PF Shaping Coils and Negative Triangularity
Poloidal-field coils are the breeder's shape actuators; L1 drives their currents to hold the negative-triangularity, delta -0.30 boundary that defines the ELM-free regime.
Shape is an actuated quantity
The breeder (Hyperion) is a spherical tokamak whose plasma boundary shape is not incidental — it is a controlled target. Poloidal-field (PF) coils produce the fields that set elongation, triangularity, and X-point location. L1 drives each PF-coil current so the reconstructed boundary matches the design shape: negative triangularity of δ = −0.30 at aspect ratio 2.5, major radius R0 1.2 m.
Why negative triangularity
A negative-triangularity boundary is pursued for its edge behavior: it can access good confinement while suppressing the edge-localized modes (ELMs) that damage plasma-facing components in conventional shapes. Holding δ = −0.30 precisely is therefore a control objective, because the favorable regime depends on maintaining the shape against the plasma's tendency to relax toward positive triangularity.
The control law
L1 measures boundary shape moments from magnetics (flux loops, Mirnov coils) via fast equilibrium reconstruction, forms the error against the target moments, and computes PF-current corrections u = -K(x - x_ref). The gain K is designed offline from the machine's response model; L1 executes the multiply-accumulate in fabric each cycle and releases the coordinated coil update on a synchronized gate.
- Command: PF-coil current set, updated coordinately.
- Target: shape moments including δ = −0.30, defined X-point.
- Feedback: fast magnetics + equilibrium reconstruction.
- Constraint: coil current, field, and force limits clamped at drivers.
Coupling to stability
Shape control shares the PF/coil system with vertical stabilization and radial position control. L1 runs these on a common clock so their commands superpose cleanly on the coils, and arbitration ensures a stability action pre-empts a shape refinement when the two compete for the same actuator.