Equilibrium and Shape Control Codes
Shape-control codes link real-time equilibrium estimates to coil commands, holding the plasma boundary and X-point at their targets throughout a discharge.
Holding the shape
A tokamak plasma will drift and deform unless its boundary is actively held. Shape-control codes take a fast estimate of the current boundary, compare it to the desired shape, and compute the poloidal-field coil currents that correct the difference, all within the control cycle.
Fast equilibrium estimation
Full reconstruction is too slow for the innermost control loop, so shape control uses reduced real-time equilibrium estimators. These fit a simplified current model to the magnetic signals fast enough to run every control cycle, giving the boundary and X-point position the controller needs.
Actuator mapping
- Relate each control point on the boundary to coil-current changes
- Invert this response to command the coils toward the target shape
- Respect coil-current and voltage limits and coupling between coils
The vertical instability
Elongated plasmas, which improve performance, are vertically unstable and will run away from their position on a fast timescale set by the surrounding conducting structures. A dedicated fast loop stabilizes this motion, and its design is a central part of shape control, especially for high-elongation shapes.
Design coupling
Controllability is a design constraint, not an afterthought. The coil set, passive structures, and diagnostic layout must together allow the intended shape to be held stably. Control codes are used during design to confirm that a candidate coil arrangement can actually hold the target equilibrium.
The Hyperion breeder's compact, high-elongation, negative-triangularity shape makes shape and vertical control an integral part of its design analysis rather than a later add-on.