Study of the Penetration of a 2 Inches Diameter YBCO Thin Film With the Travelling Magnetic Wave

This paper studies the magnetisation and ac magnetisation losses caused by a travelling magnetic wave on a single-layer YBCO thin film. This work provides thorough investigations on how the critical magnetic field gradient has been changed by the application of a travelling wave. Several conditions were studied such as zero-field cooling (ZFC), field cooling (FC) and a delta-shaped trapped field. It was found that the travelling wave tends to attenuate the existing critical magnetic field gradients in all these conditions. This interesting magnetic behaviour can be well predicted by the finite element (FEM) software with the E-J power law and Maxwell's equations. The numerical simulations show that the existing critical current density has been compromised after applying the travelling wave. The magnetisation profile caused by the travelling wave is very different from the standing wave, while the magnetisation based on the standing wave can be interpreted by the Bean model and constant current density assumption. Based on the numerical method, which has reliability that has been solidly proven in the study, we have extended the study to the ac magnetisation losses. Comparisons were made between the travelling wave and the standing wave for this specific YBCO sample. It was found that by applying the magnetic wave of the same amplitude, the ac magnetisation loss caused by the travelling wave is about 1/3 of that caused by the standing wave. These results are helpful in understanding the general magnetism problems and ac magnetisation loss in the travelling magnetic wave conditions such as inside a high temperature superconducting (HTS) rotating machine, etc.

Section: Zero-field Cooling (Zfc) Conditionsupporting

confidence: 76%

Section: Delta-shaped Trapped Fieldmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

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The magnetisation profiles and ac magnetisation losses in a single layer YBCO thin film caused by travelling magnetic field waves

Wang

Coombs

2015

“…It would introduce a travelling feature for the external field in the HTS coil. Travelling field penetrates √2−√3 times further than a standing AC field of the same amplitude and frequency [27,28]. Therefore, in the case that the AC field amplitude is less than B th , travelling field might be still able to reach the penetration depth and interact with central DC current.…”

Section: Field Decay Curve Of Closed-loop Coilmentioning

confidence: 99%

Behaviour prediction of closed-loop HTS coils in non-uniform AC fields

Zhong

Wang

et al. 2021

Field decay rate is the key characteristic of superconducting magnets based on closed-loop coils. However, in Maglev trains or rotating machines, closed-loop magnets work in external AC fields and will exhibit an evidently accelerated field decay resulting from dynamic resistances, which are usually much larger than joint resistance. Nevertheless, there has not been a numerical model capable of systematically studying this behaviour, which is the main topic of this work. The field decay curves of a closed-loop high-temperature-superconducting (HTS) coil in various AC fields are simulated based on H-formulation. A non-uniform external field generated by armature coils is considered. Reasonable consistence is found between experimental and simulation results. In our numerical model, the impact of current relaxation, which is a historical challenge, is analysed and subsequently eliminated with acceptable precision. Our simulation results suggest that most proportion of the field decay rate is from the innermost and outermost turns. Based on this observation, a magnetic shielding pattern is designed to reduce the field decay rate efficiently. This work has provided magnet designers with an effective method to predict the field decay rate of closed-loop HTS coils in external AC fields, and explore various shielding designs.

“…The superconducting coil can therefore be regarded as a separate circuit element obeying its own constitutive equation. This is conceptually different from the situation where a topologically-continuous superconducting bulk or film is magnetized by a time-dependent field [17,18], as the latter case does not require a continuous DC voltage to maintain the circulating current.…”

mentioning

confidence: 99%

Impact of flux gap upon dynamic resistance of a rotating HTS flux pump

Jiang

Bumby

Badcock

et al. 2015

HTS flux pumps enable superconducting currents to be directly injected into a magnet coil without the requirement for thermally-inefficient current leads. Here, we present results from an experimen tal mechanically-rotating HTS flux pump employing a coated-conductor stator and operated at 77 K. We show the effect of varying the size of the flux gap between the rotor magnets and coated conductor stator from 1 mm to 7.5 mm. This leads to a corresponding change in the peak applied perpendicular magnetic field at the stator from approximately 350 mT to 50 mT. We observe that our experimental device ceases to maintain a measurable output at flux gaps above 7.5 mm, which we attribute to the presence of screening currents in the stator wire.We show that our mechanically-rotating flux pump is well described by a simple circuit model which enables the output performance to be described using two simple parameters, the open-circuit voltage Voc and the internal resistance, Rd. Both of these parameters are found to be directly proportional to magnet-crossing frequency and decrease with increasing flux gap. We show that the trend in Rd can be understood by considering the dynamic resistance experienced at the stator due to the oscillating amplitude of the applied rotor field. We adopt a literature model for the dynamic resistance within our coated-conductor stator and show that this gives good agreement with the experimentally measured internal resistance of our flux pump.