Steven Wray

High-current flux pumps are an enabling technology for high-temperature superconducting (HTS) magnets operating in a quasi-persistent current mode (qPCM). For fusion energy specifically, flux pumps must be capable of delivering more than 10 kA of DC output to a superconducting magnet. Recent HTS flux pumps have yet to achieve more than 1–3 kA of current. In this report, we present progress towards a 10 kA HTS flux pump using the transformer-rectifier circuit topology at the Paihau-Robinson Research Institute. A 10 kA test solenoid has been designed and the fabrication has been commenced. The main requirements of the rectifier switches and transformer have been identified and evaluated, with a circuit simulation demonstrating 10 kA of output expected in under 15 s, with a current ripple of only 0.33%. Finally, a method of low-resistance solder joint fabrication using indium-foil is validated.

The performance of HTS transformers is integral to predictability of outputs in HTS transformer-rectifier flux pumps. To date, little attention has been given to compact HTS transformers using stacked-tape cables. Current sharing behaviour in HTS transformers with stacked-tape cable secondary windings is studied here experimentally. Secondaries of 1–4 tapes are tested with a 500:1 transformer. Two modes of coupling are identified. Firstly, primary inductive coupling determines secondary current during changes in primary current. Secondly, current is shared inductive between loops when primary current is held constant. This is caused by secondary circuit resistance. The transition between sharing modes causes strong hysteresis in the current output of the transformer. However, this is found to have little impact on the RMS current in the secondary if the resistance is sufficiently low. Outputs greater than 600 A are demonstrated from a compact transformer. These results may lead to more compact transformers in future transformer-rectifier flux pump devices.

Iron-core electromagnets are widely used as applied-field switches in high-temperature superconducting (HTS) flux pumps. It was recently reported that the critical current ( I c ) of the HTS wire is suppressed by up to 45% in the air gap of the iron core. Here, it is reported that the self-field I c of a single tape (unifilar) is suppressed from 357 A to 195 A while in the air gap of an iron core, agreeing closely with the previous study. This is extended by showing that the I c response to a perpendicular magnetic field is asymmetric, with a peak I c of 249 A found to be at 0.16 T of applied magnetic field. This effect can be eliminated using an anti-parallel current configuration (bifilar), which is shown experimentally to recover close to the no-core self-field I c of 357 A with no observed asymmetry. The results of both types of switch configurations are closely replicated in finite-element modelling using COMSOL Multiphysics^®. This is caused by the asymmetric interaction between the self-induced magnetic field of the HTS switch element and the magnetic circuit of the iron-core electromagnet.