Thermally Activated ReBCO Switches for Charging High-Current Magnets

Aiming at design improvements for the International Axion Observatory, alternative options for powering of the 20-kA magnet are under consideration. Use of high-Temperature superconductors in a transformer-rectifier system cooled by a single stage cryocooler at 40 K-50 K level is a promising option, which allows to avoid massive movable bus bars, current leads, and related services. The overall efficiency of such a magnet powering system is determined by the characteristics of high-current switches, which have to operate in a fast synchronous manner. We studied the performance of thermally activated superconducting switches made of adjusted 4-mm-wide ReBCO tapes, using etching of the tapes in a FeCl-3 bath to increase their resistance in normal state and to prepare low-mass heaters perfectly matching the tape surface. First experimental results on the 'off' state performance and time constants of the switches, when toggling between 'on' and 'off' states, are presented. Finally, we discuss a conceptual design of an all-ReBCO rectifier system comprising a superconducting transformer and thermally activated switches, which can be scaled up for charging large magnet systems like particle detector magnets where slow charging is feasible.