough to lock; and (4) resonant magnetic perturbation (RMP) coils operated in rotating phase to spin up a decelerating mode.4