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As a current compensator, we already developed a linear type magnetic flux pump (LTMFP) that is comprised of DC coils, three phase armature coils, an LTS Nb foil and laminated linear slots. The LTMFP produced a homopolar traveling magnetic field with DC bias current and 3-phase armature current and then, a pumping current is generated in the closed...
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... this model, we designed the same size of LTMFP and ad- vanced LTMFP to compare with each other. Fig. 4 shows the 3D contour of magnetic flux density of the LTMFP with AC of 7 at 60 Hz and magnetic field density of the DC bias cur- rent of 10 A. The mesh consists of 14742 elements. The Under the current conditions, the distributions of homopolar traveling magnetic field is calculated in the dot line as shown in Fig. 5. In addition, we ...
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Magnetic behaviour of superconducting Nb foil, which is utilized for the proposed linear-type magnetic flux pump, was investigated based on 3D FEM analysis. The superconducting (nonlinear) J-E expressions were taken into account for the precise analysis. It was found that there exists the appropriate range of the DC bias current for the realisation...
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Citations
... lf359@cam.ac.uk. Unlike a rotating travelling wave flux pump using permanent magnets, a linear pumping method can work with varying field strength and without vibrations or noise due to rotor unbalance [14,15]. The operational principle of a rotating type flux pump is different from that of a rectifier flux pump. ...
This letter presents a flux pumping method and the results gained when it was used to magnetize a range of different YBCO coils. The pumping device consists of an iron magnetic circuit with eight copper coils which apply a traveling magnetic field to the superconductor. The copper poles are arranged vertically with an air gap length of 1 mm and the iron cores are made of laminated electric steel plates to minimize eddy-current losses. We have used this arrangement to investigate the best possible pumping result when parameters such as frequency, amplitude and waveform are varied. We have successfully pumped current into the superconducting coil up to a value of 90% of I
c and achieved a resultant magnetic field of 1.5 T.
... C. Hoffman presented a novel flux pump for HTS magnets, which is based on using NdFeB permanent magnets mounted on a rotor to energize YBCO HTS coils [4]. Other than early-developed flux pump devices, linear flux pump works with less vibrations and electric noises [7]- [9]. Bai's system used copper solenoids with iron pole pieces which were linearly aligned separately on two sides of an air gap, and the two-side poles oppose to each other. ...
This paper presents a novel linear flux pump which could be used to magnetize 2G HTS tapes and coils. The design is based on an iron magnetic circuit together with copper solenoids and is powered by a current source driver circuit. Several applied waveforms were tested including a symmetric sine wave and an asymmetric trapezoidal wave. Both standing and travelling waves were applied. The measurements focused on the effects of frequency and magnitude of the applied field and their effect on the system pumping efficiency. It was found that a trapezoidal wave was more effective than a sine wave, producing a greater final current at the same applied frequency and field strength. The maximum induced current in the superconducting coil was 19 A which was achieved using an applied field of 50 mT, applied as a travelling trapezoidal wave. The driving current to the copper coils was 5 A in amplitude with a frequency of 10 Hz. It was found that when the applied field magnitude was less than 16 mT pumping did not occur. It proved possible to pump the system with a standing wave as well as a travelling wave. This effect needs to be investigated further as it is possible that the standing wave had travelling components. Index Terms—Applied field, flux pump, high temperature su-perconductors (HTS), superconducting coil.
... However, for some LTS materials, since the upper critical field H C2 is relatively low, it is possible to create a normal region using the magnetic field. For instance, Chung [90] used the LTS material Nb foil for experiments. Nb is a type-II superconducting material for which the lower critical field µ 0 H C1 is equal to 0.14 T, and the upper critical field µ 0 H C2 is equal to 0.31 T. The upper critical field µ 0 H C2 = 0.31 T can be easily achieved using a magnet, and thereby the flux pump method can be used with this type-II superconducting material with the same mechanism as for a type-I superconducting material. ...
... The magnet generates magnetic flux, which creates the normal region in the type-I superconducting film, while travelling of the normal region is induced by mechanically rotating the magnet. The linear three phase winding method is shown on the right of Fig. 3.6 [90,[92][93][94][95]. The linear three phase winding is normally used in a linear motor to generate linearly travelling magnetic poles [96]. ...
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.
... However, for some LTS materials, since the upper critical field H C2 is relatively low, it is possible to create a normal region using the magnetic field. For instance, Chung [90] used the LTS material Nb foil for experiments. Nb is a type-II superconducting material for which the lower critical field µ 0 H C1 is equal to 0.14 T, and the upper critical field µ 0 H C2 is equal to 0.31 T. The upper critical field µ 0 H C2 = 0.31 T can be easily achieved using a magnet, and thereby the flux pump method can be used with this type-II superconducting material with the same mechanism as for a type-I superconducting material. ...
... The magnet generates magnetic flux, which creates the normal region in the type-I superconducting film, while travelling of the normal region is induced by mechanically rotating the magnet. The linear three phase winding method is shown on the right of Fig. 3.6 [90,[92][93][94][95]. The linear three phase winding is normally used in a linear motor to generate linearly travelling magnetic poles [96]. ...
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.
High-temperature superconducting (HTS) magnets have been investigated widely for their higher upper critical magnetic field, larger engineering critical current density and simpler cryogenic system compared with low-temperature superconducting (LTS) magnets. However, in order to keep the permanent-current mode of the HTS magnets, the external power supply is usually employed to charge the magnet via copper current leads, which is a considerable heat source to the cooling system. Thus, in order to avoid the heat disturbance brought by the current leads, a new “through-wall” dynamo-type HTS flux pump using a pair of magnetic couplers is proposed, realizing the truly wireless power transfer, and exploring its possible application for the conduction cooled system. Based on the proposed structure, the heat conduction, which was calculated to be about 7.75 W, and heat convection could be minimized. In addition, to further improve the charging performance of the dynamo-type flux pump, a ferromagnetic (FM) slice was added into different positions of the system. The effect of the FM slice on charging performance was studied numerically and experimentally. According to the results of simulations and experiments, adding a FM slice under the HTS stator improves the saturated current and the charging speed of the dynamo-type flux pump by 20%~30%.
A flux pump is one possible way to reduce the heat load of a superconducting magnet system by replacing mechanical connections with magnetic coupling. However, the heat loss of the flux pump should be analyzed so that it can be applied to conduction-cooled magnets because of the low temperature margin in conduction-cooled systems. In this research, the simplified thermodynamic behavior of a linear-type flux pump system is investigated. The system is assumed to be cooled by a conduction cooling system. The heat generation of a linear type flux pump is calculated based on the AC loss theory. The temperature stability of a thermal link for the flux pumpsystem is analyzed with different design parameters. A design method is suggested for the thermal conductivity of the thermal link in a flux pump of conduction-cooled magnet by summarizing the results of the analysis.
This study introduces over 400 km/h maglev vehicle system based on electromagnetic suspension (EMS) model. Such a maglev system is composed of high temperature superconducting (HTS) magnet, normal conducting magnet and propulsion system. The hybrid electromagnet (EM) consumes little power to keep large suspension gap. The propulsion system realizes to propel the vehicle along the guide-way and assist in braking action. Especially, as the essential point of EMS-based maglev model is to keep the stable suspension force in the high speed operation, the levitation and propulsion forces should be analyzed in order to equilibrate the suspension gap. In this paper, theoretical properties of the levitation force based on electromagnetic analysis are described, as we have been constructed a proto-type electromagnetic suspension (EMS) based maglev vehicle system. Based on the simulation results, we obtained the design parameters for variation suspension gap under high speed propulsion condition.
