Flux Pump Research Articles

A flux pump driven non-soldering closed-loop HTS magnet and its electromagnetic-thermal semi-analytical modelling method

We propose a new flux pump driven non-soldering closed-loop (NSCL) high temperature superconducting (HTS) magnet free of any soldering joints throughout the magnet for persistent current mode (PCM) operation. All the superconducting parts of the magnet are wound directly with HTS closed-loop ring split from an 8 mm REBCO tape using our new cutting scheme free of any soldering requirements. The magnet contains two single-pancake coils connected in series forming a closed circuit through two parallel bridge branches. Two thermal switches set on the two bridge branches control the on–off of the two bridges. A copper coil with iron core installed around one of the bridges is employed as the flux pump to drive the HTS magnet. An electromagnetic-thermal semi-analytical modelling method is proposed to analyse the pumping process by which the transport current in the magnet is calculated. The theoretical limit equation of the saturation current is improved as well. The proposed method can predict the current of the NSCL HTS magnet during the pumping process and provide results that are close to experiments. Experiments verify both the feasibility of the proposed flux pump driven NSCL HTS magnet and the modelling method. The results show that the NSCL HTS magnet works well in the PCM, which provides inspiration to the design of PCM operation of high field HTS magnets. The proposed modelling method also helps guide the design of different forms of HTS magnets and the flux pumps driving them.

In-field electro-magnetic-force characteristics of high-temperature superconducting films containing cracks

Superconducting materials have so far been applied in the fields of superconducting motors, flux pumps and nuclear magnetic resonance, but they are prone to mechanical damage due to their material properties. In this paper, the influences of different crack types (including position, angle, size, quantity and spacing) on the characteristics of current-carrying and excited superconducting thin films are analyzed by numerical simulations, as well as their electromagnetic and mechanical characteristics. Several novel and important conclusions are found. In both states, the boundary crack presents the largest current density concentration, while the central crack produces the largest stress concentration. With the increase of crack spacing, the current density on the multi-crack film decreases and eventually tends to be stable. In the current-carrying state, compared with 3 mm cracked film, the current density of 1 mm cracked film is reduced by up to 62.5 %. The von Mises stress of high-temperature superconducting film with 0.1 mm single-spacing cracks is 22.5 % higher than that for 0.3 mm double-spacing cracks, which indicates that close spacing and a high number of crack types are most detrimental to the mechanical properties of high-temperature superconducting films. In the excited state, when the crack angle θ is 90°, the current density streamlines cannot be segmented effectively, which results in the current distribution trend closer to the defect-free film. A large amount of induced current flows into the low-flux region at the 3 mm crack, leading to an increased risk of film quench. The films with 1 mm single-spacing and double-spacing cracks form a low-flux region between the cracks, which makes the magnetic field more difficult to penetrate.

Characterization of HTS Bifilar Bridge for Self-Regulating Flux Pump: Experimental and Numerical Study

Characterization of HTS Bifilar Bridge for Self-Regulating Flux Pump: Experimental and Numerical Study

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.

Experimental evidence of magnetic flux pumping in ASDEX upgrade

In high-β scenarios with on-axis co-current electron cyclotron current drive, which normally lowers q 0 below unity, the absence of sawteeth suggests the involvement of an additional current redistribution mechanism beyond neoclassical current diffusion. This is supported by imaging motional Stark effect diagnostic measurements, which indicate that q 0 remains consistently around 1. This phenomenon is observed in the presence of a 1/1 mode, indicating its potential role in the current redistribution. It is shown that the mode’s ability to modify the central current and suppress sawteeth increases with plasma pressure. These findings align with a recent theoretical model, which predicts a pressure threshold for sawtooth avoidance by a 1/1 quasi-interchange mode and where this threshold increases with the strength of inward current diffusion. Moreover, the advantages of the flux pumping scenario for future machines are highlighted.

An Integrated Controller for Compact Rectifier-Type HTS Flux Pumps

An Integrated Controller for Compact Rectifier-Type HTS Flux Pumps

Impact of Magnet Number on the DC Output of a Dynamo-Type HTS Flux Pump

Impact of Magnet Number on the DC Output of a Dynamo-Type HTS Flux Pump

Evaluation of the Performance of Commercial High Temperature Superconducting Tapes for Dynamo Flux Pump Applications

High Temperature Superconducting (HTS) dynamo flux pumps are a promising alternative to metal current leads for energization and the persistent current mode operation of high current DC superconducting magnet systems for applications in rotating machines, such as Magnetic Resonance Imaging (MRI) or fusion systems. The viability of the flux pump concept has been widely proven by laboratory experiments and research is now in progress for the design and optimization of flux pump devices for practical applications. It has been widely established that the dependence of the critical current density (Jc) on the temperature (T), the magnetic field magnitude (B), and the orientation (θ), has a substantial impact on the overall DC voltage obtained at the terminals, as well as on the current limit and the loss of the flux pump. Since HTS tapes produced by different manufacturers, they show different dependencies of Jc with the amplitude and the orientation of the magnetic field. They also give rise to different outputs when employed in flux pumps. In this paper, we evaluate and compare the performance of several commercial HTS tapes used for flux pumping purposes through numerical simulation. We also investigate the dependence of the flux pump ‘s performance on the operating temperatures. A 2D finite element numerical model is first developed and validated against experimental data at 77 K. Afterward, the same HTS dynamo apparatus used for validation is exploited for the comparison. The Jc(B,θ,T) and n(B,θ,T) relations, which characterize each different tape in the model, are reconstructed via artificial intelligence techniques based on the open-access database of the Robinson Research Institute. It is shown in the paper that certain tapes are more suitable than others for flux pump applications and that the best overall operating temperature is in the vicinity of 77 K.

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.

Time-dependent development of dynamic resistance voltage of superconducting tape considering heat accumulation

In flux pumps, motors and superconducting magnets, the high temperature superconductor (HTS) coated conductor frequently carries a DC transport current when an oscillating magnetic field is present in the background. Under this circumstance, the interesting effect of dynamic resistance takes place, which can affect the operating performance of superconducting devices: heat accumulation can contribute to the rising temperature of the HTS tape and the dynamic resistance voltage can change accordingly. This article explores the time-dependent development of the dynamic resistance voltage using a numerical modeling considering the thermal effects. After a validation against experimental results, this work investigates the effects of several factors on the structure of the HTS tape on the time-dependent development of the dynamic resistance, thus providing insights toward a better understanding of the time-dependent behavior of HTS tapes under external magnetic fields.

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.

Charging process simulation of a coil by a self-regulating high-T superconducting flux pump

Charging process simulation of a coil by a self-regulating high-T superconducting flux pump

Magnetic Conduction Structure Design and Charging Test of a Linear-Motor Type Flux Pump for Through-Dewar Excitation of High Temperature Superconducting Motors

High temperature superconductivity (HTS) flux pump can be used as the exciter to inject direct current into the superconducting excitation coil of a synchronous motor, without current lead and with low thermal efficiency, which will reduce the cryogenic volume and improve the efficiency of HTS synchronous motor. However, the flux pump has non-negligible thermal impact when it is running continuously for a long time within the cryogenic Dewar, which will cause a heavy heat burden to the cryogenic system. Therefore, based on the linear-motor type flux pump previously invented in our laboratory, this paper improved its magnetic circuit, so that it can introduce the travelling magnetic wave into the Dewar from outside. Firstly, we have used COMSOL Multiphysics to simulate and optimize the waveform of this flux pump. Secondly, we have manufactured the flux pump and magnetic conduction structure, and have tested its performance in liquid nitrogen at 77 K, a pumped DC current of 216 A was measured with a single 12 mm wide YBCO stator. These work have further optimized the linear-motor type flux pump, which provide a contactless static excitation device (CSED) for the 16.9 kW HTS synchronous motor built in our laboratory.

Through-Wall Excitation of a Conduction Cooling HTS Magnets by a Linear-Motor Type Flux Pump

The flux pumps can realize non-contact excitation of high-temperature superconducting (HTS) magnet and eliminate heat leakage caused by current leads. However, earlier proposed flux pump needs to be installed inside the cryostat, which leads to additional heat load to the cryogenic system. Here, we report the successful demonstration of a through-wall linear-motor type flux pump, with its heating parts such as the copper windings completely outside the cryostat, while its generated heat is dissipated in the air and insulated from the cryostat. With an insulation HTS double pancake coil installed in the cryostat and cooled by a G-M cryogenic cooler at 32.6 K, we have demonstrated the injection of direct currents of 42 A into the closed-loop, without the current leads. Thus, the feasibility and reliability of using the linear-motor type flux pump as the power source to excite the HTS through the cryostat is verified, making it possible for future applications such as in MRI and superconducting motors, etc.

A Numerical Design of High-Resistance and Energy-Efficient HTS Switch Based on Dynamic Resistance

Dynamic resistance effect introduces dissipative loss to the type-II high <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">T</i> c superconductors, which is initially considered as a disadvantage for superconductors when it was first discovered. However, in some cases “dynamic resistance”, a unique effect, can be effectively exploited. Based on the dynamic resistance effect, the AC-magnetic-field-actuated high temperature superconducting (HTS) power switch is developed. It is the key component in the HTS transformer-rectifier flux pump (TRFP). The operating characteristics of power switch are vital to the performance and reliability of the transformer rectifier flux pump. In this paper, we compare three designs of HTS power switch with different stabilizer structures: striated stabilizer layers with wide and narrow strips as well as original non-striated stabilizer layers. The performance of HTS power switch is evaluated by several criteria, including off-state voltage and resistance, temperature rise, power loss, and energy conversion efficiency. This study will help design high-performance, low-loss, and energy-efficient HTS power switch based on “dynamic resistance” effect.

Temperature Dependent Performance of a Conduction-Cooled Jc(B) Transformer- Rectifier Flux Pump

Transformer-rectifier flux pumps provide an elegant solution for high current generation ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$&gt;$</tex-math></inline-formula> 1 kA) in superconducting magnets. By using step-down transformers and superconducting switches they can charge magnets with minimal heat-load between the external and cryogenic components. High temperature superconductors' type-II superconductivity enables a range of different switching approaches. It was recently shown that a switching approach based on reducing the critical current with an applied DC magnetic field - <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">J_c(B)</i> switching - is feasible. The efficiencies of this switching method are an attractive option for a conduction-cooled system where cooling power is limited. In this work we present the first conduction-cooled transformer-rectifier driven by <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">J_c(B)</i> switching where we successfully charge a magnet up to its critical current of 115 A whilst monitoring various component temperatures. We explore the parameter space of applied current and switch temperature to establish the effects on the charging time and maximum current.

Power Optimization of 50 GHz ERSFQ Circuits

The Energy-efficient Rapid Single Flux Quantum (ERSFQ) logic family offers zero-static power dissipation. Any ERSFQ circuit requires a feeding Josephson transmission line (FJTL), a relatively large active transmission line, which serves as an accurate voltage source. For proper operation, the FJTL needs to be continuously pumped at a frequency that is equal to or higher than the highest clock frequency of the ERSFQ circuit. We selected an on-chip pseudo-random binary sequence (PRBS) generator as the example circuit since it naturally facilitates quantitative measurements of operating bias margins in terms of bit error rate (BER). Using external and internal flux pump sources, we performed extensive experimental studies on BER for ERSFQ PRBS generators operating at clock frequencies up to 50 GHz. We paid special attention to the over-pumping regime, where the FJTL pump frequency is higher than the PRBS clock frequency and confirmed its superiority in improving BER. At 35.56 GHz clock frequency, a group of 4 PRBS generators and 4 individual FJTLs with the bias current ratio (BCR) equal to 1, had a BER lower than 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-12</sup> and bias margins of ±25% for internal and external flux pump sources over-pumping at 6.25% above clock frequency. The maximal operational clock frequency of a single ERSFQ PRBS generator with intrinsic pump source (+6.25% over-pumping) and BER <10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-12</sup> was 49.53 GHz. We also focused on optimization of the FJTL's size to reduce the power consumption and the area occupied by the FJTL itself. All chips were fabricated at MIT-Lincoln Laboratory using the SFQ5ee fab node. Future steps required for better understanding of ERSFQ operation are discussed.

Dynamic resistance and voltage response of a REBCO bifilar stack under perpendicular DC-biased AC magnetic fields

The dynamic resistance of REBCO (REBa2Cu3O7-d, RE stands for rare earth), coated conductors (CCs) is a key parameter in many high-temperature superconductor applications where CCs carry DC currents exposed to AC and DC magnetic fields, such as field-triggered persistent current switches, flux pumps, and fault current limiters. In this work, dynamic resistance and dynamic voltage have been studied via experiments and finite element method (FEM) simulations in a REBCO bifilar stack at 77 K, under combined AC and DC magnetic fields with different magnitudes, frequencies, and waveforms. Our results show some distinct features of dynamic resistance and voltage from those under pure AC magnetic fields. With an increasing DC magnetic field, the dynamic resistance exhibits an obvious linearity with the applied AC magnetic field, and becomes less dependent on the AC field frequency. The fundamental frequency of the dynamic voltage under a DC magnetic field becomes the same as that of the applied AC field, which completely differs from the pure AC field case where the fundamental frequency doubles. For the first time, instantaneous threshold field (B th) values are obtained from the dynamic voltage, which are substantially different in the field-increasing and field-decreasing processes. These key differences are attributed to the dominant role of DC magnetic fields in determining the critical current of the superconductor, which significantly dwarfs the influence of AC fields. These new discoveries may help researchers better understand the electromagnetism of superconductors and be useful for relevant applications.