Superconducting flux pumps are the kind of devices which can generate direct current into superconducting circuit using external magnetic field. The key point is how to induce a DC voltage across the superconducting load by AC fields. Giaever [1] pointed out flux motion in superconductors will induce a DC voltage, and demonstrated a rectifier model which depended on breaking superconductivity. Klundert et al. [2, 3] in their review(s) described various configurations for flux pumps all of which relied on inducing the normal state in at least part of the superconductor. In this letter, following their work, we reveal that a variation in the resistivity of type II superconductors is sufficient to induce a DC voltage in flux pumps and it is not necessary to break superconductivity. This variation in resistivity is due to the fact that flux flow is influenced by current density, field intensity, and field rate of change. We propose a general circuit analogy for travelling wave flux pumps, and provide a mathematical analysis to explain the DC voltage. Several existing superconducting flux pumps which rely on the use of a travelling magnetic wave can be explained using the analysis enclosed. This work can also throw light on the design and optimization of flux pumps.

We present temperature-dependent modeling of high-temperature superconductors (HTS) to understand HTS electromagnetic phenomena where temperature fluctuation plays a nontrivial role. Thermal physics is introduced into the well-developed H-formulation model, and the effect of temperature-dependent parameters is considered. Based on the model, we perform extensive studies on two important HTS applications: quench propagation and pulse magnetization. A micrometer-scale quench model of HTS coil is developed, which can be used to estimate minimum quench energy and normal zone propagation velocity inside the coil. In addition, we study the influence of inhomogeneity of HTS bulk during pulse magnetization. We demonstrate how the inhomogeneous distribution of critical current inside the bulk results in varying degrees of heat dissipation and uniformity of final trapped field. The temperature-dependent model is proven to be a powerful tool to study the thermally coupled electromagnetic phenomena of HTS.

High-T c Superconducting (HTS) flux pumps are capable of injecting flux into a superconducting circuit, which can achieve persistent current operation for HTS magnets. In this paper, we studied the operation of a rectifier-type HTS flux pump. The flux pump employs a transformer to generate high alternating current in its secondary winding which is connected to an HTS load shorted by an HTS bridge. A high frequency AC field is intermittently applied perpendicular to the bridge, thus generating flux flow. The dynamic resistance caused by the flux flow "rectifies" the secondary current, resulting in a direct current in the load. We have found that the final load current can be easily controlled by changing the phase difference between the secondary current and bridge field. Bridge field of frequency ranging 10Hz-40Hz, magnitude ranging 0-0.66T was tested. Flux pumping was observed for field magnitude of 50mT or above. We have found that both higher field magnitude and higher field frequency result in a faster pumping speed and a higher final load current. This can be attributed to the influence of dynamic resistance. The dynamic resistance measured in the flux pump is comparable with the theoretical calculation. The experimental results fully support a first order circuit model. The flux pump is much more controllable than travelling wave flux pumps based on permanent magnets, which makes it promising for practical use.

Direct current carrying type II superconductors present a dynamic resistance when subjected to an oscillating magnetic field perpendicular to the current direction. If a superconductor is under a homogeneous field with high magnitude, the dynamic resistance value is nearly independent of transport current. Hoffmann and coworkers [C. Hoffmann, D. Pooke, and A. D. Caplin, IEEE Trans. Appl. Supercond. 21, 1628 (2011)] discovered, however, flux pumping effect when a superconducting tape is under an inhomogeneous field orthogonal to the tape surface generated by rotating magnets. Following their work, we report the whole Voltage-Ampere (V-I) curves of an YBCO coated conductor under permanent magnets rotating with different frequencies and directions. We discovered that the two curves under opposite rotating directions differ from each other constantly when the transport current is less than the critical current, whereas the difference gradually reduces after the transport current exceeds the critical value. We also find that for different field frequencies, the difference between the two curves decreases faster with lower field frequency. The result indicates that the transport loss is dependent on the relative direction of the transport current and field travelling, which is distinct from traditional dynamic resistance model. The work may be instructive for the design of superconducting motors. It is well known that type II superconductors carrying a direct current present a resistance when they are under a perpendicular oscillating magnetic field. 1-10 Theoretical analysis 1,3,7,8 and experimental results 1-7,10 have shown that this resistance is due to flux flow caused by the interaction between the transport current and the applied field, and it is defined as dynamic resistance. The dynamic resistance value is proportional to the applied field magnitude and frequency if the superconductor in a homogeneous field, and it is nearly independent of transport current if the field magnitude is high. 7 Hoffmann and coworkers 11 discovered flux pumping effect when a piece of superconducting stator connecting to a superconducting load was subjected to inhomogeneous magnetic fields orthogonal to the tape surface generated by rotating permanent magnets. Researchers from VUW 12 pointed out that dynamic resistance is the main limiting factor of saturation current in the flux pump. Concerning the open circuit voltage, they proposed a geometrical explanation considering screening current. 13 All these existing researches 11-13 focus on the flux pumping effect, where the superconducting stator generates dc power. In this letter, we extend the research to the V-I characteristics of YBCO Coated Conductor (CC) under rotating permanent magnets. The experimental system is shown in Fig. 1(a), which is similar to rotating magnets flux pumps 11-15. Eight Neodymium 52 magnets with a diameter of 20mm are uniformly mounted on a round copper disc. The outer diameter of the disc is about 81mm. All magnets were mounted with their north poles fa...

The use of (Re)BCO is limited by the problems of magnetisation / demagnetisation. (Re)BCO is available in many forms but two of the most interesting for high magnetic field applications are 2G tape and Bulks (either or as grown or manufactured artificially using 2G tapes). The minimum joint resistance which can be achieved between YBCO tapes is of the order of 100 nΩ but this is still too large to operate coils in persistent mode. Bulks have potential to act as very high field magnets but in order to do this they need to be magnetised. This paper describes flux pumping methods which can be used to charge either coils or bulks.