Mechanism of the High- Tc Superconducting Dynamo: Models and Experiment
| Autor: | Mark D. Ainslie, Rod Badcock, Chris W. Bumby, Ratu Mataira, A. E. Pantoja |
|---|---|
| Rok vydání: | 2020 |
| Předmět: |
Physics
Condensed matter physics Stator General Physics and Astronomy 02 engineering and technology Internal resistance 021001 nanoscience & nanotechnology 01 natural sciences law.invention Magnetic field law Electric field Magnet 0103 physical sciences Eddy current 010306 general physics 0210 nano-technology Voltage Dynamo |
| Zdroj: | Physical Review Applied. 14 |
| ISSN: | 2331-7019 |
| DOI: | 10.1103/physrevapplied.14.024012 |
| Popis: | 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. |
| Databáze: | OpenAIRE |
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