External AC Magnetic Field Research Articles

AC loss characteristics of YBCO conductors carrying transport currents in external AC magnetic fields

In this work we present dependence of AC losses in a YBCO tape conductor carrying an AC transport current in an external AC magnetic field on the angle between the direction of the external field and the tape face of the conductor. The losses were measured by a calorimetric method whose validity was proven by simultaneous electric and calorimetric measurements of the losses in the same part of the conductor. The experiment was conducted at 77 K. The measured data show that the AC losses are dominated by the normal component of the external magnetic field to the tape surface. It is also shown that the AC losses sharply drop when the external magnetic field is parallel to the tape surface, which is due to extremely high aspect ratio of the YBCO conductor.

AC losses in HTS bulk and their influence on trapped magnetic field

A trapped magnetic field in a bulk is decayed and even erased by the application of an AC external magnetic field whose amplitude is much smaller than the trapped magnetic field. In the paper an analytic model to describe behavior of the trapped magnetic field is proposed assuming that its decay is caused by thermal effect due to AC losses in the bulk. The proposed model well explains AC loss characteristics, temperature rise and the decay of the trapped magnetic field of the bulk. Based on the proposed model, methods to suppress the decay of the trapped are shown and the effectiveness of the methods is shown.

Impact of DC Components in Transport Current and External Magnetic Field on Hysteresis Losses in an HTS Tape

The high-temperature superconductor (HTS) development is presently intensive, and one can expect that HTSs will be used in different electric devices in the future. In some of these devices, e.g., a magnetostrictive actuator, the conductor will be exposed to both ac and dc transport currents and external magnetic fields. One of the most important parameters in the design of electric devices is the power loss. It is, therefore, of importance to develop models that are able to predict the power loss in devices based on HTSs. In this paper, we present an experimental characterization of the hysteresis losses in an HTS tape exposed to ac or dc transport currents in combination with external magnetic fields. The experimental results are compared to some existing models based on the critical state model. The power loss was measured on a multifilamentary Bi-2223 tape at constant temperature (70 K) and frequency (50 Hz).

Study on suppression of decay of trapped magnetic field in HTS bulk subject to AC magnetic field

The trapped magnetic field in a bulk is decayed and even erased by the application of the AC external magnetic field whose amplitude is much smaller than the trapped magnetic field. In the previous work, an experimental result showed that the decay of the trapped magnetic field was due to temperature rise of the bulk caused by the AC losses. Based on this result, it is considered that the decay can be suppressed by (a) improvement of the cooling of the bulk and (b) reduction of the AC losses. We verified the effectiveness of these methods by conducting an experiment using a recently developed metal impregnated bulk which has high critical current density and improved thermal conductivity.

Total AC loss in Bi2223/Ag tape carrying AC transport current in external AC magnetic field of various directions

Total AC loss characteristics of a high temperature superconducting tape carrying AC transport current in external AC magnetic field with various applied directions to tape surface was experimentally investigated. We measured the AC transport current loss and the AC magnetization loss in the Bi2223/Ag-seath tape carrying the AC transport current in AC magnetic field by changing the angle to the tape surface and obtained the total AC loss as the sum of these losses. Based on the measured data, the dependence of the AC losses on the direction of the external AC magnetic field was discussed.

Effect of AC Magnetic Field on Transport Current Loss of HTS Tapes

It is known that external magnetic field not only causes the magnetization loss in a superconducting tape, but also increases the transport current loss, by the so-called "dynamic resistance". In power applications, superconductors carrying alternating current should be in alternating magnetic field. The frequency of the field is usually the same as that of the transport current, but they have phase difference. We developed an experimental system, which can measure transport current loss in external AC magnetic fields. With this equipment, we explored the effect of external field on the transport current loss with various field strengths and phase differences. This paper describes experimental results and discussions on the effect of amplitude and phase on transport current loss of HTS (high temperature superconductor) tape.

Electromagnetic field analysis of rectangular high Tc superconductor with large aspect ratio

AC loss reduction can substantially influence the economic feasibility of superconducting power devices. For AC loss reduction, understanding the AC loss characteristics of high T c superconductors is essentially required, and numerical electromagnetic field analysis of high T c superconductors is very useful tool for this purpose because the electromagnetic phenomena inside the superconductors can be visualized. This work focuses on electromagnetic field analysis of rectangular high T c superconductors with a large aspect ratio. A numerical code based on the finite element method was tuned to perform electromagnetic field analysis of high T c superconductors with a large aspect ratio. The optimization of mesh systems is required for accurate and efficient numerical analysis. AC losses in high T c superconductors with AC transport current and/or an AC external magnetic field are estimated through numerical analysis. The numerically calculated AC losses are compared with analytical values at several reference points to study the validity and error of the numerical analysis.

Finite lifetime of spiral domains in the anger state of multidomain magnetic films

The finite lifetime T g of a dynamic spiral domain in the anger state in a multidomain film with perpendicular magnetic anisotropy is estimated theoretically in the framework of a dissipative model. The finite lifetime of spiral domains is investigated as a function of the frequency of an external ac magnetic field that varies according to a harmonic law.

Response of glass and liquid phases in the vortex lattice to an external AC magnetic field at different frequencies

We individuated a method to distinguish a glass phase from a highly viscous liquid phase in a lattice of vortices, established in type-two superconductors. Our analysis is based on the study of the temperature dependence of numerically obtained 1st and 3rd harmonics curves of the AC magnetic susceptibility, by changing the frequency of the applied AC magnetic field. The harmonics are obtained by integrating the non-linear diffusion equation for the magnetic field, with different voltage–current characteristics, corresponding to the two different phases. This method could be applied to the analysis of experimental curves in order to determine if the detected magnetic response of superconducting samples may be interpreted in terms of vortex glass or Kim–Anderson model.

Nonequilibrium dynamics of quantum spin glass in an AC magnetic field

The ageing effect is studied analytically in a disordered quantum system interacting with its surroundings and subjected to an external ac magnetic field. Energy dissipation is due to the interaction of the system with a set of independent harmonic oscillators, imitating a quantum thermal bath. Dynamic equations for the autocorrelation function and linear-response function are derived using the method of closed-path integrals. The effect of an external field is studied on the correlation function and response in the spin-glass and paramagnetic phases. Both functions are found to depend on the spin interaction strength.

着磁されたバルク超電導体の交流磁界印加特性

着磁されたバルク超電導体の交流磁界印加特性

I–V curves of bifilar BSCCO tapes exposed to AC magnetic field

Abstract A study of the DC I–V curves of bifilar BSCCO/Ag tapes at 77 K is presented. Tapes under study were carrying DC transport current and exposed to external AC (0–20 mT, 50–400 Hz) or DC (0–20 mT) magnetic fields perpendicular to the plane of the tape. The measurements were carried out with bifilar tape to avoid the influence of the self-field. The measured I–V curves are well described by power law in the range of electric field 0.1–10 μV/cm. The AC field affects the I–V curves in a much stronger way than the DC field. The application of the AC field results in a linear decrease of the critical current (criterion 1 μV/cm) independent of the frequency of the magnetic field. The exponent n also exhibits a strong dependence on the AC magnetic field decreasing with increasing AC field amplitude and increasing with increasing frequency. The results are interpreted by assuming a confinement of the DC transport current in the central AC field free region of the tape. A low current density is assumed to flow in the region to which the AC magnetic field has penetrated.

AC losses in a HTS coil carrying DC current in AC external magnetic field

We electrically measured AC losses in a Bi2223/Ag-sheathed pancake coil excited by a DC current in AC external magnetic field. Losses in the coil contain two kinds of loss components that are the magnetization losses and dynamic resistance losses. In the measurement, current leads to supply a current to the coil were specially arranged to suppress electromagnetic coupling between the coil current and the AC external magnetic field. A double pick-up coils method was used to suppress a large inductive voltage component contained in voltage signal for measuring the magnetization losses. It was observed that the magnetization losses were dependent on the coil current and that a peak of a curve of the loss factor vs. amplitude of the AC external magnetic field shifted to lower amplitude of the AC magnetic field as the coil current increased. This result suggests the full penetration magnetic field of the coil tape decreases as the coil current increases. The dynamic resistance losses were measured by measuring a DC voltage appearing between the coil terminals. It was observed that the DC voltage appearing in the coil subject to the AC external magnetic field was much larger than that in the coil subject to DC magnetic field.

Influence of vacuum region on AC losses in superconducting wires with 2D FEM formulated by self-magnetic field

The influence of effective vacuum part is numerically evaluated on AC losses in superconducting wires calculated by means of a two-dimensional finite element method, which is formulated with a self-magnetic field due to an induced current as unknown. In the present method, the high-resistivity metal around the wires is needed for the sake of the simplicity in program coding. The superconducting wires carry an alternating transport current or are exposed to an external AC magnetic field. The AC losses are numerically calculated for square and circular effective vacuum parts with wide ranges of the sizes and the resistivity. Relative errors in the obtained results are discussed as compared with conventional analytical predictions.

Decay of trapped magnetic field in HTS bulk caused by application of AC magnetic field

In our previous work it was observed that trapped magnetic field in a YBCO bulk was decayed and even erased by application of AC external field whose amplitude was much smaller than the peak value of the trapped magnetic field. To study a reason for decay we investigated temperature rise of a YBCO bulk subject to the AC external magnetic field by use of thermocouples. A case was observed that the temperature rise of the bulk reached almost the critical temperature. This result shows that the decay of the trapped magnetic field was related to the temperature rise due to AC losses in the bulk.

Numerical evaluation of measured AC loss in HTS tape in AC magnetic field carrying AC transport current

Numerical study to evaluate the validity of the measurement method of AC loss in high temperature superconducting tapes in external AC magnetic field carrying AC transport current was done. The AC transport current loss and the magnetization loss in a conventional Bi2223/Ag-sheathed multifilamentary tape in the external AC magnetic field carrying the AC transport current were measured by the four terminal method and the pick-up coil method respectively. Numerical calculations on the AC losses in the tape were performed by using our previously developed analytical model. The measured losses were compared with the analytical results and the errors of the AC loss measurement were evaluated.

Measurement of AC current distributions in HTS tapes exposed to an external AC magnetic field by a compensated pickup coil array

A new system was developed for measuring AC current distributions in high T/sub c/ superconducting tapes under an external AC magnetic field. In our measurement the current distributions are indirectly found from the results of the self-field distributions measured by an array of pickup coils located around the tape. For improved measurement accuracy under the external magnetic field, a compensation coil was used to cancel the component of the external magnetic field from the signal of pickup coil. In order to confirm the validity of this system, measurement of current distributions in a test conductor composed of insulated copper wires was carried out under an external AC magnetic field. Using this system, we measured the current distributions in multifilamentary Ag alloy sheathed Bi-2223 tape as a function of the angle of the external AC magnetic field at 77 K. Further, we investigated the characteristics of the current distributions for the case of a simultaneous application of AC transport current and transverse AC magnetic field. On the basis of the obtained results the practicality of our measurement system is discussed.

AC losses in YBCO coated conductors carrying AC transport currents in perpendicular AC external magnetic field

The paper reports an experimental study on AC loss characteristics of a YBCO coated tape conductor carrying AC transport current in AC external magnetic field. In the experiment we electrically measured total losses that were transport current losses plus magnetization losses in AC external magnetic fields perpendicular to the tape surfaces. The transport current losses were measured by a four terminal method and the magnetization losses by a method using a pick-up coil. To measure the total losses electrically we need to pay proper attentions to prevent electromagnetic coupling between the sample currents and the external magnetic fields and to suppress inductive voltage in signals from the pick-up coil and voltage taps. The total losses were purely hysteretic for the amplitude of the external field up to 20 mT and the peak transport current up to the critical current. The loss characteristics can be explained by a theory based on the Bean model if the dependence of the critical current on the external magnetic field is taken into consideration.

Trapped field characteristic of HTS bulk in AC external magnetic field

Although immutable trapped field is required in superconducting bulk applications as a quasipermanent magnet, the trapped field is influenced and changed by time-varying external magnetic field in a realistic operational environment of electrical devices. This means that shielding current distribution within bulk is changed by the time-varying magnetic field and the transient magnetic flux movement results in temperature rise and finally reduction of the trapped field. In this paper, we experimentally investigated the transition of trapped field while applying external AC magnetic field with various amplitude and frequency to a disk-shaped YBCO bulk. And we also numerically investigate the relationship between characteristic of trapped-field attenuation and shielding current distribution within the bulk using a newly developed simulation program. This program is based on the finite element method (FEM) considering the voltage-current (E-J) characteristics. It is found that the shielding current distribution is dependent on frequency of AC external field and trapped field attenuation is closely related to the shielding current.

AC losses of multifilamentary Bi-2223/Ag conductors with different geometry and filament arrangement

This paper presents results from numerical modeling of nontwisted multifilamentary Bi-2223/Ag conductors with various geometry and filament arrangement. Anisotropic models of J/sub c/(B) and n(B) have been employed in finite element method simulations under different operating conditions - with applied ac transport current, in external parallel and perpendicular ac magnetic field, and with combined current and field of varying orientation. AC losses and current distributions in square and round Bi-2223/Ag wires with different filament arrangement have been calculated and compared to those of a 7-filamentary tape. It is demonstrated that the shape factor of the conductors and the orientation of the local magnetic field with respect to the individual filaments are of primary importance for the ac loss magnitude in different applications. Outlined are the advantages of using conductors with specific filament configuration for a given application.