Hyperion Central Solenoid
The central solenoid sits on the machine axis and drives plasma current inductively, acting as the primary of a transformer.
The transformer at the core
The central solenoid is a tall stack of coils running up the machine axis inside the center stack. It works as the primary winding of a transformer: changing its current induces a loop voltage around the torus, and that voltage drives the plasma current. The plasma itself is the single-turn secondary.
Why current matters
- Plasma current, 9.86 MA on Hyperion, produces the poloidal field that, with the toroidal field, twists the field lines needed for confinement.
- It also heats the plasma ohmically in the early phase.
- Its magnitude and profile are central to stability and confinement quality.
The flux limit
A solenoid can only drive current inductively while its own current is still ramping; once it saturates, the inductive drive ends. In a compact spherical tokamak the solenoid is squeezed for space in the center stack, so it holds limited flux. This is why the design pairs the solenoid with non-inductive current drive to sustain the plasma beyond the inductive phase.
In the model
The central solenoid is selectable within the center stack. The exploded view lifts it clear along the axis, showing the coil stack and its position relative to the inboard TF legs and inboard shielding it shares the tight central volume with.
Working with current drive
Because the solenoid alone cannot sustain a long pulse, Hyperion relies on external current drive to hold the plasma current after startup. See also the poloidal-field coils that share the poloidal system.