Kunishige Kuroda

Eddy currents in flat two-layer superconducting cables

The principle of minimum energy dissipation rate is applied for calculation of steady interstrand eddy currents in flat two-layer superconducting cables subjected to transverse time-dependent magnetic fields which are non-uniform along the cable. The resulting eddy current patterns and corresponding energy losses are found to depend strongly on whether the longitudinal profile of the magnetic field contains a significant harmonic term with a period equal to the cable twist pitch. The limitations imposed by the sample geometry and the ramp rate on the experimental confirmation of results are discussed.

Pin fluctuation and the scaling of transport characteristics in superconductors

The summation problem in a random pin network, where pin-position and-strength were distributed randomly, have been solved numerically by the Monte-Carlo method, then the current-voltage curves were obtained. It has been shown that the scaling of the transport characteristics can be expressed as a thermally induced percolative transition by taking into account the random pin distribution.

Flux creep characteristics in the presence of a pinning distribution for Y/sub 1/Ba/sub 2/Cu/sub 3/O/sub 7-/spl delta// superconducting thin films

Nonlinear current-voltage I-V characteristics in Y/sub 1/Ba/sub 2/Cu/sub 3/O/sub 7-/spl delta// thin films have been studied for flux creep taking into account the inhomogeneous pinning strength. By measuring the frequency dependence of the I-V characteristics, we separated the flux creep and flux flow properties. I-V curves split in the low electric-field region depending on the measurement frequency. From the frequency dependant I-V curves, we obtained the maximum creep field, at which flux creep crossed over to flux flow. By use of the Weibull function, which describes the depinning probability of flux lines in pinning sites, we determined the pinning distribution from the flow nonlinearity obtained from AC measurements. The flux creep electric-field was calculated by integrating the Arrhenius equation according to the pinning distribution. The obtained expressions agreed will with the measurement over more than six decades of electric field.<<ETX>>

Influence of sample geometry on amplitude of eddy current oscillation in Rutherford-type cables

Interstrand eddy current induced in the samples of flat two-layer superconducting cables of finite length by ramping up uniform magnetic field are found to be periodic. Corresponding energy losses are compared with those calculated for eddy currents being uniform along the sample length. Dependence of amplitude of eddy current oscillations on the sample geometry is considered.<<ETX>>

Frequency dependences of Ac losses in (Bi1-xPbx)2Sr2Ca2Cu3Oy bulk superconductors in Ac magnetic field

Abstract It is shown that the observed ac loss in a cylindrical (Bi 1-x Pb x Sr 2 Ca 2 Cu 3 O y (Bi-2223) bulk superconductor with the diameter of 9.6mm in the ac magnetic field depends on the frequency of ac field contradicting the conventional theory based on the critical state model. The numerical simulation of electromagnetic field was performed by solving the Maxwell equations using the observed magnetic field dependence of critical current density and voltage-current characteristics. As the results, it is clarified that this deviation originates from the fact that the internal distribution of flux density indifferent from that predicted by the critical state model.

Development of a quick response persistent current switch [for SMES]

The paper describes the test results of a 100 A persistent current switch and the design of a 1.5 kA persistent current switch for use in SMES applications. These switches are of transformer-type, based on the principle of a current transformer. In order to increase resistance in the switch-off mode, bifiliar windings with a negligible small inductance are inserted into the secondary circuit of the transformer. Tests of 100 A persistent current switches have been performed with satisfactory results such as switch-off resistance of 50 /spl Omega/, switch-off time delay of 20 ms and operational current of 100 A. A 1.5 kA persistent current switch has been also designed to have a response time shorter than 100 ms and a switch-off resistance larger than 100 /spl Omega/.

Dynamics of the current penetration in the finite samples of flat two layer superconducting cables subjected to the time dependent magnetic field

Currents induced by the time dependent magnetic field in short samples of flat two layer superconducting cables are calculated. To simplify the problem, mutual inductances between elements of the cables were neglected, while all self-inductances were attributed to the strands constituting the cable. Evolution of screening currents in the strands and crossover currents overflowing between them is studied to estimate characteristic time constants of the process of current penetration. Dependences of the maximum characteristic time constant on the sample sizes are found.

Calculation of composites ramp rate sensitivity based on two layer model

Assuming that the current distribution between layers of multifilamentary composite superconductor, subjected to both increasing transport current and background magnetic field, is neither equal nor uniform along the length of the composite, the dependence of the quench current of the composite on the current ramp rate is calculated. Two competing processes are found to define the maximum current achieved in the composite during the current increase; heat release in the crucial points and expelling of the current from these points by the electric field in the superconductor.

Longitudinal distribution of normal zone in flat superconducting cables

Abstract Calculations have been performed to determine stationary normal zone distribution in flat multistrand superconducting cables. It was found that short pieces of normal zone located at the cable edge are stable in the relatively wide range of transport currents in the cable. Contact electrical resistances between the strands of the superconducting part of the cable can be considered as a distributed shunt.