Persistent Current Mode
A superconducting coil can hold current in a closed loop with no supply connected, using a superconducting switch, which stabilizes the field.
The idea
Because a superconducting circuit has no resistance, a current set up in a closed loop persists on its own. A persistent-current switch is a short length of superconductor across the coil terminals with a small heater. Open the switch by heating it normal, drive current in from the supply, then cool the switch back superconducting to close the loop and disconnect the supply.
Why use it
With the supply removed, the field is extremely stable because there is no supply ripple or lead heat, and the current leads can be retracted or run at low current, cutting the largest conduction heat load into the cold mass. Persistent mode is standard in steady-field magnets such as MRI systems and is attractive wherever a constant field is needed for long periods.
Control sequence
The controller heats the switch, ramps supply current until the coil current matches, holds while the switch is confirmed normal, then removes heater power and waits for the switch to recover superconductivity before ramping the supply down. Mismatch between supply and coil current at switch closure causes a current redistribution that must be kept small to avoid a disturbance.
- Heat switch normal to open the loop
- Ramp coil current from the supply
- Cool switch superconducting to close the loop
- Ramp supply down; current persists in the coil
Limits for fusion
Persistent mode suits coils that hold a fixed field, such as toroidal-field or plug magnets. It does not suit coils whose current must change during a pulse, like equilibrium, shaping, or vertical-stability coils, which stay supply-driven. In the Hyperion breeder the toroidal-field system holds a steady field and is the natural candidate for a persistent or near-persistent scheme in the design model, while the poloidal coils remain actively driven.
A small residual current decay from joint resistance is monitored, but in a well-made circuit it is negligible over an operating campaign.