Tearing-Mode and NTM Control
Detecting and suppressing magnetic islands that degrade confinement and can lock into disruptions.
Islands in the field
Tearing modes break the nested magnetic flux surfaces and form magnetic islands at rational surfaces where the safety factor q is a ratio of small integers. Islands short-circuit heat across themselves, flattening the pressure profile and degrading confinement. Neoclassical tearing modes (NTMs) are a pressure-driven variant that, once seeded, can grow and persist at high beta.
Why they matter for control
A growing island degrades performance; a large island can slow the plasma rotation until the mode locks to the wall, and a locked mode frequently precedes a disruption. Tearing-mode control aims to keep islands small or suppress them entirely, both to preserve confinement and to remove one of the most common disruption pathways.
Suppression by localized current drive
The primary tool is localized electron-cyclotron current drive (ECCD): a steerable radio-frequency beam deposits current precisely at the island's rational surface, replacing the missing current that lets the island grow and shrinking it. Success depends on aiming the deposition accurately at the island - which requires real-time knowledge of where the rational surface is.
The control loop
- Detect the mode and its rational surface from magnetics and profile diagnostics
- Locate the island radially via equilibrium reconstruction
- Steer the current-drive deposition onto the island
- Modulate the drive in phase with the island rotation for efficiency
- Confirm suppression and hold to prevent regrowth
Prevention over cure
Because NTMs are easier to prevent than to remove once large, control also works to avoid seeding them: managing sawteeth and other triggering events, and keeping the current profile away from conditions that make islands grow. Suppression is the backstop when prevention is incomplete.