AI Plasma Control

Plasma Shape Control

Holding the plasma boundary to a target shape by regulating the currents in the poloidal field coils that surround it.

What shape means

The plasma boundary is the last closed flux surface. Its shape - elongation, triangularity, and the gaps to the wall and divertor - sets confinement quality, stability, and how heat reaches plasma-facing components. Shape control regulates the poloidal field (PF) coil currents so that this boundary tracks a designer-specified target throughout the discharge.

Isoflux control

Kronos motion — control room

The dominant method is isoflux control: instead of directly commanding a shape, the controller drives the poloidal flux at a set of chosen boundary control points to a common value, so those points lie on one flux surface. Choosing where the control points sit defines the shape. Isoflux is robust because it works directly with the measured magnetic flux rather than an inferred geometric outline.

The coupled, over-determined problem

Each PF coil influences the entire boundary, so shape control is strongly coupled and typically over-actuated. Controllers use the plasma-response matrix - how each coil current shifts each control point's flux - and invert it (in a regularized, least-squares sense) to distribute demand across coils. Singular-value methods identify which shape combinations are easy to control and which are nearly uncontrollable.

Interaction with position and current

Shape, vertical position, and plasma current all draw on the same coils, so their loops must be coordinated. Fast vertical stabilization is usually decoupled onto dedicated fast coils; slower shape and current loops share the main PF set through the actuator-management layer, which allocates the shared authority.

Design note

The Kronos breeder Hyperion is designed as a spherical tokamak with strong shaping, including negative triangularity of -0.30 in its reference design. Negative triangularity changes both the achievable stability and the shape-control problem; here it is a simulated design parameter, not a measured result from hardware.