tearing mode is a resistive plasma instability that tears magnetic field lines apart and reconnects them at rational surfaces, forming magnetic islands that degrade confinement.
The defining contrast is with the 📝kink instability, its ideal cousin. In a perfectly conducting 📝plasma, field lines can bend but never break, so an ideal instability must displace plasma and field together — fast and violent. The tearing mode exploits the small but finite electrical resistance of a real plasma: within a narrow layer around a surface where the 📝safety factor (q) is rational — most dangerously q = 2 — resistivity lets field lines slip, tear, and rejoin in a new topology. The result is a chain of 📝magnetic island structures that grows on the slow resistive timescale, milliseconds to seconds rather than microseconds, fed by free energy stored in the gradient of the 📝plasma current profile.
Slow growth does not mean low stakes. A saturated tearing mode quietly taxes confinement, but a large one interacts with the vessel wall, brakes its own rotation, and locks — and a locked mode is among the most reliable precursors of 📝disruption in a 📝Tokamak. Tokamak operators therefore track tearing activity in real time, shaping current profiles to avoid the mode and driving localized current at the rational surface to heal the islands it creates.