DC Transport Current Research Articles

A superconducting switch using induced current in the closed loop circuit

Superconducting magnets have been applied in many fields because of the high magnetic field and the low loss. In order to energize superconducting magnets and operate them in the persistent current mode, the superconducting switch is essential. Recently, a superconducting switch using the interaction between the DC transport current and induced current in a closed loop circuit has been proposed, which cannot only respond fast, but also operate efficiently. In this work, an analytical model based on equivalent circuit is proposed to explain the working principle of this type of superconducting switch. An experimental protype is built to investigate its electromagnetic behaviors and working performance. Impacting factors of the voltage across the superconducting switch have been clarified based on experimental results. The method based on asymmetric AC magnetic field is proposed to enhance the average voltage across the superconducting switch and its performance has been verified through experimental tests. Besides, the advantages and application prospects of the superconducting switch are discussed.

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.

Dynamic resistance loss of the high temperature superconducting coil for superconducting magnetic energy storage

The Superconducting Magnetic Energy Storage (SMES) has excellent performance in energy storage capacity, response speed and service time. Although it's typically unavoidable, SMES systems often have to carry DC transport current while being subjected to the external AC magnetic fields.Under this circumstance, the dynamic resistance loss occurs in conjunction with the magnetization loss. Accordingly, the dissipative heat originating from the dynamic resistance loss will cause thermal overhead and even threaten the stability of the SMES. To ensure the safe operation of the SMES, this study examines dynamic resistance losses of high temperature superconducting (HTS) coils. In addition, this study also compares it with other losses, including magnetization loss, under various conditions such as different DC transport currents, amplitudes, frequencies, and angles of external AC magnetic fields. Furthermore, the analysis of dynamic resistance loss has also taken into account thermal coupling. This research offers valuable insights into the electromagnetic-thermal characteristics of the HTS coil and can be used as a reference for designing SMES systems.

Effect of a DC transport current on the AC loss in no-insulation ReBCO racetrack coils exposed to AC parallel magnetic field at 77 K and 4.2 K

ReBCO coils are developed as DC field coils in linear motor systems to increase the force density, in favor of permanent magnets. Such coils have to sustain a relatively large heat load stemming from the AC magnetic field environment in which they operate. The use of no or partial turn-to-turn insulation can make them more stable against the effects of local heating. Conversely, the radial electrical connections in no-insulation (NI) coils allow for large coupling currents, causing additional AC loss on top of the already significant heat load. Here we report on the AC loss in sub-scale NI, 4 mm wide single-tape, ReBCO racetrack coils exposed to parallel-to-the-tape magnetic field in the frequency range of 10−4 to 1 Hz at 77 K and 4.2 K, while carrying a DC transport current. AC loss is measured magnetically and electrically. The main goal of these experiments is to validate our 2D numerical model, which provides more insight into the origin of the AC loss. At low frequencies, inter-turn coupling currents are spread more or less homogeneously throughout the winding pack. Whereas at high frequencies, the skin effect causes shielding of the interior of the coil and large induced currents only occupy the coil’s outer surface.

Optimisation of Energy Efficiency: Dynamic Voltages in Superconducting Tapes to Energise Superconducting Power/Energy Applications

The evolution from low-temperature superconductors (LTSs) to high-temperature superconductors (HTSs) has created a great amount of opportunities for superconducting applications to be used in real life. Dynamic voltage is a special superconducting phenomenon, and it occurs when the superconductor takes a DC transport current while simultaneously exposed to an AC magnetic field. The dynamic voltage is crucial for some superconducting applications as it is the energy source by which to energise the load, such as flux pumps. This article investigates the missing aspects that previous studies have not deeply exploited: the optimisation of energy efficiency for the dynamic voltage in an HTS tape with different working conditions (e.g., currents and magnetic fields). First, the mechanics of superconducting dynamic voltage were explicated by typical analytical solutions, and the modelling method was validated by reproducing the behaviours of the Bean model and analytical solutions of dynamic voltage. After the feasibility of the modelling was proved, in-depth modelling was performed to optimise the energy efficiency of an HTS tape with different DC transport currents and AC magnetic fields. Owing to the physical limitations of the superconducting tape (e.g., quench), a safe operating region was determined, and a more delicate optimisation was performed to discover the optimal operating conditions of the HTS tape. The novel conceptualisation and optimisation approaches for the superconducting dynamic voltage in this article are beneficial for the future design and optimisation of superconducting energy/power applications under complicated electromagnetic conditions.

Numerical Modelling of the Dynamic Voltage in HTS Materials under the Action of DC Transport Currents and Different Oscillating Magnetic Fields.

The dynamic voltage is a unique phenomenon of superconducting materials. It arises when the superconductor is carrying a DC transport current and spontaneously in subject to an AC magnetic field. This study excavates the aspects that previous studies have not comprehensively investigated: the dynamic voltage in a DC-carrying superconducting tape exposed to different oscillating AC magnetic fields. First, the fundamental physics of dynamic voltage/flux of superconductors is reviewed and further analysed in detail. We used the superconducting modelling method using the H-formulation merged into the finite-element method (FEM) software, to re-produce the typical dynamic voltage behaviour of a superconducting tape. The modelling was verified by both the analytical and experimental results, in order to precisely prove the reliability of the modelling. Afterwards, the modelling was performed for a DC-carrying superconducting tape under four different oscillating magnetic fields (sine, triangle, sawtooth and square), and their corresponding dynamic voltages and energy losses were analysed and compared. Results show the sinusoidal magnetic field can lead to the optimal combination of reasonable dynamic voltage but relatively lower loss, which is suitable for those superconducting applications requiring dynamic voltage as the energy source, e.g., flux pumps. This article presents novel investigation and analysis of the dynamic voltage in superconducting materials, and both the methodology and results can provide useful information for the future design/analysis of superconducting applications with DC transport currents and AC magnetic fields.

Time-varying magnetic field induced electric field across a current-transporting type-II superconducting loop: beyond dynamic resistance effect

The emergence of a potential drop across a current-transporting type-II superconducting loop under a perpendicular oscillating magnetic field is revealed. We have derived analytical formulae to describe the effect under DC transport current in 1D, based on Bean’s critical state model. The analytical formulae are verified by a finite element model. To exploit this effect, we have developed a transformer-like ‘resistive switch’, and experimentally observed a switching effect. This work demonstrates a physically important general insight of the interaction between DC transport currents and time-varying magnetic fields in type-II superconducting loops, which extends beyond the well-known ‘dynamic resistance’ effect. It also provides a useful view on the interaction between a ‘transport-current’ and a ‘screening-current’ in the superconductor. The resulting demonstrated switch has the potential to be used in a variety of applications including superconducting rectifiers, fault current limiters, and superconducting magnetic energy storages.

Intrinsic Influence of Dynamic Resistance on Current Distribution of Two HTS Parallel Branches Without Terminal Contact Resistance

If a superconductor carries DC transport current and exposes to an alternating current (AC) magnetic field whose amplitude is more than its threshold field, a DC resistance so called dynamic resistance occurs on the superconductor because of the dissipative interaction between the DC transport current and the external magnetic field, which possibly appears in many kinds of high temperature superconductor (HTS) apparatus, such as linear synchronous motors. Occurrence of the dynamic resistance significantly affects the current distribution of the HTS components connected in parallel such as cables and conductors except for increasing the AC loses. In this paper, we employ a partially slit second-generation (2G) HTS tape with two branches to study the influence of dynamic resistance on current distribution such that the terminal contact resistances can be avoided. In experiment, DC transport current is applied to the whole 2G HTS tape and branch 1# is only exposed to an applied AC magnetic field. In addition, we use an analytical formulation of the dynamic resistance of branch 1# based on Bean critical state model and voltage calculation of branch 2# relies on E-I curve to build analytical model by using Newton Iteration. It is shown that DC current value of branch 1# decreases and branch 2# increases with the increasing of the dynamic resistance of branch 1#. The results are important for HTS DC devices with carrying ripple current or exposed to AC magnetic field.

The instantaneous dynamic resistance voltage of DC-carrying REBCO tapes to AC magnetic field

When a REBCO tape carrying DC transport current is exposed to a perpendicular AC magnetic field with amplitude above certain value, flux could be triggered to traversing the superconducting tape. During the process, a biased DC response voltage, called the dynamic resistance voltage will be induced. In the paper, the calculation on the time-dependent dynamic resistance voltage was carried out by the H-formulation model implemented in the finite-element method (FEM) software package COMSOL Multiphysics. Fourier analysis on the waveform of dynamic resistance voltage was utilized to figure out the main components of the response voltage. The DC component, second harmonic component and ratio of the two components can reflect how the instantaneous dynamic resistance responds to the applied AC magnetic field and the DC transport current. The findings in the paper can be used to monitor the state of DC-carrying REBCO tapes subject to an AC magnetic field.

AC Losses of Quasi-Isotropic Conductor Carrying a DC Current in an AC Magnetic Field

The quasi-isotropic conductor stacked by the second generation (2G) high temperature superconducting (HTS) tapes is a promising choice for current transmission with low AC losses. In this paper, AC losses of quasi-isotropic conductor carrying a DC current in an AC magnetic field is studied by numerical and experimental methods at 77 K. AC losses are calculated by a two-dimensional finite element method (FEM) based on <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">T-A</i> formulation. Meanwhile, the total AC losses dissipated in the quasi-isotropic conductor can be obtained from experimental data of dynamic loss and the magnetization loss measured by electrical means. The amplitude of the external AC magnetic field and the DC transport current are varied. Simulation results are in good agreement with the experimental ones and the magnetic field distribution of the entire system is discussed in detail. The dependence of AC losses on external AC magnetic field and DC transport current is obtained, which is very helpful for fully understanding AC losses characteristic of quasi-isotropic conductors.

Numerical Study on Dynamic Resistance of an HTS Switch Made of Series-Connected YBCO Stacks

When a superconductor is exposed to an AC magnetic field whilst carrying a constant DC transport current, a DC electrical resistance can be observed, commonly referred to as “dynamic resistance”. This dissipative effect can play a critical role in many potential high-temperature superconducting (HTS) applications, especially for the AC-field-actuated HTS switch, which is the key component in the HTS transformer-rectifier flux pump (TRFP). The characteristics of the HTS switch determine the performance and reliability of TRFP. In this paper, we report a numerical study of a new HTS switch design made of series-connected YBCO stacks. This study aims to obtain insights into the characteristics of dynamic resistance and power loss, including their relationships with the amplitude and the frequency of the AC magnetic field. The energy conversion efficiency of the HTS switch is also investigated. This work will help design a high-performance AC-field-actuated HTS switch.

Effect of Stack Geometry on the Dynamic Resistance Threshold Fields for Vertical Stacks of Coated Conductor Tapes

The expanding capabilities of HTS flux pumps and rectifiers to provide kA+ currents necessitates the exploration of high-current switching phenomenon. One such phenomenon, the dynamic resistance, occurs in type-II devices carrying dc transport currents while exposed to ac magnetic fields. In the following, finite element analysis of the threshold field for dynamic resistance in superconducting cables comprised of N tapes connected in parallel and stacked vertically is presented. Cables are modelled using the commercial software COMSOL and the H-formulation. The models employ I <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> (B, θ) and n(B, θ) data obtained on short samples at 77 K as inputs to more accurately reflect the variation in local properties within the superconductor. The finite element results are then compared with calculations made using analytical models assuming a critical state. The finite element data closely resembles that predicted for a strip for a single tape, rapidly tending towards the slab result as N increases.

Magnetoresistance investigation in the mixed state of a high temperature superconductor

In this work, we analyze the behavior of the magnetoresistance R (H, T) of a high temperature superconductor YBa 2 Cu 3 O 7−δ thin film optimally doped. Measurements of the magnetoresistance were carried out in the mixed state for a magnetic field up to 14 T applied parallel to c − axis then parallel to ab − plane with a dc transport current of 100 and 500 nA, 0.1 and 0.3 mA perpendicular to the magnetic field direction in both cases. The obtained results show that the magnetoresistance is in a good agreement with the thermally assisted flux creep. However, the magnetoresistance in the flux flow regime cannot be described by the Barden-Stephen model which conjectures another origin of these results. In this sense, the quantum fluctuations of the order parameter is a good alternative to understanding the shape of the magnetoresistance. The comparison of the magnetoresistance of the two directions shows that [see formula in PDF] is larger than [see formula in PDF] with anisotropy factor [see formula in PDF] depending on temperature and magnetic field. The irreversibility line H irr (T) as well as H C2 (T) are determined and the first one is fitted with the phenomenological model H irr (T) = H irr (0) (1 − T/T o ) α where H irr (0), T o and α are parameters obtained from the fit. The investigation of the scaling law allows us to prove that a crossover from 3D to 2D behavior is a feature of our sample which justifies the existence of the decoherence effect.

Numerical Modelling of Dynamic Resistance in a Parallel-Connected Stack of HTS Coated-Conductor Tapes

Dynamic resistance is observed in type-II superconductors carrying a DC transport current while simultaneously exposed to an alternating magnetic field. The appearance of a non-zero resistance is attributed to the interaction between the transport current and moving fluxons. This effect is relevant to many superconductor applications such as high-temperature-superconductor (HTS) flux pumps, DC/AC magnets, synchronous machines, and persistent current switches. Here, we present a finite element method (FEM) analysis of both the time averaged dynamic resistance and the instantaneous current sharing behaviour in a cable comprised of a stack of four YBCO thin films connected in parallel. Numerical modelling was performed using the H -formulation method implemented in the commercial software COMSOL. The model employs experimentally measured values of the angular dependence of the critical current I c( B, θ ) and the flux creep exponent n ( B, θ ). A single threshold field is observed, above which a finite dynamic resistance is observed in all tapes simultaneously. The time-averaged dynamic resistance of individual tapes tends to be larger for the exterior tapes than the interior tapes, but this difference decreases as the total transport current in the cable increases. We attribute this to shielding currents flowing in the exterior tapes during the majority of the cycle, which displace net DC current into the interior tapes. However, the relative proportion of DC transport current flowing in the exterior and interior tapes is also observed to vary periodically once per half cycle of the applied field. This is due to the periodic trapping of return screening currents in the interior tapes.

Parallel magnetic field dependence of AC losses in periodically arranged superconductors with ferromagnetic substrates

Unlike superconductors on nonmagnetic substrates (SC/NM tapes) in a parallel magnetic field, the ferromagnetic substrates can cause obvious change of current density and magnetic field distributions in superconductor coated conductor (CC) tapes. In this case AC losses of CC tapes exposed to a parallel magnetic field cannot be neglected. In this paper, we investigate the electromagnetic response and AC losses of CC tapes using H-formulation model. The ferromagnetic loss of the substrate in a CC tape and the effect of magnetic field dependent permeability on AC loss are also considered. It can be found that the AC loss of a CC tape is much larger than that of a SC/NM tape, especially in weak applied magnetic fields. It is an effective way to significantly reduce AC loss of a CC tape by appropriately increasing the width of the ferromagnetic substrate. We also consider the CC tape carrying DC transport current and simultaneously exposed to a parallel AC magnetic field. The influence of the DC transport current on the AC loss is investigated. Moreover, AC losses of CC tape-array systems mainly depend on the arrangement and the vertical interval between CC tapes. We demonstrate that AC loss can be reduced effectively by decreasing the vertical interval between CC tapes.

Voltage–Current Characteristics and Temperature Variation by DC Power Source for the YBCO Coated Conductor Under Spray Cooling Method

This paper proposes the spray cooling method in conducting the extremely low temperature cooling of the superconducting wire, and presents the results of cooling performance tests and characteristics of the spray cooling method. To that end, extremely low-temperature containers were designed and manufactured to enable the testing of immersion, conduction, and spray cooling methods. Then, a cooling performance test and dc transport current test were carried out according to individual cooling methods. The test results are outlined below for each cooling method. Similar trends were shown in the initial cooling performance and cooling maintenance performance of the existing immersion cooling method and the spray cooling method. Similar trends were also shown in voltage-current characteristics according to dc transport current. Thus, in order to observe the cooling characteristics of the spray cooling method further, additional tests were carried out to define temperature characteristics according to spray pressure, and the voltage-current characteristics based on spray duration. The test results confirmed that higher spray pressure resulted in better cooling performance, and longer refrigerant time resulted in more stable conduction of dc. Accordingly, quench voltage was detected as low.

Gyrotropic Hall effect in Berry-curved materials

We study the ac Hall response induced by passage of dc transport current in two- and three-dimensional metals with gyrotropic point groups -- the gyrotropic Hall effect -- and consider the phenomenon of current-induced optical activity in noncentrosymmetric metals as a physical application of our theory. While the effect is expected to be present in single crystals of any noncentrosymmetric metal, we expect it to be strongest in enantiomorphic Weyl semimetals. Using the semiclassical kinetic equation approach we present several mechanisms underlying the gyrotropic Hall effect. Amongst them, the intrinsic mechanism is determined by the Berry curvature dipole, while extrinsic impurity-induced processes are related to skew scattering and side jump phenomena. In general, the intrinsic and extrinsic contributions can be of similar magnitude. We discuss the gyrotropic Hall effect for all frequencies of practical interest, from the DC transport limit, to optical frequencies. We show that for frequencies that are small compared to relevant band splittings, the trace of the gyrotropic Hall tensor in three-dimensional materials is proportional to a topological, quantized Berry charge, and therefore is robust in gyrotropic Weyl systems. This implies that polycrystals of strongly gyrotropic Weyl semimetals will demonstrate strong current-induced optical activity, whereas the response vanishes for polycrystalline ordinary metals. Therefore, the current-induced optical activity can be considered a valuable tool in identifying the topological nature of a material.

Charge noise acting on graphene double quantum dots in circuit quantum electrodynamics architecture**Project supported by the National Key Research and Development Program of China (Grant No. 2016YFA0301700), the National Natural Science Foundation of China (Grant Nos. 61674132, 11674300, 11575172, and 11625419), and the Anhui Initiative in Quantum information Technologies, China (Grant No. AHY080000). This work was partially carried out at

We investigate the dephasing mechanisms induced by the charge noise and microwave heating effect acting on a graphene double quantum dot (DQD) capacitively coupled to a microwave resonator. The charge noise is obtained from DC transport current, and its contribution to dephasing is simultaneously determined by the amplitude response of the microwave resonator. A lowfrequency 1/f-type noise is demonstrated to be the dominant factor of the dephasing of graphene DQD. Furthermore, when the applied microwave power is larger than −90 dBm, the dephasing rate of graphene DQD increases rapidly with the increase of microwave power, and fluctuates slightly with the applied microwave power smaller than −90 dBm. Our results can be applied to suppress the impeditive influence on the dephasing of graphene-based devices associated with microwave input in the perspective investigations.

Numerical modelling of dynamic resistance in high-temperature superconducting coated-conductor wires

The use of superconducting wire within AC power systems is complicated by the dissipative interactions that occur when a superconductor is exposed to an alternating current and/or magnetic field, giving rise to a superconducting AC loss caused by the motion of vortices within the superconducting material. When a superconductor is exposed to an alternating field whilst carrying a constant DC transport current, a DC electrical resistance can be observed, commonly referred to as ‘dynamic resistance.’ Dynamic resistance is relevant to many potential high-temperature superconducting (HTS) applications and has been identified as critical to understanding the operating mechanism of HTS flux pump devices. In this paper, a 2D numerical model based on the finite-element method and implementing the H-formulation is used to calculate the dynamic resistance and total AC loss in a coated-conductor HTS wire carrying an arbitrary DC transport current and exposed to background AC magnetic fields up to 100 mT. The measured angular dependence of the superconducting properties of the wire are used as input data, and the model is validated using experimental data for magnetic fields perpendicular to the plane of the wire, as well as at angles of 30° and 60° to this axis. The model is used to obtain insights into the characteristics of such dynamic resistance, including its relationship with the applied current and field, the wire’s superconducting properties, the threshold field above which dynamic resistance is generated and the flux-flow resistance that arises when the total driven transport current exceeds the field-dependent critical current, Ic(B), of the wire. It is shown that the dynamic resistance can be mostly determined by the perpendicular field component with subtle differences determined by the angular dependence of the superconducting properties of the wire. The dynamic resistance in parallel fields is essentially negligible until Jc is exceeded and flux-flow resistance occurs.

Dynamic resistance of a high-Tc coated conductor wire in a perpendicular magnetic field at 77 K

Superconducting high-Tc coated conductor (CC) wires comprise a ceramic thin film with a large aspect ratio. This geometry can lead to significant dissipative losses when exposed to an alternating magnetic field. Here we report experimental measurements of the ‘dynamic resistance’ of commercially available SuperPower and Fujikura CC wires in an AC perpendicular field. The onset of dynamic resistance occurs at a threshold field amplitude, which is determined by the total DC transport current and the penetration field of the conductor. We show that the field-dependence of the normalised magnetisation loss provides an unambiguous value for this threshold field at zero transport current. From this insight we then obtain an expression for the dynamic resistance in perpendicular field. This approach implies a linear relationship between dynamic resistance and applied field amplitude, and also between threshold field and transport current and this is consistent with our experimental data. The analytical expression obtained yields values that closely agree with measurements obtained across a wide range of frequencies and transport currents, and for multiple CC wires produced by different wire manufacturers and with significantly differing dimensions and critical currents. We further show that at high transport currents, the measured DC resistance includes an additional nonlinear term which is due to flux-flow resistance incurred by the DC transport current. This occurs once the field-dependent critical current of the wire falls below the DC transport current for part of each field cycle. Our results provide an effective and simple approach to calculating the dynamic resistance of a CC wire, at current and field magnitudes consistent with those expected in superconducting machines.