Neoclassical Tearing Modes (NTMs)

NTMs are magnetic islands seeded by the bootstrap current that degrade confinement. They are controllable with localized current drive.

What

Magnetic islands driven by a bootstrap perturbation

Effect

Degrade confinement; can trigger disruptions

Control

Localized current drive to heal the island

Relevance

A managed risk at high beta

Neoclassical tearing modes (NTMs) are a form of instability in which small magnetic islands grow because a perturbation to the bootstrap current reinforces them. Once established, the islands flatten the pressure profile across them, degrading confinement — and in the worst case they can seed a disruption. They tend to appear at higher beta, exactly where reactors want to run.

NTMs are, however, a well-understood and controllable risk: injecting a small, localized current precisely at the island (electron-cyclotron current drive) replaces the missing bootstrap current and heals it. For a high-beta design like Kronos MetroVolt, NTM control is part of the routine stability tool set that keeps the plasma inside its operating window, alongside RWM feedback and vertical control.