Shape and Position Control

Shape control holds the plasma boundary on its target outline and gaps, keeping it clear of walls and on the divertor strike points.

The controlled boundary

The plasma boundary must sit where the scenario intends: at the right major radius, with the right elongation and triangularity, with adequate gaps to the wall, and with its exhaust landing on the divertor targets rather than the main wall. Shape and position control regulates all of this using the poloidal field coils.

Control points

Rather than control the whole boundary as a continuous curve, the system regulates a set of control points and gaps: the outer gap, the inner gap, the top and bottom positions, and the strike-point locations. Each is measured through real-time equilibrium reconstruction and driven by a combination of coil currents.

Coupling between coils

Each poloidal field coil affects several control points at once, so shape control is a multi-input, multi-output problem. Controllers use a model that maps coil currents to boundary changes, then invert it to decide which coils to move for a desired shape correction. Decoupling the interactions is central to steady shape control.

Relation to vertical control

Position control and vertical stabilization overlap: both move the plasma with coils. In practice the fast vertical loop is separated out to run at high rate, while the slower shape loop handles the rest of the boundary. Coordinating the two so they do not fight is part of the control design.

In the Kronos program

The Hyperion breeder is shaped to a negative triangularity of -0.30, an outward-curved boundary at the outboard midplane, so its shape control must hold that unusual outline while managing the gaps and strike points. Achieving and holding negative triangularity through the discharge is a design objective validated in simulation ahead of first plasma.