Rotation and Momentum Control

Plasma rotation stabilizes some instabilities and shears turbulence; control adjusts it through momentum input and magnetic braking.

Why rotation matters

A rotating plasma resists certain instabilities. Toroidal rotation and its radial shear suppress turbulence, improving confinement, and rotation keeps some modes from locking to the wall, which would otherwise lead to disruption. Rotation is therefore a controlled quantity in advanced scenarios, not just a byproduct.

Sources of momentum

Neutral beams injected in the direction of the current impart toroidal momentum and are the main external drive. Radio-frequency systems can also drive rotation indirectly. Intrinsic rotation, generated by the plasma itself without external torque, adds a baseline that the controller must account for.

Braking

Rotation can also be reduced deliberately. Applying a static magnetic perturbation with correction coils brakes the plasma through a drag force. Because the same coils are used for error-field correction and edge control, rotation braking competes for them, another item for the allocator.

Locking and its danger

If rotation falls too low, a magnetic island can lock to the wall and stop rotating, after which it grows and often disrupts the plasma. Rotation control includes watching for the conditions that precede locking and maintaining enough rotation, or enough correction, to prevent it. This links rotation control to tearing-mode and error-field control.

In the Kronos program

For the Hyperion breeder, maintaining rotation is part of avoiding mode locking during the high-performance phase, working alongside error-field correction and tearing-mode control. The relative contributions of beam-driven and intrinsic rotation are studied in simulation to set control targets ahead of operation.