Equilibrium and Radial-Field Coil Control
Equilibrium coils supply the vertical field that balances the plasma's outward force and set the plasma current profile through the central solenoid.
Radial force balance
A toroidal plasma carrying current pushes itself outward in major radius, from hoop force and internal pressure. A vertical magnetic field crossed with the plasma current produces an inward force that balances this. Equilibrium coils generate that vertical field, and their current must track the plasma current and pressure to hold the plasma at its target radius.
The central solenoid
In a tokamak the central solenoid acts as the primary of a transformer: ramping its current induces the toroidal plasma current. It also contributes to the poloidal field shaping the plasma. Control schedules the solenoid to induce and sustain plasma current during the pulse, and its available flux swing limits how long a purely inductively driven pulse can run.
Coordinated coil control
Equilibrium, solenoid, and shaping coils are driven together. A position loop uses the equilibrium field to hold major radius, while shape loops trim the boundary. Because the plasma current and pressure evolve during the pulse, the coil currents follow a scenario feedforward with feedback correction, all mediated by the fast power supplies.
- Vertical field balances the plasma's outward force
- Central solenoid induces the plasma current
- Coil currents track evolving current and pressure
- Feedforward scenario plus feedback trim
Non-inductive sustainment
Because the solenoid's flux is finite, long or steady pulses need non-inductive current drive from beams and radio-frequency systems to replace the transformer action, and bootstrap current from the plasma's own pressure gradient helps. In the Kronos breeder design study, the modeled 9.86 MA plasma current is sustained by a combination of inductive and auxiliary drive; the machine is a simulation and design case, and the coil control is described generically.
Equilibrium control is the steady hand that keeps the plasma where every other system expects it to be.