Increasing The Current Output Of High-Temperature-Superconducting Transformer-Rectifier Circuits

Superconducting power supplies – sometimes referred to as ‘flux pumps’ – are devices which can generate large dc currents in superconducting electromagnets. They operate from within the cryogenic environment of the magnet, which precludes the need for current leads to a room-temperature current supply. The use of superconducting power supplies is especially important for electromagnets that utilize high-temperature superconducting (HTS) wire. Since fully-superconducting joints between HTS wires are currently still impractical, solder joints with finite resistance are necessary. Resistive losses prevent the operation of HTS magnets in a persistent current mode (PCM), as current decays too quickly to be of practical use. Instead, the current in the magnet must be maintained by a power supply in a quasi-persistent current mode (qPCM).

Broadly, there are two categories of superconducting power supply: the travelling-wave topology and the transformer-rectifier. Transformer-rectifiers are superconducting circuit devices. A transformer injects ac current into a superconducting rectifier circuit to produce a dc current output. It has the advantages of higher efficiency at a wider range of current outputs. The separation of circuit components also allows for more detailed understanding and predictable scaling. For this reason, it is identified as a useful approach to powering HTS magnets with large currents. This thesis explores the challenges in producing a HTS transformer-rectifier circuit with as much current output as possible. Specifically, a goal of 10 kA is identified as relevant to large-scale applications of high-field, HTS magnets. To date, no other studies have given a comprehensive understanding of the full-wave HTS transformer-rectifier. Here, device performance and operation is approached first using circuit theory. An experimental device is presented to validate theoretical expectations and identify avenues to higher current output. A pair of studies are presented on the individual switch and transformer components in the circuit, with recommendations given for future devices. These works identify a number of advantages on offer by the full-wave circuit. Specifically, a non-commutative operation method is found that can increase the current output of the circuit by an order of magnitude. Finally, these studies are used to provide a design methodology for specifying the electrical circuit required for a given application. A 10 kA HTS transformer-rectifier is designed for use with fusion energy magnets.

From these studies, full-wave HTS transformer-rectifiers can be approached from a more practical perspective. The design methodology presented can be applied to almost any HTS magnet application. Future studies can leverage such findings to rapidly prototype transformer-rectifiers for a specific magnet system.