Direct measurement of the vortex migration caused by traveling magnetic wave

Wang, Wei; Spaven, F.; Zhang, Min; Baghdadi, Mehdi; Coombs, Tim

doi:10.1063/1.4862788

Cited by 22 publications

(23 citation statements)

References 20 publications

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“…High-T c superconducting (HTS) dynamos [1][2][3][4][5][6][7][8][9][10][11][12][13] and other similar HTS "flux pumps" [14][15][16][17][18][19][20][21][22] have been receiving continuing attention recently, as they offer a potential solution to the dc current-injection problem in a wide range of superconducting machines [23] and magnets [24,25]. Specifically, the HTS dynamo is of interest for its predicted output produced by a HTS dynamo arises naturally from Maxwell's laws [12] when applied to a situation in which eddy currents flow in a thin sheet exhibiting a highly nonlinear local resistivity.…”

Section: Introductionmentioning

confidence: 99%

Mechanism of the High- Tc Superconducting Dynamo: Models and Experiment

et al. 2020

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High-T c superconducting (HTS) dynamos are experimentally proven devices that can produce large (more than a kiloamp) dc currents in superconducting circuits, without the thermal leak associated with copper current leads. However, these dc currents are theoretically controversial, as it is not immediately apparent why a device that is topologically identical to an ac alternator should give a dc output at all. Here, we present a finite-element model and a comparison of it with experiment that fully explain this effect. It is shown that the dc output arises naturally from Maxwell's laws when time-varying overcritical eddy currents are induced to circulate in a HTS sheet. We first show that our finite-element model replicates all of the experimental electrical behavior reported so far for these devices, including the dc output characteristics and transient electrical waveforms. Direct experimental evidence for the presence of circulating eddy currents is also obtained through measurements of the transient magnetic field profile across the HTS tape, using a linear Hall array. These results are also found to agree closely with predictions from the finite-element model. Following this experimental validation, calculated sheet current densities and the associated local electric fields are examined for a range of frequencies and net transport currents. We find that the electrical output from a HTS dynamo is governed by the competition between transport and eddy currents induced as the magnet transits across the HTS tape. The eddy currents are significantly higher (approximately 1.5 times) than the local critical current density, and hence experience a highly nonlinear local resistivity. This nonlinearity breaks the symmetry observed in a normal ohmic material, which usually requires the net transport current to vary linearly with the average electric field. The interplay between local current densities and nonlinear resistivities (which both vary in time and space) is shown to systematically give rise to the key observed parameters of experimental HTS dynamo devices: the open-circuit voltage, the internal resistance, and the short-circuit current. Finally, we identify that the spatial boundaries formed by each edge of the HTS stator tape play a vital role in determining the total dc output. This offers the potential to develop alternative designs for HTS dynamo devices, in which the internal resistance is greatly reduced and the short-circuit current is substantially increased.

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Section: Introductionmentioning

confidence: 99%

Mechanism of the High- Tc Superconducting Dynamo: Models and Experiment

et al. 2020

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“…The numerical model has shown that, the ac travelling magnetic waves have caused the superconducting current densities with "intermediate values," which were smaller than its critical value. 1 In this paper, we have built an updated CTMFP device, shortening the produced wavelengths to half and quarter of the previous experiment. 10 An updated numerical model have also been built to reveal the causation of the induced intermediate values of current densities.…”

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confidence: 99%

“…The detailed experimental setup can be found elsewhere. 1,10 In the updated CTMFP magnets, the width of the teeth and slots of each phase winding is 1.25 mm, which is one quarter of the original CTMFP magnets. 10 In this work, we defined a parameter d to describe the relative size of the YBCO sample to the wavelength…”

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confidence: 99%

“…An updated numerical model was built based on the FEM method with Maxwell's Equations and the E À J power relationship, to simulate the complete magnetization process during the experiments. The governing equation is expressed as 1,11…”

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confidence: 99%

“…3(a) shows a clear degradation of pre-established magnetic gradients after applying the ac travelling waves, similar to the results in the previous CTMFP experiment. 1 That is to say, the ac travelling waves tend to flatten out the pre-established critical magnetic gradient in the YBCO film, making the value of magnetic gradient smaller than the critical value. The magnetic gradient degrades further with the decrease of wavelength k from 30 mm, 15 mm to 7.5 mm (d increases from 0.83, 1.67 to 3.33).…”

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confidence: 99%

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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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