Origin of dc voltage in type II superconducting flux pumps: field, field rate of change, and current density dependence of resistivity

Geng, Jianzhao; Matsuda, Koichi; Fu, Lin; Fagnard, Jean-François; Zhang, H; Zhang, X; Shen, Boyang; Dong, Qihuan; Baghdadi, Mehdi; Coombs, Tim

doi:10.1088/0022-3727/49/11/11lt01

Cited by 91 publications

(66 citation statements)

References 29 publications

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“…In this case, the open circuit voltage is the same as we reported in Ref. 20. Most flux will enter the CC from one edge and leaves at the other, so the open circuit voltage magnitude is proportional to field frequency and its polarity is determined by field traveling direction, as described by Eq.…”

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confidence: 83%

“…The general explanation for the dc voltage of type I and type II superconducting films under travelling magnetic field has been reported in Ref. 19,20. The dc open circuit voltage can be expressed as 20 :…”

mentioning

confidence: 99%

“…Where R edge (t) represents the total resistance of either edge, and R edge (dH/dt) is the dynamic resistance of each edge which is caused by the interaction between the changing field at each edge and the global screening current i, 20,22 and R edge (I) is caused by the de-pinning effect. When the transport current I is smaller than 200A, the pinning energy is large, so R edge (I) is negligibly small compared with R edge (dH/dt).…”

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Voltage-ampere characteristics of YBCO coated conductor under inhomogeneous oscillating magnetic field

Geng

Shen

et al. 2016

Applied Physics Letters

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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...

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Voltage-ampere characteristics of YBCO coated conductor under inhomogeneous oscillating magnetic field

Geng

Shen

et al. 2016

Applied Physics Letters

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“…Bumby et al [19] attributed the open circuit DC voltage in the HTS dynamo to the rectifying effect due to varying resistances in forward and back screening current paths of the superconducting stator. Geng et al [20] proposed a generalized transformer-rectifier model to explain travelling wave based flux pumps, in which the magnetic fields were considered to play the role of magnetic induction as well as switching, and the switching effect is due to field strength, field rate of change, and current density dependence of flux motion in the superconductors. Based on the understanding of the physics, Geng and Coombs proposed two HTS transformer-rectifier flux pumps.…”

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confidence: 99%

A kilo-ampere level HTS flux pump

Geng

Painter

Long

et al. 2019

Supercond. Sci. Technol.

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This paper reports a newly developed high current transformer-rectifier High-T c Superconducting (HTS) flux pump switched by dynamic resistance. A quasi-persistent current of over 1.1 kA has been achieved at 77 K using the device, which is the highest reported operating current by any HTS flux pumps to date. The size of the device is much smaller than traditional current leads and power supplies at the same current level. Parallel YBCO coated conductors are used in the transformer secondary winding as well as in the superconducting load coil to achieve high current. The output current is limited by the critical current of the load rather than the flux pump itself. Moreover, at over 1 kA current level, the device can maintain high flux injection accuracy, and the overall flux ripple is less than 0.2 mili-Weber. The work has shown the potential of using the device to operate high field HTS magnets in ultra-high quasi-persistent current mode, thus substantially reducing the inductance, size, weight, and cost of high field magnets, making them more accessible. It also indicates that the device is promising for powering HTS NMR/MRI magnets, in which the requirement for magnetic field satiability is demanding.

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“…6,7 The physics of HTS flux pump phenomenon is still under extensive discussions. 8 Explanations are required from studying its unique magnetization behaviours and vortex dynamics.…”

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Magnetization of YBCO film with ac travelling magnetic waves of relatively short wavelengths

Wang

Coombs

2017

Applied Physics Letters

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The magnetizations of the YBCO film with ac travelling magnetic waves of relatively short wavelengths were studied. The results have verified that the reported “intermediate value” of the superconducting current density [Wang et al., Appl. Phys. Lett. 104(3), 032602 (2014)] was caused by the existence of multiple transition regions in the sample: the magnetic poles induce ±JC in the pole regions, which produces two transition regions within each wavelength λ (+JC→−JC→+JC, and vice versa, while the symbol → indicates the transition region). The current densities in the transition region are with intermediate values, which are smaller than the critical value. In case of relatively short wavelength, there are multiple transition regions, which occupy a large fraction of the YBCO sample with intermediate current values. Moreover, the wavelike current distributions might help explain the flux transportation and dc output voltage in HTS flux pump.

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Origin of dc voltage in type II superconducting flux pumps: field, field rate of change, and current density dependence of resistivity

Cited by 91 publications

References 29 publications

Voltage-ampere characteristics of YBCO coated conductor under inhomogeneous oscillating magnetic field

Voltage-ampere characteristics of YBCO coated conductor under inhomogeneous oscillating magnetic field

A kilo-ampere level HTS flux pump

Magnetization of YBCO film with ac travelling magnetic waves of relatively short wavelengths

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