Neoclassical Tearing Mode Control

Neoclassical tearing modes form magnetic islands that degrade confinement; control locates and shrinks them with targeted current drive.

How NTMs arise

A neoclassical tearing mode (NTM) is a magnetic island that grows on a rational surface where the safety factor takes a simple ratio such as 3/2 or 2/1. Once a seed island forms, a self-driven current deficit inside the island lets it grow further. NTMs flatten the pressure profile across the island, reducing confinement, and a large 2/1 island can lock and lead to disruption.

The control principle

The standard countermeasure is to replace the missing current inside the island. Electron cyclotron current drive (ECCD) deposits current at a precise location if the beam is aimed at the island's rational surface. Filling the current deficit stops the island's growth mechanism and can shrink it away.

Aiming and tracking

The hard part is aiming. The rational surface moves as the profile evolves, and the island rotates. Control uses real-time equilibrium reconstruction to place the surface, then steers a mirror or adjusts the magnetic field so the deposition lands on target. Modulating the current drive in phase with the rotating island improves efficiency by depositing only in the island's low-current region.

Detection

Magnetic pickup coils sense the rotating perturbation and give the mode number and frequency. Electron temperature diagnostics can locate the island radially. The controller fuses these to decide whether to pre-emptively aim current drive at a likely surface or to react once an island appears.

In the Kronos program

For the Hyperion breeder, NTM control is part of the stability toolkit for the high-performance phase, using targeted current drive kept ready to stabilize low-order rational surfaces before an island can lock. The control design is exercised in simulation against the machine's profiles ahead of first operation.