Plasma Current Control

Regulating the toroidal plasma current with the central solenoid and, in steady state, with non-inductive current drive.

Why current is fundamental

The toroidal plasma current generates the poloidal field that confines the plasma and sets the safety factor q, which governs stability. Controlling the current means controlling the flux linked by the plasma: inductively, current is driven by changing the central solenoid flux; the plasma responds through its resistance and inductance like a large, hot circuit element.

The circuit model

To first order the plasma is an L/R circuit coupled to the poloidal-field coils. The loop voltage drives current against resistance; inductance sets how fast current can change. Current control regulates the solenoid to track a reference waveform - rising during ramp-up, held during flat-top, lowered during ramp-down - while respecting the available volt-seconds.

Volt-second budget

Inductive current drive consumes solenoid flux (volt-seconds), a finite resource. The scenario must fit the whole discharge inside the available flux, spending it on ramp-up and resistive maintenance. When flux runs low, the current must ramp down. Non-inductive current drive (from neutral beams or radio-frequency waves) extends flat-top by sustaining current without spending solenoid flux.

Coupling to shape and position

Because current and shape both use the poloidal-field system, their loops share actuators and must be coordinated through actuator management. Changes in current redistribute the internal inductance, which shifts the shape and position, so the loops disturb one another and are designed as a coupled system.

Design context

The Kronos breeder Hyperion is designed for a plasma current of 9.86 MA in its reference scenario, a demanding target for a spherical tokamak that places heavy emphasis on current ramp control and volt-second economy. These are simulated design figures, not measurements from operating hardware.