Maximising the current output from a self-switching kA-class rectifier flux pump

High current operation of HTS magnets is desirable for many applications, but operating currents are typically limited by resistive heating of the normal-conducting current leads, which connect the cryogenic magnet coil to a room-temperature electronic power supply. Rectifying flux pumps are a feasible alternative solution as they enable current to be directly injected into the HTS magnet coil via a closed superconducting circuit. In this work, we have developed and demonstrated two prototype HTS self-rectifying flux pump devices capable of delivering very high output currents. One of these devices can deliver a maximum direct current of >2 kA. This is a new record for the flux pump excitation of an HTS coil. Unlike other rectifying flux pumps, the self-rectifying flux pumps reported here do not employ an active switching component. Instead passive 'self-switching' rectification is achieved by applying an assymmetric current waveform to the primary winding. This significantly reduces the complexity of the system compared to other flux pump architectures, and removes the need for dissipative electronic components within the cryogenic environment. We demonstrate that these devices also function when the iron core linking the superconducting secondary and the normal conducting primary is broken by a pair of air gaps. In these conditions DC currents of >600 A can be maintained with a total air gap length of 10 mm. This indicates that this approach represents a feasible practical solution for through-wall excitation of high current HTS magnets.