Plasma Current Control

Plasma current control regulates the total toroidal current, the quantity that anchors confinement, stability, and the safety factor.

Why total current is fundamental

The toroidal plasma current sets the poloidal magnetic field that confines the plasma, fixes the edge safety factor for a given shape and field, and largely determines confinement quality. It is one of the first quantities any control system regulates and it is present in every scenario phase.

Inductive drive

The primary actuator is the central solenoid, which acts like the primary of a transformer with the plasma as the secondary. Changing the solenoid current induces a loop voltage that drives plasma current. The controller regulates plasma current by commanding the solenoid, subject to the finite volt-second budget the solenoid can supply.

Non-inductive contributions

Neutral beams, radio-frequency current drive, and the self-generated bootstrap current also carry part of the total. In steady operation these reduce the demand on the solenoid, extending the pulse. The current controller must account for them so it commands only the remaining inductive share, avoiding overshoot when non-inductive current changes.

Coupling to profile control

Total current control and current-profile control are related but distinct: the first regulates the integral, the second regulates the distribution. They must be consistent, since driving more total current changes the profile. Well-designed systems separate the fast total-current loop from the slow profile loop while keeping them coordinated.

In the Kronos program

The Hyperion breeder targets a flat-top current of 9.86 megaamperes. As a spherical tokamak with a slim central column, its solenoid volt-second budget is limited, so current control is designed to reach and hold this value efficiently, leaning on non-inductive drive during flat-top. The scheme is validated in simulation before hardware operation.