High temperature superconductors (HTS) allow the construction of innovative magnets beyond the capabilities of existing magnet technologies. Utilising the high current density of HTS would allow the creation of systems with higher magnetic fields [1]–[3] or unique field topologies. However, conventional charging techniques lead to large thermal loads into the cryogenic environment when charging to high current levels [4]. HTS flux pumps (FPs) offer an alternative and have been shown to generate currents greater than 1kA with minimal to no heat leak to the cryogenic environment. Various FP methodologies have been demonstrated experimentally including dynamos [4], [5], linear type [5], [6], and transformer-rectifier FPs [7]–[9]. Transformer-rectifier FPs appear to offer the best alternative for large current applications as they offer the highest output voltages and efficiencies.
All HTS transformer-rectifiers operate under the same fundamental concept: an AC signal is transmitted via a transformer into a superconducting circuit which is rectified into a load using superconducting switching elements. In HTS switch rectifiers, the switching element can remain superconducting throughout the process enabling the generation and maintenance of high currents without significant resistive loss. The required switching is generated by manipulating the superconducting state. In the existing literature, this process is driven by dynamic resistance [10] – where an AC magnetic field is applied to the switching element and AC loss mechanisms drive the resistive behaviour. However, the dynamic resistance magnitude induced is inversely proportional to the HTS critical current density, therefore, dynamic resistance FPs may become less effective as the load current requirements increase.
In this work, we propose and experimentally verify a transformer–rectifier type FP that exploits the Jc(B) characteristics of HTS. In this FP, the superconducting switching element is subjected to a DC magnetic field, which suppresses its critical current below the transformer generated transport current level. This drives the switch into the flux flow regime, generating the required resistive behaviour. Over repeated cycles flux with systematic control of the switching elements in sync with the applied current, we generate a quasi-persistent current of 54.5 A at 77 K into a load coil. This output current is limited by the critical current of the load rather a fundamental limitation of the FP itself. This advancement will overthrow the common knowledge that HTS flux pumps could only be used for maintaining rather than fast ramping magnetic fields.
[1] H. W. Weijers et al., “Progress in the Development and Construction of a 32-T Superconducting Magnet,” IEEE Trans. Appl. Supercond., vol. 26, no. 4, pp. 1–7, Jun. 2016, doi: 10.1109/TASC.2016.2517022.
[2] H. W. Weijers et al., “High Field Magnets With HTS Conductors,” IEEE Trans. Appl. Supercond., vol. 20, no. 3, pp. 576–582, Jun. 2010, doi: 10.1109/TASC.2010.2043080.
[3] C. Li, J. Geng, B. Shen, J. Ma, J. Gawith, and T. A. Coombs, “Investigation on the Transformer-Rectifier Flux Pump for High Field Magnets,” IEEE Trans. Appl. Supercond., vol. 29, no. 5, pp. 1–5, Aug. 2019, doi: 10.1109/TASC.2019.2900623.
[4] A. Ballarino, “Current Leads, Links and Buses,” Proc CAS-CERN Accel. Sch. Supercond Accel., pp. 547–558, 2013.
[5] L. Fu, K. Matsuda, M. Baghdadi, and T. Coombs, “Linear Flux Pump Device Applied to High Temperature Superconducting (HTS) Magnets,” IEEE Trans. Appl. Supercond., vol. 25, no. 3, pp. 1–4, Jun. 2015, doi: 10.1109/TASC.2015.2406294.
[6] Z. Bai, S. Ding, C. Li, C. Li, and G. Yan, “A Newly Developed Pulse-Type Microampere Magnetic Flux Pump,” IEEE Trans. Appl. Supercond., vol. 20, no. 3, pp. 1667–1670, Jun. 2010, doi: 10.1109/TASC.2010.2040728.
[7] J. Geng and T. A. Coombs, “An HTS flux pump operated by directly driving a superconductor into flux flow region in the E– J curve,” Supercond. Sci. Technol., vol. 29, no. 9, p. 095004, Sep. 2016, doi: 10.1088/0953-2048/29/9/095004.
[8] J. Geng, K. Matsuda, L. Fu, B. Shen, X. Zhang, and T. A. Coombs, “Operational research on a high- T c rectifier-type superconducting flux pump,” Supercond. Sci. Technol., vol. 29, no. 3, p. 035015, Mar. 2016, doi: 10.1088/0953-2048/29/3/035015.
[9] P. Zhou et al., “A Contactless Self-Regulating HTS Flux Pump,” IEEE Trans. Appl. Supercond., vol. 30, no. 4, pp. 1–6, Jun. 2020, doi: 10.1109/TASC.2020.2978787.
[10] J. Geng and T. A. Coombs, “Mechanism of a high- T c superconducting flux pump: Using alternating magnetic field to trigger flux flow,” Appl. Phys. Lett., vol. 107, no. 14, p. 142601, Oct. 2015, doi: 10.1063/1.4932950.
Keywords: Flux pump, HTS magnet, Coated conductor