An HTS flux pump operated by directly driving a superconductor into flux flow region in the E- J curve

High $\textit{T}$$_C$ Superconducting (HTS) flux pumps are capable of compensating the persistent current decay in HTS magnets without electrical contact. In this paper, following the work of an LTS self-switching flux pump, we propose a new HTS flux pump by directly driving high $\textit{T}$$_C$ superconductor into flux flow region in the $\textit{E-J}$ curve. The flux pump consists of a transformer which has a superconducting secondary winding shorted by an YBCO coated conductor bridge. A high alternating current with much higher positive peak value than the negative peak value is induced in the secondary winding. The current always drives the bridge superconductor into flux flow region only at around its positive peak value, thus resulting in flux pumping. The proposed flux pump is much simpler than existing HTS flux pumps.Cambridge Trust (Cambridge International Scholarship)This is the author accepted manuscript. It is currently under an indefinite embargo pending publication by IOP Science

Modeling methodology for a HTS flux pump using a 2D H-formulation

Flux pumps are the kind of devices that can magnetize closed superconducting magnets in a gradual manner. High-Tc Superconducting (HTS) flux pumps are particularly promising for high field applications, due to the fact that lossless HTS coils are unavailable. The physics of these devices is also attractive. In this paper, we propose a modeling methodology for a transformer-rectifier HTS flux pump switched by dynamic resistance. A finite element model is built in Comsol and solved by 2D H-formulation. The simulation result is verified by experimental data. The simulation will give a clear picture of how flux pumping occurs in the superconductor. It will show flux motion across a superconductor by shifting the electric central line, which is a unique nature of type-II superconductors. This work may be interesting in the understanding of magnetization of High-Tc Superconductors

Magnetization of 2-G Coils and Artificial Bulks

The use of (Re)BCO is limited by the problems of magnetization/demagnetization. (Re)BCO is available in many forms, but two of the most interesting for high magnetic field applications are 2-G tape and bulks (either or as grown or manufactured artificially using 2-G tapes). The minimum joint resistance that can be achieved between YBCO tapes is on the order of 100 nΩ, but this is still too large to operate coils in persistent mode. Bulks have potential to act as very high field magnets, but in order to do this, they need to be magnetized. This paper describes flux pumping methods, which can be used to charge either coils or bulks.This is the accepted manuscript. The final published version is available from IEEE at http://ieeexplore.ieee.org/xpls/abs_all.jsp?arnumber=6845330. © 2014 IEEE

General approach for the determination of the magneto-angular dependence of the critical current of YBCO coated conductors

The physical understanding and numerical modelling of superconducting devices which exploit the high performance of second generation high temperature superconducting tapes (2G-HTS), is commonly hindered by the lack of accurate functions which allow the consideration of the in-field dependence of the critical current. This is true regardless of the manufacturer of the superconducting tape. In this paper, we present a general approach for determining a unified function I $_{c}$(B, θ), ultimately capable of describing the magneto-angular dependence of the in-field critical current of commercial 2G-HTS tapes in the Lorentz configuration. Five widely different superconducting tapes, provided by three different manufacturers, have been tested in a liquid nitrogen bath and external magnetic fields of up to 400 mT. The critical current was recorded at 90 different orientations of the magnetic field ranging from θ = 0°, i.e., with B aligned with the crystallographic ab-planes of the YBCO layer, towards ±90°, i.e., with B perpendicular to the wider surfaces of the 2G-HTS tape. The whole set of experimental data has been analysed using a novel multi-objective model capable of predicting a sole function I $_{c}$(B, θ). This allows an accurate validation of the experimental data regardless of the fabrication differences and widths of the superconducting tapes. It is shown that, in spite of the wide set of differences between the fabrication and composition of the considered tapes, at liquid nitrogen temperature the magneto-angular dependence of the in-field critical current of YBCO-based 2G-HTS tapes, can be described by a universal function I $_{c}$($\textit{f}$(B), θ), with a power law field dependence dominated by the Kim's factor B/B $_{0}$, and an angular dependence moderated by the electron mass anisotropy ratio of the YBCO layer.This work was supported by the Engineering and Physical Sciences Research Council (EPSRC) project NMZF/064. X Zhang acknowledges a grant from the China Scholarship Council (No. 201408060080)

Investigation of demagnetization in HTS stacked tapes implemented in electric machines as a result of crossed magnetic field

This paper investigates the practical effectiveness of employing superconducting stacked tapes to superconducting electric machinery. The use of superconducting bulks in various practical applications has been addressed extensively in the literature. However, in practice, dramatic decrease in magnetization would occur on superconducting bulks due to the crossed field effect. In our study, we employed the superconducting stacked tapes in a synchronous superconducting motor, which was designed and fabricated in our laboratory, aiming to lessen demagnetization due to crossed field effect in comparison with superconducting bulks. Applying the transverse AC field, the effects of frequency, amplitude, and number of cycles of the transverse magnetic field are discussed. Furthermore, a stack of 16 layers of superconducting tapes is modelled and the consequences of applying the crossed magnetic field on the sample are evaluated. The confrontation between experiments and simulation allows us to thoroughly understand the crossed field effects on stacked tapes. At the end, a preventive treatment, based on the shielding characteristic of superconductor and materials with high permeability, i.e. -metal and metalic glass, is suggested. On the other hand, the shielding feature of aforementioned materials will hinder the penetration of magnetic field and, consequently, reduction of the demagnetization will be attained.This is the accepted manuscript. The final version is available from IEEE at https://ieeexplore.ieee.org/xpl/articleDetails.jsp?arnumber=6965587&sortType%3Dasc_p_Sequence%26filter%3DAND%28p_Publication_Number%3A77%29%26pageNumber%3D16

Operational research on a high-T <inf>c</inf> rectifier-type superconducting flux pump

High-T$_{c}$ superconducting (HTS) flux pumps are capable of injecting flux into a superconducting circuit, which can achieve persistent current operation for HTS magnets. In this paper, we studied the operation of a rectifier-type HTS flux pump. The flux pump employs a transformer to generate high alternating current in its secondary winding, which is connected to an HTS load shorted by an HTS bridge. A high frequency ac field is intermittently applied perpendicular to the bridge, thus, generating flux flow. The dynamic resistance caused by the flux flow 'rectifies' the secondary current, resulting in a direct current in the load. We have found that the final load current can easily be controlled by changing the phase difference between the secondary current and the bridge field. The bridge field of frequency ranging from 10 to 40 Hz and magnitude ranging from 0 to 0.66 T was tested. Flux pumping was observed for field magnitudes of 50 mT or above. We have found that both higher field magnitude and higher field frequency result in a faster pumping speed and a higher final load current. This can be attributed to the influence of dynamic resistance. The dynamic resistance measured in the flux pump is comparable with the theoretical calculation. The experimental results fully support a first order circuit model. The flux pump is much more controllable than the traveling wave flux pumps based on permanent magnets, which makes it promising for practical use.JG would like to acknowledge Cambridge Trust for offering a Cambridge International Scholarship to support his study in Cambridge.This is the author accepted manuscript. The final version is available from the Institute of Physics via http://dx.doi.org/10.1088/0953-2048/29/3/03501

Flux pumping for non-insulated and metal-insulated HTS coils

High-temperature superconducting (HTS) coils wound from coated conductors without turn-to-turn insulation (non-insulated (NI) coils) have been proven with excellent electrical and thermal performances. However, the slow charging of NI coils has been a long-lasting problem. In this work, we explore using a transformer-rectifier HTS flux pump to charge an NI coil and a metal-insulated coil. The charging performance comparison is made between different coils. Comprehensive study is done to thoroughly understand the electrical-magnetic transience in charging these coils. We will show that the low-voltage high-current flux pump is especially suitable for charging NI coils with very low characteristic resistance

Measurements and calculations of transport AC loss in second generation high temperature superconducting pancake coils

Theoretical and experimental AC loss data on a superconducting pancake coil wound using second generation (2 G) conductors are presented. An anisotropic critical state model is used to calculate critical current and the AC losses of a superconducting pancake coil. In the coil there are two regions, the critical state region and the subcritical region. The model assumes that in the subcritical region the flux lines are parallel to the tape wide face. AC losses of the superconducting pancake coil are calculated using this model. Both calorimetric and electrical techniques were used to measure AC losses in the coil. The calorimetric method is based on measuring the boil-off rate of liquid nitrogen. The electric method used a compensation circuit to eliminate the inductive component to measure the loss voltage of the coil. The experimental results are consistent with the theoretical calculations thus validating the anisotropic critical state model for loss estimations in the superconducting pancake coil