Isoflux versus Rigid Shape Control
Two ways to frame the shape-control problem: matching flux at boundary points, or moving a rigid outline.
Two framings
Shape control can be posed in two ways. Rigid control treats the plasma boundary as a fixed outline that is moved and scaled as a body - simple, but a poor match to how the plasma actually deforms. Isoflux control instead drives the poloidal flux at chosen boundary points to a common value, defining the shape implicitly through the flux the plasma sits on.
Why isoflux won
Isoflux control works directly with measured and reconstructed flux, the quantity the coils actually influence, rather than an inferred geometric boundary. It handles the plasma's real deformation naturally, gracefully manages the transition between limited and diverted shapes, and controls the position of the X-point and strike points, which rigid control cannot cleanly express.
How isoflux works
- Choose control points on the desired boundary and at the X-point
- Compute the flux at those points from real-time reconstruction
- Drive the coil currents so all boundary points share one flux value
- Position the X-point by regulating flux and field there directly
- Adjust the chosen points to change the target shape
Where rigid control still appears
Rigid or gap-based control - regulating the distance from the plasma to specific wall locations - is simple, intuitive, and still used for straightforward position tasks or as a fallback. It can be adequate when the shape is not strongly deforming and the priority is keeping the plasma off the wall rather than precise boundary shaping.
The practical choice
Modern shaped, diverted plasmas are controlled predominantly by isoflux methods because they match the physics and the available measurements. The choice is not merely academic: it determines how the control points, diagnostics, and reconstruction are set up, and it shapes the whole magnetic-control design.