Radial Position Control

L1 regulates the breeder plasma's major-radius position against the design R0 of 1.2 m, keeping wall gaps and the equilibrium centered.

Holding the plasma centered

The plasma's radial position — its major radius R relative to the design R0 of 1.2 m — must be regulated so the boundary keeps proper clearance from the inner and outer walls and the equilibrium stays where the shape controller expects it. Radial drift changes the plasma-wall gaps and couples into shape and stability, so L1 controls it as a dedicated loop.

Force balance

Radial position is set by the balance between the plasma's outward hoop and tire-tube forces and the vertical (radial-field) component produced by the PF system. L1 estimates R from magnetics and adjusts the equilibrium field so the plasma sits at R0. Because the spherical tokamak has a tight aspect ratio of 2.5, inner-wall clearance is precious and radial control must be accurate.

The loop

• Estimate R and radial velocity from flux loops and Mirnov coils.
• Compute the equilibrium-field correction to restore R0.
• Command the relevant PF-coil currents, coordinated with shape.
• Clamp to coil and gap limits at the drivers.

Radial and vertical control share the PF/coil system and the same fast magnetics, so L1 runs them on a common clock and lets them superpose on the coils. Arbitration ensures that when vertical stability — the faster, more dangerous instability — needs the coils, it takes precedence over radial trimming.

Coupling to shape and current

Radial position, boundary shape, and plasma current are coupled through the same equilibrium. A change in plasma current or internal profile shifts the natural radial position, which the loop corrects. Keeping R at R0 is a precondition for the shape controller to hold δ = −0.30 accurately, since shape moments are referenced to a centered equilibrium.