High-temperature Superconducting Flux Pump Research Articles

Modeling methodology for the transformer-rectifier flux pump considering electromagnetic and thermal coupling

High-temperature superconducting (HTS) magnets are promising in the application of high-intensity magnetic field. HTS flux pumps are devices that can charge closed HTS magnets without direct electrical contact. Simulation is an effective way to clarify the physical mechanism and provide further insight into the design of the device. In this work, we propose an accurate and efficient modeling methodology to simulate the transformer-rectifier HTS flux pump, which has considered electromagnetic and thermal coupling. The validity of the model has been verified by experimental results and theoretical calculations. The working characteristics of the HTS flux pump are investigated based on the proposed model, including DC bias component in the charging loop, the voltage recovery delay of the dynamic bridge and the temperature distribution in the dynamic bridge. The simulation results clearly depict working details of the device, in terms of electricity, magnetism and heat. The proposed model can serve as a powerful tool to design the HTS flux pump in practical applications.

Temperature dependent behavior of a kA-class superconducting flux pump with a continuous cylindrical stator

A high temperature superconducting (HTS) dynamo is a type of device known as a “flux pump” that can inject DC into a closed superconducting circuit. Here, we report experimental results from a variable-temperature dynamo-type HTS flux pump operated within a cryo-cooled chamber. This device employs a “continuous stator” topology, whereby an HTS “coated conductor” is wrapped to form a cylinder around a mechanical rotor such that applied flux from the rotor magnet must always penetrate the stator. This leads to a high current device that can inject >1 kA into a series-connected HTS coil at 53 K. The open-circuit DC output voltage (Voc) from this HTS dynamo has been studied at stator temperatures between 35 and 95 K and attained a maxima at a temperature ∼5 K lower than the stator Tc. At lower temperatures, Voc decreases and falls to zero below ∼40 K. This non-intuitive effect is found to be due to flux-screening by critical currents flowing with the HTS stator, which increase with decreasing temperature. These shielding currents prevent flux from penetrating the HTS stator and, hence, reduce the magnitude of locally induced emf (and thus DC output) within the HTS film. A key implication of these results is that all magnetically driven HTS flux pumps should be operated at temperatures well above their flux-screening point, and this consideration must be taken into account for future designs of multi-kA class HTS flux pumps.

MOSFET-based HTS flux pump

We have developed a high-temperature superconducting (HTS) flux pump using high-power metal-oxide-semiconductor field-effect transistors (MOSFETs) for switching. For its primary coil, two commercial transformers are utilized, each consisting of two copper coils wound on a single iron core, which enables changing the load current over primary current ratio (101 or 194) between the primary and secondary coils. Full-wave rectification is achieved with two half-wave secondary circuits, each of them having six HTS one-turn coils to lower the resistance. Each secondary coil is composed out of nickel-reinforced BSCCO tapes, where 12 MOSFETs have been soldered in parallel straight to the tapes and controlled with analog electronics. Secondary coils are clamped to custom-made copper-stabilized HTS current leads. A support structure for keeping the HTS coils in place was 3D-printed using cryogenic-compatible composite material PETG-CF20. Resistances of the two secondary circuits were measured to be 4 and 7 at 77 K with a total critical current of 980 A. We successfully ramped up a 50 µH Conductor on Round Core solenoid at 77 K using our HTS flux pump with 50 Hz AC voltage source. We achieved a maximal load current of 900 A and exceeded the 715 A critical current of the solenoid. During the thermal runaway of the magnet, the increased load voltage limits the maximum load current supplied by the flux pump.

Characterization of flux pump-charging of high-temperature superconducting coils using coupled numerical models

Flux pumps provide a promising solution for contactless charging of high-temperature superconducting (HTS) coils, eliminating the need for bulk current leads and reducing the heat burden for the cryogenic system. Characterizing the nonlinear effects of an HTS coil charged by a flux pump and understanding the dynamics of the charging process is essential for promoting the practical application of flux pumps. Numerical models provide a fast and cost-effective way of achieving this. In this study, we propose a methodology for coupling HTS coil and flux pump models using an electrical circuit, resulting in reduced computation costs. We validate the effectiveness of our approach against the experimental results of an HTS coil charged by a dynamo-type flux pump. Specifically, we obtain the voltage produced by the HTS dynamo using a 3D model based on the minimum electromagnetic entropy production method and apply this voltage to the load HTS coil using a formulation finite-element method model coupled via an electrical circuit. The simulated charging current shows good agreement with experimental observations, validating our modeling strategy. The results demonstrate that the flux flow state in the HTS coil is the primary factor limiting the charging performance of the HTS dynamo as the charging current approaches the coil’s critical current. Furthermore, based on the simulation, we demonstrate that, when using flux pumps, it is advisable to leave a margin between the operating current and the critical current of the coil. Overall, our approach has the potential to be applied to HTS coils charged by any device.

A full-wave HTS flux pump using a feedback control system

Transformer-rectifier flux pumps are DC superconducting power supplies capable of charging superconducting magnets to high currents and stored magnetic energies. Here, we demonstrate a full-wave superconducting flux pump assembled from high-temperature superconducting (HTS) wire that utilizes superconducting switches controlled by applied magnetic field. A negative DC offset occurs in the superconducting secondary of the circuit during operation which is related to the output load current. A feedback control system is proposed and demonstrated to account for the negative DC offset. Increasing the primary current proportional to the load current during operation allowed for the maximum output of the flux pump to be increased from 35 A to more than 275 A. These results are reproduced using a coupled electrical- and magnetic–circuit model formulated in the MATLAB Simulink® package.

A statistical model for the design of rotary HTS flux pumps based on deep-learning neuron network

Rotory high temperature superconducting (HTS) flux pumps can consistently generate a DC voltage by rotating magnets over superconducting tapes, and thus energize the circuit if a closed loop is formed. The voltage output is a crucial factor to reflect the performance of such an HTS flux pump, which is determined by a set of design specifications, and some of them have been investigated extensively in the current literature. However, no work has been done yet to study the HTS dynamo output voltage by efficiently integrating all the design parameters together. In this paper, a well-trained deep-learning neuron network (DNN) with back-propagation algorithms has been put forward and validated. The proposed DNN is capable of quantifying the output voltage of an HTS dynamo instantly with an overall accuracy of approximately 98% with respect to the simulated values with all design parameters explicitly specified. The model possesses a powerful ability to characterize the output behavior of HTS dynamos by considering multiple design parameters, e.g., airgap, superconductor tape width, operating frequency, remanent flux density, rotor radius, and permanent magnet width, which have covered all the typical design considerations. The output characteristics of an HTS dynamo against each of the design parameters have been successfully demonstrated using this model. Compared to conventional time-consuming finite element method (FEM) based numerical models, the proposed DNN model has the advantages of automatic learning, fast computation, as well as strong programmability. Therefore the DNN model can greatly facilitate the design and optimization process for HTS dynamos. An executable application has been developed accordingly based on the DNN model, which is believed to provide a useful tool for learners and designers of HTS dynamos.

Review of high temperature superconducting flux pumps

High temperature superconducting (HTS) magnets conduct DC currents ranging from hundreds to tens of thousands of amperes. To achieve such DC output amplitudes, conventional power supplies are unsuitable, owing to their extreme cost, energy consumption, and bulkiness. The indispensable current leads of conventional power supplies carrying large DC current cause an extra heat leakage into the cryogenic system, thus increasing the number of required cryocoolers. A potential solution to tackle this problem, however, is to use HTS flux pumps that inject a large amount of DC current into the HTS magnet in a wireless fashion, thereby eliminating the need for current leads, and allow the magnets to work in the quasi-persistent current mode. Compared with the conventional power supplies, the flux pumps offer the advantages of low cost, low energy consumption, and compact size, etc., which essentially have broad application prospects in nuclear magnetic resonance (NMR/MRI), fusion, particle accelerators, superconducting electric machine, maglev train, etc. Over the last decade, a variety of HTS flux pumps have been invented with improved DC outputs, reaching over kilo-amperes. Moreover, those flux pumps have different working principles, structures and operation strategies. In this paper, we provide an in-depth review on the HTS flux pumps developed in the last decade. In particular, for the HTS travelling wave flux pumps and HTS transformer-rectifier flux pumps, the discussions are focused on their working principles and technical advances. In the end, we discuss the present applications of HTS flux pumps, along with their potential future applications.

The Performance Test of an Updated Linear-Motor Flux Pump With Single-Sided DC Winding

In order to further optimize the size of the linear-motor type flux pump, an updated linear-motor type motor flux pump device with single side DC winding was designed and fabricated. Compared with the previous generation of linear-motor type flux pump with traveling wave length of 21 mm, the volume of this generation of linear-motor type flux pump with traveling wave length of 15 mm is reduced by nearly 30% and the pumping current capacity is increased by more than 30%, which generated the same DC biased AC travelling magnetic wave in the airgap, and pumped DC current into the high temperature superconducting (HTS) closed-loop in a non-contacting manner, based on the macroscopic magnetic coupling effect. In this work, we tested the performances of this updated flux pump device. We pumped current into two parallel superconducting stators and a YBCO double pancake coils. The pumping current of two parallel superconducting stators reached 219 A, while pumped current in YBCO double cake coil (DPC) reached 73 A, mainly due to the limit of the small critical current of the HTS DPC. The experimental results show that using one-sided DC coil can achieve effective bias effect, which further verified the rationality of explanation based on the macroscopic magnetic coupling effect. These results are helpful to optimize the HTS flux pump device and further understand the working mechanism of the flux pump.

A Novel Switch Design for Compact HTS Flux Pump

High temperature superconducting (HTS) magnets are becoming more and more attractive for many commercial applications. HTS magnets, however, need an external power supply to operate in persistent current mode. One way to charge an HTS magnet is using a conventional power supply. This, however, is not the most efficient way to charge an HTS magnet due to losses from the current leads. Flux pumping is an alternative method to charge an HTS magnet. Over the years different types of flux pumps have been proposed specific to different applications. Transformer-rectifier flux pump is one of the many types of flux pumps that have been researched widely. The design of switches plays a central role in a transformer-rectifier flux pump. In this paper, we propose a novel switch design for a half-wave transformer-rectifier flux pump. The switch proposed in this study will be used in compact and mobile HTS Magnetic Resonance Imaging (MRI) instruments.

Thermal Behavior Modelling of a Fast AC Field Controlled HTS Switch

When a type II superconductor carrying a direct current is subjected to a perpendicular AC magnetic field, a DC voltage can be observed, commonly referred to as “dynamic resistance effect”. This dc voltage appears in a very short time when AC field is applied and disappears quickly when AC field is removed. These advantages make the type II superconductors to be a fast AC-field-actuated HTS switch, which plays an important role in many potential high-temperature superconducting (HTS) applications, especially for HTS flux pumps or HTS magnetic energy storage systems. However, the temperature in the superconductor varies due to this dc voltage and may result in quench for the superconductor. Therefore, it is important to investigate the thermal behavior of an AC-field-actuated HTS switch. In this paper, we build and experimentally verified a 2D electromagnetic-thermal coupled model to investigate the electromagnetic-thermal characteristics of HTS switches in different working conditions. This model can estimate the dynamic voltage rise and the temperature rise of the superconductor. This work is indicative for designing an AC-field-actuated HTS switch in terms of increasing the dc voltage, and enhancing the safety and stability.

Design of a 60 kA Flux Pump for Fusion Toroidal Field Coils

High-temperature superconducting (HTS) magnets offer a novel pathway to magnetically confined fusion energy generation. Reactor-scale fusion devices require tens of kA of current to generate large toroidal fields for magnetic confinement. Operating HTS magnets at kA-current levels is difficult due to unavoidable joint resistances that require active driving of current. The large footprint and energy demand of a solid-state power supply and multi-kA current leads present significant constraints on the size, cost and efficiency of a fusion tokamak. An alternative approach is to use an HTS flux pump, which can generate and sustain persistent currents in HTS coils at the kA-levels required by fusion magnet systems. Here, we outline the design of a 60 kA transformer-rectifier flux pump for energizing and sustaining an 80 mH HTS fusion magnet. The flux pump is driven by an iron-core transformer with copper primary and HTS secondary windings. This is rectified by switching elements in the superconducting circuit into a uni-directional dc voltage output which charges the inductive load. To charge the load coil to 60 kA in 24 hours, an output dc voltage of 55 mV is needed. However, only 6 mV is required to maintain a long-term steady current of 60 kA. Such a device can significantly reduce the cost and footprint of future fusion reactor designs.

High Temperature Superconducting Flux Pumps for Contactless Energization

The development of superconducting technology has seen continuously increasing interest, especially in the area of clean power systems and electrification of transport with low CO2 emission. Electric machines, as the major producer and consumer of the global electrical energy, have played a critical role in achieving zero carbon emission. The superior current carrying capacity of superconductors with zero DC loss opens the way to the next-generation electric machines characterized by much higher efficiency and power density compared to conventional machines. The persistent current mode is the optimal working condition for a superconducting magnet, and thus the energization of superconducting field windings has become a crucial challenge to be tackled, to which high temperature superconducting (HTS) flux pumps have been proposed as a promising solution. An HTS flux pump enables current injection into a closed superconducting coil wirelessly and provides continuous compensation to offset current decay, avoiding excessive cryogenic losses and sophisticated power electronics facilities. Despite many publications regarding the design and analyses of various types of HTS flux pumps, the practical application of HTS flux pumps in a high-performance superconducting machine has been rarely reported. Therefore, it is of significance to specify the main challenges for building and implementing a reliable HTS flux pump. In addition, the physical mechanisms of distinct HTS flux pumps have caused some confusion, which should be clarified. Above all, a systematic review of the recent development and progress of HTS flux pumps remains lacking. Given the above-mentioned issues, this paper summarized the most up-to-date advances of this emerging technology, clarified the working mechanisms and commonly adopted modeling approaches, presented objective analyses of the applicability of various HTS flux pumps, specified the primary challenges for implementing HTS flux pumps, and proposed useful suggestions to improve this wireless excitation technology. The overall aim of this work is to bring a deep insight into the understanding of HTS flux pumps and provide comprehensive guidance for their future research and applications.

Modeling and characteristics investigation of self-regulating HTS flux pump

Self-regulating high temperature superconducting (HTS) flux pump is a promising candidate for realizing the quasi-persistent current mode operation of closed-loop HTS magnets. To characterize its performance and optimize accordingly, in this work, we have established a model in Matlab/Simulink. The effectiveness of the model has been validated based on the data obtained from the previous experimental counterpart. The calculated results show that this model is able to reproduce the experimental results with a satisfactory agreement. Based on the exploration of the model, we found that the charging performance of self-regulating HTS flux pump is mainly dependent on a range of typical design considerations, including the length of HTS bridge, the applied frequency of power source, the coupling coefficient of magnetic couplers, as well as the scale of HTS coils. Additionally, we proved that the charging speed can be accelerated and the saturated load current will correspondingly increase with the appropriate system parameters. The results obtained by this work provide some useful references for further optimization design of self-regulating HTS flux pump devices.

Influence of ferromagnetic slice on the charging performance of a through-wall HTS flux pump employing a magnetic coupler

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 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 at different positions of the system. The effect of the FM slice on charging performance is studied numerically and experimentally. According to the results of simulations and experiments, adding an FM slice under the HTS stator improves the saturated current and the charging speed of the dynamo-type flux pump by 20%–30%.

A closed-loop high temperature superconducting magnet energized by flux pump and its piecewise semi-analytical solution

High temperature superconducting (HTS) magnets energized by flux pumps can carry heavy current without heat leakage from current leads and are therefore appropriate for a wide range of applications. However, the soldering resistance remains an obstacle to the persistent operations of the HTS magnets. Here, we propose a closed-loop HTS magnet magnetized by flux pump with thermal switches. The magnet consists of a closed-loop single pancakes (SPs) coil wound by a rare-earth barium copper oxide (REBCO) tape on which a slit is cut to form a closed-loop ring within which current can flow without encountering the soldering resistance. Another REBCO tape is soldered on the closed-loop ring to form a pump bridge and a flux pump consists of a copper coil with iron core is installed around the pump bridge. There exists a group of manganese copper wires wound on the closed-loop ring and the pump bridge respectively as thermal switches. A piecewise semi-analytical solution is suggested to investigate the charging process by which the transport current in the magnet is calculated. Experiments are carried out to verify the feasibility of the magnet and the solution. The results show that the magnet can be operated in a persistent current mode, and thus, this study represents a practical solution for persistent operation of the HTS magnets. Besides, the proposed solution can effectively explain and predict the saturation current of the suggested magnet, which can, therefore, guide the design of other forms of HTS magnets and flux pumps.

A half-wave superconducting transformer-rectifier flux pump using J c(B) switches

High temperature superconducting (HTS) flux pumps can generate ultra-high currents (>1 kA) without the requirement for thermally inefficient room temperature current leads. Ultra-high currents enable physically smaller magnets with significantly less inductance unlocking new design opportunities. However, limited by intrinsically low electrical power efficiencies, existing HTS flux pumps cannot output high voltage or high power. In this work, we design, devise, and experimentally verify a transformer–rectifier type HTS flux pump using Jc(B) switching. We show that the rectification can be achieved by exploiting the HTS E-J relation with the application of DC magnetic fields. A quasi-persistent current of 54.5 A has been achieved at 77 K only limited by the load coil critical current. In addition, the electrical power efficiencies of both half-wave and full-wave flux pump are derived. We illustrate that the fundamental J c(B) mechanism provides significantly higher efficiency than existing HTS flux pumps. This advancement will overthrow the common knowledge that HTS flux pumps could only be used for maintaining rather than fast ramping magnetic fields.

Sensitivity analysis and machine learning modelling for the output characteristics of rotary HTS flux pumps

High temperature superconducting (HTS) rotatory flux pump, or so called HTS dynamo, can output none-zero time-averaged DC voltage and charge the rest of the circuit if a closed loop has been formed. This type of flux pump is often employed together with HTS coils, where the HTS coils can potentially work in persistent current mode, and act like electromagnets with considerable magnetic field, having wide range of applications in industry. The output characteristics of HTS rotary flux pumps have been extensively explored through experiments and finite element method (FEM) simulations, yet the work on constructing statistical models as an alternative approach to capture key characteristics has not been studied and published. A 2D FEM program has been used to model the HTS rotatory flux pumps and evaluate the effects of different factors upon the output voltage through parameter sweeping and analysis of variance. Typical design considerations, including operation frequency, air gap, HTS tape width and remanent flux density have been investigated, in particular the bilateral effect of HTS tape width has been explained by looking at the averaged integration of the electric field over the tape. Based on the data obtained from various simulations, regression analysis has been conducted through a collection of machine learning methods and demonstrated that the output voltage of a rotary flux pump can be obtained promptly with satisfactory accuracy via Gaussian process regression, aiming to provide a novel approach for future research and powerful design tool for industrial applications using HTS rotary flux pump devices.

A Linear-Motor Type HTS Flux Pump With Pumped Current Exceeding 640 A

In this paper, we present the fabrication and performance of a linear-motor type flux pump with a pumped current larger than 640 A. This high-temperature superconducting (HTS) flux pump can be used for magnetic resonance imaging (MRI) magnets and HTS electric machines. We use the flux pump to charge 10 superconducting closed-loops, and show the tested results of pumping current under different applied frequencies and magnetic field amplitudes. This flux pump device can output direct current larger than 640 A at 77 K. The output of this flux pump equals to a DC power source with low output voltage and high output current. Comparing with the traditional power supply of the same rated current, the size of this HTS flux pump is much smaller, while it also eliminates the harmful metal current leads that cause heat leakage. We show that, by increasing the length of the travelling wave region of this flux pump, there is no limit to the output of the current output, which can reasonably reach the kilo-ampere level. The purpose of this paper is to test the reliability of the methodology for the large current HTS flux pump.

3D modeling of a superconducting dynamo-type flux pump

High temperature superconducting (HTS) dynamos are promising devices that can inject large DC currents into the winding of superconducting machines or magnets in a contactless way. Thanks to this, troublesome brushes in HTS machines or bulky currents leads with high thermal losses will be no longer required. The working mechanism of HTS dynamo has been controversial during the recent years and several explanations and models have been proposed to elucidate its performance. In this paper, we present the first three-dimensional (3D) model of an HTS flux pump, which has good agreement with experiments. This model can be beneficial to clarify the mechanism of the dynamo and pinpoint its unnoticed characteristics. Employing this model, we delved into the screening current and electric field distribution across the tape surface in several crucial time steps. This is important, since the overcritical screening current has been shown to be the reason for flux pumping. In addition, we analyzed the impact of both components of electric field and screening current on voltage generation, which was not possible in previous 2D models. We also explored the necessary distance of voltage taps at different airgaps for precise measurement of the voltage across the tape in the dynamo.

Performance Investigation of Contactless Self-Regulating HTS Flux Pump

Self-regulating high temperature superconducting (HTS) flux pump is one of the most effective methods to compensate the current degradation of a closed HTS magnet for its stable operation. In this study, to understand the working principle as well as the influential factors regarding to its operation performance, a basic circuit model has been established. Based on that, we found the saturation load current is strongly related to the added secondary resistance, which could offset the DC bias current induced in the charging loop. Then, we built a prototype experimentally, in which, part of the load coil was used as a HTS bridge and bifilar bridge design has been adopted to minimize the possible flux fluctuation in the load loop. In the experiment, the dependence of the charging performance on the bridge length as well as the added secondary resistance at different frequencies has been clarified. The results show that, the load current is remarkably impacted by the aforementioned parameters. Specifically speaking, with an increasing added secondary resistance, the operating frequency needs to be higher to ensure the secondary current can follow the primary current, which is crucial for maintaining a larger load current.