Coils generate a magnetic field proportional to current, can adjust up to 125A.
A pair of parallel power FETs in common source configuration (low side switching) control the current through the coils. A current sensor feedsback the current to an opamp which compares the current to an analog voltage and adjusts the FETs' gates accordingly. The complication is that the load is inductive and theoretically will never be stable. In practice, the coils are a mess of inductors, resistors and capacitors so the circuit did work when assembled.