The rapid development of second generation (2G) high temperature superconducting (HTS) wires in the last decade has made it possible to wind high quality 2G HTS coils. These 2G HTS coils show promise for future applications such as magnetic resonance imaging (MRI) magnets, electrical machines, magnetic levitation trains, energy storage, etc. 2G HTS coils can be operated using either dc current or ac current. Several important issues have yet to be resolved, such as how to properly magnetise an HTS coil under dc conditions, or how to minimise losses under ac conditions. These problems should be carefully studied before the 2G HTS coils can be widely applied in scientific and industrial applications. This thesis focuses on emerging HTS flux pump technology for HTS coils operating in a dc environment. HTS flux pump technology applies a travelling magnetic wave to fully magnetise an HTS coil, which is both efficient and economical, and has in recent years been proven feasible. However, the underlying physics of this technology are so far poorly understood. In order to study the influence of a travelling magnetic wave on HTS films such as YBa2Cu3O7-δ, two types of circular-type magnetic flux pump (CTMFP) devices were proposed and built. These novel devices generate an annular-shape travelling magnetic wave. The first type was the original CTMFP magnet, which produces the longest wavelength of travelling wave. The second type was the updated CTMFP magnet, which can produce a shorter wavelength of travelling wave (1/2 of the original CTMFP magnet in the six phase connection and 1/4 in the three phase connection). A 2 inch diameter round shape YBCO thin film (200 nm thick of the YBCO layer) and a 46 mm× 46 mm square shape YBCO tape (1.0 µm thick of the YBCO layer, with a hole of Φ26 mm in the centre) were tested. When using a round shape YBCO thin film and the original CTMFP magnet, it was found that the travelling wave tends to decrease the existing critical magnetic gradient inside the YBCO film. The experiment was repeated under different conditions, such as zero-field cooling (ZFC), field cooling (FC), delta-shape trapped field, etc. A simulation based on the H-formulation using FEM software revealed that, after application of the travelling wave, the current density distribution inside the round shape YBCO sample was disturbed, becoming much lower than its critical current density JC. This discovery is interesting because the Bean model suggests that the current density inside a type-II superconductor should be equal to either +JC or - JC (the critical state model). It was found that a round shape YBCO sample follows the Bean model prediction for the homogeneous oscillating field (homogeneous in space), which suggests that the travelling wave is more efficient for transporting the magnetic flux inside YBCO film, compared to a homogeneous oscillating field. An updated CTMFP magnet was designed and built to investigate the influence of the degree of field inhomogeneity on the change of an existing critical magnetic gradient. The results were compared between the six phase connection (1/2 wavelength of the original CTMFP magnet) and the three phase connection (1/4 wavelength of the original CTMFP magnet). It was found that with a travelling wave of consistent amplitude, by shortening the wavelength, the change of magnetic gradient is made stronger. The result supports the assumption that the field inhomogeneity in space may have an important influence on the magnetisation of a YBCO sample. Additionally, in the case of a three phase connection (1/4 wavelength), by reversing the direction of the travelling wave, a different magnetisation profile was obtained, which suggests that the experiment may have detected a macroscopic “magnetic coupling” phenomenon. However, this result needs further study before it can be confirmed. The square shape YBCO sample was tested by applying a travelling wave in a dc background field under FC conditions. The square shape YBCO sample has a centre hole (Φ26 mm), which is closest to the condition of an HTS coil (single layer instead of multi-layer). However, in the experiment there was no clear change of magnetic flux inside the superconducting loop after application of the travelling wave. This might be attributed to the fact that, the field inhomogeneity is not strong enough to cause flux migration in the experiments, and the YBCO layer is relatively thicker which increases the difficulties. Moreover, the width of the superconducting region is relatively small (10 mm), in order to help magnetic flux migrate into the superconducting loop, the field inhomogeneity must be strong enough in the superconducting region, which increases the technical difficulties. However, this might be able to be accomplished by increase the amplitude of the travelling waves. Some experiments will be carried out in the future. The experimental findings in this thesis can not only aid in understanding the mechanism of HTS flux pump technology for an HTS coil, but also can help in understanding ac loss from a coil exposed to a travelling wave. As was suggested by the experimental results, the magnetisation of the YBCO film due to the travelling wave is very different from the magnetisation induced by a homogeneous oscillating field. Under operational conditions, such as inside an HTS motor, the HTS coils experience a travelling wave rather than a homogeneous oscillating field. This thesis discusses the difference in resultant ac loss from a travelling wave and a homogeneous oscillating field of the same amplitude. It was found that, for the round shape YBCO sample, the ac loss from a travelling wave is about 1/3 of the loss from a homogeneous oscillating field. The regions in which the ac loss occurred are also different between a travelling wave and a homogeneous oscillating field. These results suggest that the travelling wave cannot be equated to a homogeneous oscillating field when calculating ac loss. In conclusion, this thesis studies two novel experimental devices, built to study the magnetisation of YBCO films under the influence of a travelling wave. Several novel electromagnetic behaviours were observed in the YBCO films under the influence of a travelling wave, which may help improve understanding of HTS flux pump technology for an HTS coil, and the ac loss induced by a travelling wave.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:607603 |
| Date | January 2014 |
| Creators | Wang, Wei |
| Publisher | University of Cambridge |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
| Source | https://www.repository.cam.ac.uk/handle/1810/245420 |
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