Perpendicular Ac Magnetic Field Research Articles

Numerical simulation of dynamic loss and total loss in the REBCO tapes under perpendicular AC magnetic fields up to 8 T at 20 K and 50 K

REBCO tapes carry DC current under AC magnetic fields in proposed HTS fusion applications. AC loss will be generated in the process and it is important to understand the AC loss behaviour for safe operation of the fusion magnets. In this work, magnetisation loss (Qm), dynamic resistance (Rdyn), and total loss (Qtotal) in four different REBCO tapes are numerically studied, using the measured JcB,θ and nB,θ, for the magnetic field amplitude applied perpendicularly up to 8 T at 20 K and 50 K, where JcB,θ represents the magnetic field and field angle (θ) dependent critical current density. The peak of Theva JcB,θ data is different from that of other tapes. We artificially shifted the ab-plane peak of Theva JcB,θ to the left by 25° to match the peak value. The newly shifted data is named as Theva-shift, which was also investigated to study the influence of the Theva peak shift on AC loss. The normalised DC transport current level (i = It/Ic0) ranges from 0.05 to 0.9, where the DC current amplitude and the self-critical current of the tape are represented by It and Ic0, respectively. The simulation results show that the AC losses deviate significantly from the Brandt-Indenbom (BI) equation at high magnetic fields. Jc and instantaneous loss curves for different tapes show correlation at high magnetic fields. The simulation results also show how different JcB,θ characteristics for different tapes influence AC losses. When AC loss values are scaled by the self-field critical current, Qm without current and Qtotal with current in the different tapes show a good agreement. It implies that the temperature dependence of the two types of loss can be calculated from a known loss at one temperature and the self-field critical current.

Numerical Analysis of Dynamic Resistance and Total Loss in REBCO-Coated Conductors at Low Temperature Under High Perpendicular AC Magnetic Fields

In several applications of high-temperature superconductors (HTS), a REBCO-coated conductor (CC) carries a DC current under varying magnetic fields. This generates AC loss which can be an important factor to consider in the thermal design of the application. In this work, the dynamic resistance (Rdyn), and total loss in a 4 mm wide Superpower REBCO CC are numerically investigated for AC perpendicular magnetic fields ( <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">B</i> m) up to 1 T and temperatures ranging from 30 K to 50 K. For <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">I</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">dc</sub> / <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">I</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c0</sub> > 0.1 (where Idc is the DC current level and <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">I</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c0</sub> is the self-field critical current of the conductor), either of the previously developed equations, the non-linear or the simple linear expression for determining B <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">th</sub> (threshold magnetic field), can be used to predict R <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">dyn</sub> . Finite dynamic loss is observed even below the B <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">th</sub> points due to the emergence of resistance. The resistance arises from the superposition of subcritical shielding and transport currents which introduces a finite electric field in the CC. The total loss at higher fields decreases as the temperature increases from 30 K to 50 K for all current values, except for <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">I</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">dc</sub> / <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">I</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c0</sub> = 0.9. A lift-off in the total loss is evident for fields around 1 T for all temperatures investigated.

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.

Simulation of Dynamic Resistance and Total Loss of HTS CORC Cables

In some high temperature superconducting (HTS) applications, HTS coated conductors carry DC current under external AC magnetic fields. Dynamic resistance occurs when the amplitude of the magnetic field is greater than the threshold magnetic field of the coated conductors. The resulting AC loss, termed as total loss, consists of dynamic loss due to dynamic resistance and magnetization loss due to the shielding currents caused by the AC magnetic field. Conductor on round core (CORC) cables wound with HTS coated conductors have attracted broad attention due to their large current-carrying capability and mechanical flexibility for coil applications. However, there has been no report on dynamic resistance and total loss in CORC cables. In this work, we present 3D FEM simulation results on the dynamic resistance and total loss of a spiral tape and three CORC cables, based on the <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${\boldsymbol{T-A}}$ </tex-math></inline-formula> formulation. The number of layers of the CORC cables, the amplitude of the AC magnetic field and transport DC current levels have been varied to study the impact of those parameters on dynamic resistance and total loss of the three CORC cables. The simulation results show that magnetization loss without current in a spiral tape can be analytically estimated by Brandt and Indenbom’s theoretical equation for a superconducting strip under perpendicular AC magnetic field with a geometric coefficient 2/ <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\pi $ </tex-math></inline-formula> . Furthermore, dynamic resistance of the spiral tape and each tape in a single-layer cable can be predicted by the analytical equation for a strip carrying DC current under perpendicular AC magnetic field, also taking into account the geometric coefficient 2/ <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\pi $ </tex-math></inline-formula> . The simulation results also show that the difference of total loss values in the three CORC cables depends on the shielding effect: the more layers of CORC cables, the lower each loss component. The two-layer Cable with each tape in the outer layer sitting on top of the tape in the inner layer has the lowest loss and the highest threshold magnetic field.

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.

Impact of Magnetic Substrate on Dynamic Loss and Magnetization Loss of HTS Coated Conductors

Direct current carrying Type II superconductors present a dc electric field under external perpendicular AC magnetic field, commonly referred to as “dynamic resistance effect”. Two kinds of AC losses occur in this situation: dynamic loss due to the interaction between the DC current and moving fluxons, and magnetization loss due to shielding current caused by the external AC magnetic fields. In general, HTS coated conductors have one substrate layer, which can be non-magnetic (Hastelloy) or magnetic (NiW). In this paper, we present the impacts of the magnetic substrate and their strength of ferromagnetism on dynamic loss and magnetization loss by using a finite-element method model based on <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">H</i> -formulation. This work reveals that the magnetic substrate layer does influence the dynamic loss and magnetization loss of HTS coated conductors. The dynamic loss is reduced by the adjacent magnetic substrate in the low field region and is increase in the high field region. The magnetization loss is reduced significantly by the magnetic substrate. This work will help choose HTS coated conductors with different substrate layers in different applications.

Dynamic resistance and total loss in a three-tape REBCO stack carrying DC currents in perpendicular AC magnetic fields at 77 K

In many high-temperature superconducting (HTS) applications, HTS-coated conductors carry a DC current under an external AC magnetic field. In such operating conditions, dynamic resistance will occur when the traversing magnetic flux across the HTS conductors. Consequently, AC loss within the superconductors is composed of the dynamic loss component arising from dynamic resistance and the magnetization loss component due to the AC external magnetic field. This AC loss is one of the critical issues for HTS applications, such as persistent current switches, flux pumps, and rotating machines. In this work, the dynamic resistance and the total loss in a three-tape HTS coated conductor stack were measured at 77 K under perpendicular AC magnetic fields up to 80 mT and DC currents (I dc) up to the critical current (I c). The stack was assembled from three serial-connected 4 mm wide Superpower wires. The measured dynamic resistance results for the stack were well supported by the results from 2D H-formulation finite element modelling (FEM) and broadly agree with the analytical values for stacks. The FEM analysis shows asymmetric transport DC current profiles in the central region of the superconductor. We attribute the result to the superposition of DC currents and the induced subcritical currents which explains why the measured magnetization loss values increase with DC current levels at low magnetic field. The onset of dynamic loss for the stack for low i (I dc/I c) values is much slower when compared to that of the single tape and hence the contribution of the dynamic loss component to the total loss in the stack is much smaller than that of the single tape. Dynamic loss in the stack becomes comparable to the magnetization loss at i = 0.5 and becomes greater than the magnetization loss at i = 0.7. Both magnetization loss and dynamic loss in the stack are smaller than those of the single tape due to shielding effects. The difference between the Q total behaviours in the stack and single tape is due to the variation of the penetration depths of the stack and single tape at the different magnetic field amplitudes.

Distribution and characterization of screening current and induced field in HTS quasi-isotropic and simply stacked strands

With the development of the second-generation high temperature superconducting tapes (2G HTS tapes), there have been several high current-carrying conductors proposed by different groups. However, since the conductors are generally stacked by 2G HTS tapes with a high aspect ratio, the perpendicular AC magnetic field can induce the screening current on the wide surface of tapes, then generates a magnetic field during the process of excitation and demagnetization, which affect the spatial uniformity and temporal drift as well as field quality in HTS magnets. In this paper, as potential conductors can be used in fusion magnets, the screening effect characteristics of quasi-isotropic strand (Q-IS) and simply stacked strand (SSS) are performed numerically and experimentally. We plot the penetration diagrams of the screening current for both strands under applied changing vertical magnetic field. The dependence of the screening current induced field (SCIF) on the amplitude of the external magnetic field is analyzed, the SCIF hysteresis loop is obtained. We also discuss the spatial distribution of SCIF for both strands. It is shown that SCIF of Q-IS has quasi-isotropic distribution and considerable amplitude reduction comparing with the SSS. The study provides a valuable reference for the application of Q-IS in superconducting magnets, especially nuclear fusion magnets.

Total loss measurement and simulation in a REBCO coated conductor carrying DC current in perpendicular AC magnetic field at various temperatures

In many high-temperature superconducting (HTS) applications, HTS coated conductors carry DC currents under external AC magnetic fields. There are two AC loss mechanisms in this situation: magnetization loss due to the external magnetic field and dynamic loss due to the interaction between the DC current and the external magnetic field. The sum of these two loss components is referred to as total loss. In this work, the total loss in a 4 mm wide REBCO coated conductor is measured under perpendicular AC magnetic fields up to 105 mT at 77 K, 70 K, and 65 K, with reduced DC current level, i (I dc/I c0), from 0.025 to 0.98, where I dc is the transport DC current value and I c0 is the self-field critical current of the coated conductor at each temperature. The experimental results show a good quantitative agreement with an analytical equation for each loss component, as well as 2D finite element modelling (FEM) results from H -formulation. For any given temperature, we observe that the total loss is mostly dominated by magnetization loss at i< 0.2, while dynamic loss makes a comparable, even greater contribution to total loss at i > 0.5. Electromagnetic analysis from the FEM modelling shows the evolution process of total loss, where the dynamic loss region and magnetization loss region vary across the conductor width at high magnetic fields or high DC current level. The simulation results also reveal the superposition of (positive) DC current and the anti-parallel (negative) shielding current, which occurs at high DC current level. The superposition drives the current density of one conductor edge to subcritical stage, and it leads to one-sided loss generation in each half-cycle. Our results provide a valuable reference for total loss behaviours in REBCO coated conductors.

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.

A temperature-dependent multilayer model for direct current carrying HTS coated-conductors under perpendicular AC magnetic fields

When a type II superconductor carrying a direct current is subjected to a perpendicular oscillating magnetic field, a direct current (DC) voltage will appear. This voltage can either result from dynamic resistance effect or from flux flow effect, or both. The temperature variation in the superconductor plays an important role in the nature of the voltage, and there has been little study of this so far. This paper presents and experimentally verifies a 2D temperature-dependent multilayer model of the second generation (2G) high temperature superconducting (HTS) coated conductors (CC), which is based on H-formulation and a general heat transfer equation. The model has coupled the electromagnetic and thermal physics, and it can simulate the behavior of 2G HTS coated conductors in various working conditions where the temperature rise has a significant impact. Representative electromagnetic phenomena such as the dynamic resistance effect and the flux flow effect, and thermal behavior like quench and recovery have been simulated. This thermal-coupled model is a powerful tool to study the thermal-electromagnetic behaviors of 2G HTS coated conductors in different working conditions, especially when the impact of temperature rise is important. This multilayer model is also very useful in analyzing the impact of different layers in the 2G HTS CCs, especially the metal stabilizer layers. It has been proven to be a very powerful tool to help understand more complicated characteristics in the CCs which could not be accurately measured or simulated by previous numerical models. The work is indicative and very useful in designing ac magnetic field controlled persistent current switches and flux pumps, in terms of increasing the off-state resistance, analyzing different sources of losses, minimizing detrimental losses, and enhancing the safety and stability.

Impact of Stabilizer Layers on the Thermal-Electromagnetic Characteristics of Direct Current Carrying HTS Coated Conductors under Perpendicular AC Magnetic Fields

When a type-II high Tc superconductor carrying a direct current is subjected to a perpendicular AC magnetic field, a direct current voltage will appear. This phenomenon is called dynamic resistance effect. In general, the high temperature superconducting (HTS) coated conductor (CC) has two stabilizer layers. However, the impacts of two stabilizer layers on the dynamic resistance, DC electrical field, losses, and temperature rise haven't been studied yet. This paper presents the impacts of the stabilizer layers and their resistivity on the dynamic resistance effect and HTS CC tape's thermal-electromagnetic behaviors by using a temperature dependent FEM model. This work reveals that the stabilizer impacts significantly on the dynamic resistance, dc voltage, power loss, and temperature rise. It is will help design high-performance AC magnetic field-controlled PCS and switches based HTS devices.

Dynamic resistance measurement in a YBCO wire under perpendicular magnetic field at various operating temperatures

Dynamic resistance plays an important role in certain high-Tc superconducting (HTS) applications where an HTS coated conductor carries a DC current exposed to an AC magnetic field. Here, we report measurements of the dynamic resistance in a 4 mm-wide YBCO coated conductor under a perpendicular AC magnetic field at 77 K, 70 K, and 65 K. Dynamic resistance was measured at three different frequencies for the reduced current, i (It/Ic0), ranging from 0.04 to 0.9, where It is the DC current level and Ic0 is the self-field critical current of the conductor at each temperature. At all three temperatures, the threshold magnetic field (Bth) values increase with reducing DC current. These results show that, for a given set of applied conditions, dynamic resistance decreases with decreasing operating temperature, which we attribute to the temperature dependent increase in the critical current of the wire. We show that measured Bth values at all three temperatures agree well with the analytical values from nonlinear Mikitik and Brandt equation for i ≤ 0.2 and with a simple linear expression that assumes a current-independent penetration field for i &amp;gt; 0.2. We further show the measured Bth curves at different temperatures normalized by critical current density collapse into one common curve. The above result implies that dynamic resistance in coated conductors at different temperatures under perpendicular AC magnetic fields can be scaled simply using measured Ic0 values at those temperatures and analytical equations.

Optimal Design for a Portable NMR- and MRI-Based Multiphase Flow Meter

In this paper, an optimal design for nuclear magnetic resonance and magnetic resonance imaging based multiphase flow meter is presented. The overall apparatus consists of an electromagnetic coil surrounded by 12 rings of Halbach arrays of 12 magnets each. Both the coil and the magnets arrays were simultaneously and optimally designed using a three-dimensional (3-D) finite-element method (FEM) package and particle swarm optimization algorithm. The aim of the design is to uniformly prepolarize the hydrogen spins of the atoms composing the flowing fluid, with a predefined static magnetic field, before they enter into the measurement area within which they are homogenously polarized with a perpendicular ac magnetic field. Cheap and lightweight hardware are the other two design parameters of the design. Results of extensive 3-D simulations of the design indicate that an optimized and homogenous static magnetic field distribution could be achieved within an area of 40 mm diameter and 606 mm length. In addition, using a multiturn coil of 6 cm diameter and 2000 turns, an ac magnetic field of 13 mT amplitude and 112 ppm homogeneity could be achieved, which is enough to handle fluids flowing at speed of up to 2 m/s. Experimental validation, which was done using newly constructed two Halbach arrays of cuboid and trapezoid magnet elements, respectively, indicates a good match with FEM simulations.

The Frequency Dependence of AC Susceptibility for Bi-2223/Ag Tapes With Varying Lengths

An experimental investigation on ac susceptibility χ = χ′ − j χ′′ of Bi-2223/Ag superconducting tapes with different lengths L is carried out under a perpendicular ac magnetic field. A series of n -value is deduced from the ac susceptibility scaling law, which is used to systematically analyze the induced current in Bi-2223/Ag superconducting tapes. Based on the experiment results, a critical length L c is defined for the tapes as ten times the width of them. AC susceptibility χ shows conspicuous frequency dependence when the length L is smaller than L c. This phenomenon is attributed to the induced transverse current in silver. In addition, the proportion of transverse current to total induced current increases with the decreasing length dimension in our research. The results help understand the electromagnetic properties of Bi-2223/Ag superconducting tapes better and lay down important experimental foundation for practical applications.

Numerical Modeling of Dynamic Loss in HTS-Coated Conductors Under Perpendicular Magnetic Fields

High-Tc superconducting (HTS)-coated conductors are a promising option for the next-generation power devices. However, their thin-film geometry incurs dynamic loss when exposed to a perpendicular external ac magnetic field, which is difficult to predicate and estimate. In this paper, we propose and verify a numerical simulation model to predict the dynamic loss in HTS-thin-coated conductors by taking into account their Jc-B dependence and I-V characteristics. The model has been tested on a SuperPower YBCO-coated conductor, and we observed a linear increase of dynamic loss along the increasing field amplitude after the threshold field. Our simulation results agree closely with experimental measurements as well as an analytical model. Furthermore, the model can predict the nonlinear increase of dynamic loss at high current, while the analytical model deviates from the measurement results and still shows a linear correlation between the dynamic loss and the external magnetic field. In addition, we have used this model to simulate the distributions of magnetic field and current density when dynamic loss occurs. Results clearly show the flux traversing the coated conductor, which causes dynamic loss. These distributions have also been used to analyze the change of dynamic loss when either the transport current or the magnetic field increase individually, while the other factor remains constant. The simulation analysis on dynamic loss is done for the first time in this paper, and our results clearly demonstrate how dynamic loss changes as well as its dependence on transport current and magnetic field.

A Fast AC Field Controlled Impedance in HTS Coated Conductors: Response Speed and Electric Field Value

Direct current carrying Type II superconductors present a dc electric field under external perpendicular ac magnetic field. This effect can also be considered as an equivalent dynamic resistance. The minimum field magnitude required to generate the dynamic resistance is less than the fully penetration field of the superconductor, which is much less than its upper critical field. Thus, the superconductor can remain superconducting, leading to fast recovery times and meaning that it is feasible to use dynamic resistance to add a controlled impedance into the system. In this study, we built a demonstrator using a section of YBCO coated conductor controlled by an external ac field of a wide frequency range. We tested the electric field value and response time of the system. The work may be indicative for designing impedances for flux pumps or superconducting magnetic energy storage systems.

AC magnetic response of superconducting YBa2Cu3O7/PrBa2Cu3O7 superlattices

Vortex activation energy UAC in the critical-state related AC magnetic response of superconductors (appearing in the vicinity of the DC irreversibility line) takes large values, as often reported, which is not yet understood. This behavior is essentially different from that of the vortex-creep activation energy at long relaxation time scales, and may become important for AC applications of superconductors. To elucidate this aspect, we investigated the AC signal of almost decoupled [Y Ba2Cu3O7]n/[PrBa2Cu3O7]4 superlattices (with n = 11 or 4 units cells) in perpendicular DC and AC magnetic fields. In these model samples, the length of the hopping vortex segment is fixed by the thickness of superconducting layers and vortices are disentangled, at least at low DC fields. It is shown that the high UAC values result from the large contribution of the pinning enhanced viscous drag in the conditions of thermally activated, non-diffusive vortex motion at short time scales, where the influence of thermally induced vortex fluctuations on pinning is weak.

Effect of the magnetic material on AC losses in HTS conductors in AC magnetic field carrying AC transport current

This paper presents an investigation on the AC losses in several typical superconducting composite conductors using the H-formulation model. A single superconducting strip with ferromagnetic substrate or cores and a stack of coated conductors with ferromagnetic substrates are studied. We consider all the coated conductors carrying AC transport currents and simultaneously exposed to perpendicular AC magnetic fields. The influences of the amplitude, frequency, phase difference and ferromagnetic materials on the AC losses are investigated. The results show that the magnetization losses of single strip and stacked strips have similar characteristics. The ferromagnetic substrate can increase the magnetization loss at low magnetic field, and decrease the loss at high magnetic field. The ferromagnetic substrate can obviously increase the transport loss in stacked strips. The trends of total AC losses of single strip and stacked strips are similar when they are carrying current or exposed to a perpendicular magnetic field. The effect of the frequency on the total AC losses of single strip is related to the amplitude of magnetic field. The AC losses decrease with increasing frequency in low magnetic field region while increase in high magnetic field region. As the phase difference changes, there is a periodic variation for the AC losses. Moreover, when the strip is under only the transport current and magnetic field, the ferromagnetic cores will increase the AC losses for large transport current or field.

Mechanism of a high-Tc superconducting flux pump: Using alternating magnetic field to trigger flux flow

High-Tc Superconducting (HTS) magnets operating in persistent current mode suffer a current decay due to flux creep of superconductor and joint resistance. Flux pumps are able to inject direct current into superconducting circuit to compensate the current decay, without the thermal loss caused by current leads. In this work, we proposed a flux pumping mechanism for HTS coils, with an experimental verification and an analytical model. The basic principle we have used is that flux flow can be triggered when the superconductor carrying a direct current is subjected to a perpendicular AC magnetic field. Low frequency alternating current is induced in a loop of YBCO tape using an AC field. A portion of the tape which we refer to as the “bridge” shorts a superconducting coil. A high frequency AC field is applied perpendicular to the bridge tape when alternating current in the tape reaches one polarity. This triggers a net flux flow and results in a current increase in the coil. The proposed flux pump has clear physics and is easily controllable, which may make it promising in practical use.