neoclassical tearing mode is a tearing instability seeded and sustained by the loss of 📝bootstrap current inside a magnetic island, one of the principal performance limits of high-pressure tokamak plasmas.
The distinction against the classical 📝tearing mode is the energy source. A classical tearing mode grows from free energy in the current profile and can be designed away; the neoclassical tearing mode (NTM) feeds on the plasma's own pressure. Tokamak plasmas spontaneously generate bootstrap current from their pressure gradients — a welcome effect, since it reduces the external current drive a reactor needs. But when a seed 📝magnetic island forms at a rational surface — often thrown off by a 📝sawtooth oscillation crash — the pressure flattens across the island, the local bootstrap current vanishes, and the missing current makes the island grow. The mechanism is self-reinforcing: a plasma pressed hard enough toward high performance carries the seeds of its own confinement loss.
NTMs are metastable — stable until a large enough seed appears — which makes them a defining concern for high-pressure operation in any 📝Tokamak, and a grown NTM can lock and end in 📝disruption. The established countermeasure is surgical: electron cyclotron waves drive current precisely inside the island, replacing the missing bootstrap current and shrinking the island back to nothing.