A dynamo for generating a persistent current in a superconducting circuit

Volger, J.; Admiraal, P.S.

doi:10.1016/0031-9163(62)90253-6

Cited by 50 publications

(11 citation statements)

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“…When there is still room for the transport current to displace subcritical currents, the penetrated magnetic field contributes only to the driving direction (forward current under the magnet). This situation is likely to be much more similar to the normal spot operation seen in LTS dynamos [53], where the penetrated flux can migrate in penetrated normal zones.…”

Section: A Origin Of the DC Voltage V Ocmentioning

confidence: 71%

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: A Origin Of the DC Voltage V Ocmentioning

confidence: 71%

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

et al. 2020

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“…Superconducting dynamos employing low-temperature superconductors (LTS) have been reported by various previous authors. 2,[18][19][20][21][22] These devices utilise materials with a low B c2 such that a "normal-conducting spot" is considered to form in the high field region beneath the rotor magnet. 1 However, a normal spot is not formed in our HTS dynamo, as B c2 (at 77 K) of the YBCO coated conductor used in this work is much greater 23 than the applied rotor field of $0.2 T. Instead, flux vortices will penetrate in the high field region beneath the rotor magnet and move through the HTS tape.…”

mentioning

confidence: 99%

Anomalous open-circuit voltage from a high-Tc superconducting dynamo

Bumby

Jiang

Storey

et al. 2016

Applied Physics Letters

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We report on the behavior of a high-T c superconducting (HTS) homopolar dynamo which outputs a DC open-circuit voltage when the stator is in the superconducting state, but behaves as a conventional AC alternator when the stator is in the normal state. We observe that this time-averaged DC voltage arises from a change in the shape of the AC voltage waveform that is obtained from a normal conducting stator. The measured DC voltage is proportional to frequency, and decreases with increasing flux gap between the rotor magnet and the HTS stator wire. We observe that the DC output voltage decreases to zero at large flux gaps, although small differences between the normal-conducting and superconducting waveforms are still observed, which we attribute to screening currents in the HTS stator wire. Importantly, the normalised pulse shape is found to be a function of the rotor position angle only. Based on these observations, we suggest that the origin of this unexpected DC effect can be explained by a model first proposed by Giaever, which considers the impact of time-varying circulating eddy currents within the HTS stator wire. Such circulating currents form a superconducting shunt path which "short-circuits" the high field region directly beneath the rotor magnet, at those points in the cycle when the rotor magnet partially overlaps the superconducting stator wire. This reduces the output voltage from the device during these periods of the rotor cycle, leading to partial rectification of the output voltage waveform and hence the emergence of a time-averaged DC voltage.

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“…high-T c superconducting (HTS) dynamo [1][2][3] enables the injection of large DC currents into a superconducting circuit, without the requirement for current leads [4], which can impose a substantial heat load on the cryogenic system. This has significant potential to be used to energise HTS coils in NMR/MRI magnets [5] and superconducting rotating machines [4].…”

Section: Introductionmentioning

confidence: 99%

Modelling the Frequency Dependence of the Open-Circuit Voltage of a High-T _c Superconducting Dynamo

Ainslie

Quéval

Mataira

et al. 2021

IEEE Trans. Appl. Supercond.

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A high-Tc superconducting (HTS) dynamo enables the injection of large DC currents into a superconducting circuit, without the requirement for current leads. In this work, we attempt to explain the frequency dependence of such dynamos/flux pumps reported in the literature, where it is observed that the rate at which the open-circuit DC voltage increases reduces with increasing frequency, in contrast to the expected linear behaviour. Heat generated in the HTS wire has been the common explanation given to date for this phenomenon. Here we offer an alternative explanation: the interaction between and current flow in the different layers of the HTS wire as the frequency of the dynamo increases. Our claim is based on numerical analysis using a segregated H-formulation finite-element model of the HTS dynamo benchmark problem that is extended to include the full HTS wire architecture and coupled with a thermal model. This framework enables us to efficiently model the relative movement between the rotating room-temperature permanent magnet and the stationary HTS wire and to study the impact of the frequency of rotation and temperature on the open-circuit DC voltage of the dynamo.

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A dynamo for generating a persistent current in a superconducting circuit

Cited by 50 publications

References 1 publication

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

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

Anomalous open-circuit voltage from a high-Tc superconducting dynamo

Modelling the Frequency Dependence of the Open-Circuit Voltage of a High-T _c Superconducting Dynamo

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A dynamo for generating a persistent current in a superconducting circuit

Cited by 50 publications

References 1 publication

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

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

Anomalous open-circuit voltage from a high-Tc superconducting dynamo

Modelling the Frequency Dependence of the Open-Circuit Voltage of a High-T c Superconducting Dynamo

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Product

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Modelling the Frequency Dependence of the Open-Circuit Voltage of a High-T _c Superconducting Dynamo