Abstract
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.
Highlights
IntroductionHigh-Tc 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]
The dc output observed from experimental High-Tc superconducting (HTS) dynamo devices [7] has presented physicists with a conundrum, as this device is topologically identical to an ac alternator described by Faraday [33] and should not be expected to produce a dc electromotive force
Through interpreting results from the FE model, we find that both the short-circuit current (Isc) and the apparent internal resistance (Rint) of the dynamo arise from competition between the net transport current IT and the local circulating eddy currents
Summary
High-Tc 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]. Ability to drive a large [26,27] dc current without the associated heat leak of traditional current leads [28] This current is provided by a near-zero source impedance, which reduces the risk of an externally initiated “driven” quench [29,30]. The dc output observed from experimental HTS dynamo devices [7] has presented physicists with a conundrum, as this device is topologically identical to an ac alternator described by Faraday [33] and should not be expected to produce a dc electromotive force (emf) This has led some authors to propose explanations based upon exotic phenomena such as quantum flux coupling [34] or flux ratcheting [21].
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