A.V. Kalinov

Superposition of currents in hard superconductors placed into crossed ac and dc magnetic fields

The superposition of currents in YBCO melt-textured samples placed into crossed ac and dc magnetic fields is predicted and observed. This superposition is a direct consequence of the critical state model. The dc magnetic field distribution is shown to become uniform wherever the ac field has penetrated. Owing to this nonlinear process, the area of the dc magnetization loop diminishes and eventually disappears completely with an increase of the ac field magnitude. This means that under the action of the external ac field, the static magnetic properties of hard superconductors change and tend to the well known properties of soft ones.

Collapse of the magnetic moment in a hard superconductor under the action of a transverse ac magnetic field

Abstract The suppression of the static magnetic moment M of a superconducting plate by an alternating magnetic field applied perpendicularly to a dc magnetic field has been studied both experimentally and theoretically. Complete disappearance of the hysteresis of the magnetization curve in YBCO melt-textured samples was observed. Nonmonotonous behavior of M with the increase of the ac magnetic field amplitude was found. The dynamics of the collapse of the magnetic moment was studied in the pulse regime of the transverse field. The experimental results indicate homogeneity of the static magnetic induction in the sample regions where the ac field penetrates. The data obtained are interpreted within the framework of a two-liquid hydrodynamic model.

Phase Separation in La-Pr Manganites and Its Evolution in a Magnetic Field

The effect of the magnetic field H on the electrical resistivity ρ and magnetic moment M of (La0.25Pr0.75)0.7Ca0.3MnO3 ceramic samples with various concentrations of 16O and 18O isotopes is studied. The behavior of ρ(H) and M(H) is characterized by an unusual shifted hysteresis loop and slow relaxation at some of its sections. The effects observed are interpreted within the framework of a model that takes into account the coexistence of regions of a ferromagnetic metal and an antiferromagnetic insulator and their evolution in an external magnetic field.

Paramagnetism of a hard superconductor in crossed magnetic fields

The d.c. magnetic moment along the d.c. magnetic field is observed to be suppressed under the action of the perpendicular a.c. magnetic field. Moreover, this moment changes its sign at some value of the a.c. magnetic field amplitude and becomes paramagnetic. This behavior can be explained in the framework of a two velocity hydrodynamic model of the critical state of a hard superconductor in crossed magnetic fields. The physical nature of this phenomenon is connected with the homogeneity of the vortex density inside a superconductor at these condition.

Size effect in anisotropic hard superconductors

Abstract The amplitude dependence of the relative AC losses q in anisotropic superconducting plate-like samples has been studied experimentally and theoretically. Contrary to the isotropic case where there exists one maximum in the dependence of q = χ ′′ on the amplitude h 0 of the AC magnetic field, two maxima in the dependence q ( h 0 ) were observed. Both maxima are found to be related to the penetration of the AC field up to the middle of a sample. This size effect occurs if the direction of the AC magnetic field differs significantly from the principal axes of symmetry. Such an unusual manifestation of the size effect can be understood under the assumption that two components of the AC field along the principal axes of symmetry are independently shielded by the two principal components of the anisotropic critical current density. The obtained results are interpreted within the framework of the critical state model generalized to the anisotropic case.

Modeling Thermal Process in a Resistive Element of a Fault Current Limiter

The thermal regime of the Ni tape immersed into liquid nitrogen and heated up due to Joule losses generated by a step-like direct current is studied experimentally and theoretically. The temporal dependence of the temperature rise in Ni tape is measured. An adiabatic approach is shown to give a conservative result. According to our results, taking into account the heat transfer by thermal conduction into liquid nitrogen is necessary. This circumstance allows one to reduce a significantly necessary amount of high-temperature-superconductor material for the superconducting part of a resistive fault current limiter.

Colossal magnetoresistance and relaxation phenomena

The magnetization and magnetoresistance of (A = Sr, Ca) single crystals have been studied in the static regime. The same characteristics have been investigated as functions of time in response to a sharp change of the magnetic field. The magnetic moment M, dynamic magnetic susceptibility , and resistivity are found to relax noticeably in the samples with A = Sr (x = 0.3, 0.35). These parameters are shown to relax approximately logarithmically over time. The relaxation is temperature dependent, and diminishes when the temperature is far from that of the magnetic transition. The resistivity and magnetization do not relax when the field direction is reversed instantaneously. The static and relaxation properties observed are interpreted in the framework of a simple phenomenological model.

Critical current anisotropy in YBCO superconducting samples

Abstract The critical current densities J c ab and J c c in YBCO superconductors are shown to be simultaneously determined by means of measurements of a rectangular sample response to an external low-frequency ac magnetic field. A method is based on the correlation of the critical current density and ac magnetic susceptibility and takes into account for both shielding currents J c ab and J c c . The magnetic field dependencies of J c ab and J c c have been measured for melt-textured YBCO samples. The anisotropy parameter J c ab / J c c is found to increase with the dc magnetic field H .

The anisotropic response of the single crystal and textured HTS to the rotating AC magnetic field

Abstract An anisotropic response to the slowly rotating AC magnetic field was studied for YBCO melt-textured and twinned single-crystal samples. The results have been treated assuming the existence of two independent critical current densities in the sample plane ( J c ab and J c c for the textured sample; J c|| along the twin boundaries (TBs) and J c,⊥ across the TBs for the single crystals).

Losses in Bi-2223/Ag tapes and in the 1 kA AC transmission line model

We present here results of our study of the 5 m long AC transmission line model with 1 kA current capability at 77 K. Primary attention was paid to the current characteristics and AC losses in individual tapes and in the core of the cable. Problems of tape transposition are considered. Our study was undertaken in several stages. At the beginning, we investigated current capability and AC losses in individual Bi-2223/Ag tapes. The goal was to determine how many tapes were needed to construct a 5 m long transmission line model with a current capability of kA; we then evaluated total AC losses in the model. Subsequently, we designed a current core for the power cable and a cryogenic envelope to test the cables performance.