Plasma Shape and Position Control
Poloidal field coils sculpt the plasma boundary and hold it in place, controlled from a real-time reconstruction of the plasma equilibrium.
What is controlled
The plasma boundary, its major-radius position, elongation, triangularity, and the location where the last closed flux surface meets the divertor, is set by currents in the poloidal field and shaping coils. Shape control regulates these boundary features to a target so the plasma sits correctly relative to the wall, divertor, and heating systems.
Real-time equilibrium
The controller cannot see the boundary directly, so it reconstructs the plasma equilibrium in real time from arrays of magnetic sensors measuring field and flux around the vessel. From this it computes control points, gaps between the plasma and the wall, or shape parameters, and drives the coils to bring them to their targets. The reconstruction must run every control cycle, so it is a fast, simplified solve.
Multivariable coupling
Each coil affects many boundary features at once, so shape control is inherently multivariable. Controllers use a matrix that maps desired boundary changes to coil-current changes, decoupling the responses. The fast vertical-stability loop is embedded within or prioritized above the slower shape loops because losing vertical control loses the plasma.
- Poloidal coils set boundary shape and position
- Real-time equilibrium reconstruction from magnetics
- Control gaps, elongation, and triangularity to targets
- Multivariable coil mix decouples the responses
Kronos context
The Hyperion breeder is a spherical tokamak run at a canonical negative triangularity of -0.30, a boundary shape chosen for its favorable edge behaviour; achieving and holding that shape is a shape-control task in the design model. Strike-point placement for divertor heat-flux control is part of the same job. The machine is a design and simulation case, so these are modeled control behaviours, not operating records.
Shape control is where the coil set, the magnetics diagnostics, and the plasma geometry all meet.