IEEE TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY, VOL. 23, NO. 3, JUNE 2013 8201104 The Investigation of Sweeping Speed on the Magnetization of a 2 Inches Diameter YBa 2 Cu 3 O 7−δ Thin Film With the Circular-Type Magnetic Flux Pump W. Wang, Member, IEEE, R. Semerad, F. Spaven, M. Zhang, Member, IEEE, C. H. Hsu, Z. Zhong, Y. Chen, Z. Huang, and T. A. Coombs Abstract—A circular-type magnetic flux pump (CTMFP) device was built to study the flux dynamics on a 2-inch-diameter YBCO thin film. This CTMFP is composed of two CTMFP coils, with each CTMFP coil containing concentric three-phase windings and a dc winding. We connected the three-phase windings to the output of a commercial inverter. By changing the output frequency of the inverter, the sweeping speed of the circular-shaped travelling magnetic wave can be changed. The connection of the phase coils follows the forward consequence, so that the circular-shaped travelling magnetic wave travels inward to the center. The output frequency f was changed from f =0.01 to 1000.0 Hz. The YBCO sample was sandwiched between the two CTMFP coils to experience the circular-shaped travelling magnetic wave. It was found that the increase of the flux density in the center of the film is independent of the sweeping frequency. In high frequency f = 1000.0 Hz, even if the waveform had changed a lot, the increment is still the same as in low frequencies. Index Terms—High-temperature superconductors, super- conducting thin films, three-phase electric power, type II superconductors. I. I NTRODUCTION H IGH temperature superconducting (HTS) films such as YBa 2 Cu 3 O 7−δ film have wide potentials in magnet tech- nology. The production of 2nd generation HTS wire is increas- ing annually to produce HTS coils working at 77 K. HTS coils which can carry very large current with very low cryogenic cost are ideal in possible applications such as MRI, motor, generator, etc. However, the large flux creep phenomenon is not avoidable since a zero resistance HTS-HTS contact is not available at this stage. A flux injection device or flux pump device is necessary to maintain a constant current in a HTS loop. Flux pump technology for type-II superconductors should be based on a clear understanding of the vortex dynamics in type-II superconductors. Vortex dynamics, especially for HTS, is still an area for open discussion [1]. Type-I superconducting Manuscript received October 5, 2012; accepted December 11, 2012. Date of publication January 3, 2013; date of current version January 23, 2013. W. Wang, F. Spaven, M. Zhang, C. Hsu, Z. Zhong, Y. Chen, Z. Huang, and T. Coombs are with the Electric Engineering Division, Department of Engineer- ing, University of Cambridge, CB3 0FA, U.K. (e-mail: ww283@cam.ac.uk). R. Semerad is with Ceraco Ceramic Coating GmbH, 85737 Ismaning, Germany (e-mail: semerad@ceraco.de). Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/TASC.2013.2237734 flux pump was widely investigated over half a century [2]. The basic idea is to introduce a normal spot in type-I superconductor either by breaching the critical field H C [3], [4] or critical temperature T C [5]. This normal spot, which contains a large amount of magnetic flux, shunts the superconducting loop and induces a constant superconducting current [2]. However, in type-II superconductors, the area within the coherence length ξ of a single fluxon can also be considered as a normal spot, though in a microscopic scale. Giaever wrote a clear discussion on the induced electric field due to the creation, transportation and annihilation of vortices in type-II superconducting film [6]. In order to study the flux dynamics in HTS film and find a possible new way to magnetize a HTS coil or film, we had proposed a circular-type magnetic flux pump (CTMFP) [7]. This new device is the combination of the linear-type magnetic flux pump [8] and the thermally actuated flux pump [9], [10]. It includes concentric three phase windings and a dc coil. The three phase windings are connected with a commercial three phase inverter to generate circular-shape travelling magnetic wave. The dc coil is connected to a dc power supply which provides dc background field. We optimized the device by overlapping two identical CTMFP coils. A 2 inches diameter YBCO thin film, provided by Ceraco Ceramic Coating GmbH, Germany, was investigated and some results are reported in [11]. The possible application of this device is for an annulus MRI machine [12]–[16]. In this paper, we report experimental results on a 2 inches diameter YBCO thin film with different sweeping speed of the circular-shape travelling magnetic wave. The directions of the sweeping wave are set as from outside into the center (inwardly). II. EXPERIMENTAL SETUP A. CTMFP Coils Each of the CTMFP coils is comprised of a three concentric phase windings and a dc coil. The phase windings are separated by electric iron lamination. We had improved previous design [7] by overlapping two identical CTMFP coils, so that the magnetic field in between the gap is mostly perpendicular to the surface of YBCO sample. The CTMFP coils are inserted in TUFNOL support. The design is shown in Fig. 1. As marked 1051-8223/$31.00 © 2013 IEEE