Transport Current Distribution Research Articles

酸化物超伝導多芯線材の電流一様化による通電損失の低減 新しい線材構造の提案とその電磁特性の理論解析

A new-type structure of multifilamentary oxide-superconducting wire and tape is proposed. It is expected that this structure will be effective in reducing transport losses produced when they carry changing currents. The fabrication method of such a wire or tape with low transport losses has more flexibility in the design of its parameters, such as the pitch and the direction of twisting or the number of filament layers, than the existing method does. Theoretical analyses of transport current distributions and resultant transport losses are made for a cylindrical two-layer wire with a twist in the same or the opposite direction for each layer. The theoretical result shows that uniformity in transport current distribution is achieved by optimizing the wire parameters regardless of the twist direction. Moreover, the possibility that the obtained uniformity reduces transport losses by one order compared with the existing type, which has strongly localized current distribution, is theoretically shown for a new multilayer wire or tape.

Spatial distribution of transport current in low and self field in BSCCO tapes

The spatial current distribution in monofilamentary BSCCO-2223 tapes is evaluated numerically using the critical state model with a field dependent critical current density. The overall transport current density is the sum of the local currents which perceive different magnetic fields depending on their location within the tape. In self field a current peak in the center of the specimen occurs. This is caused by the field dependence of j/sub c/. If a monofilamentary tape is sliced longitudinally the self field effect leads to an enhancement of j/sub c/ in the slices that formed the tape edges, even though the microstructure of the monofilament itself is assumed homogeneous. Thus the fact that this enhanced current flow at the tape edges is found experimentally does not necessarily mean that the microstructure in the tape edges is better, but could be-at least in part-due to the self-field effect. In twisted multifilamentary tapes the current distribution is expected to be homogeneous since the filaments cross over the tape. The self field effect also leads to a reduction of the overall current density for use of stacks of tapes as current leads.

Current-voltage characteristics and transport current distributions in Ag/Bi-2223 monocore tapes

Abstract Current-voltage characteristics of Ag-sheathed Bi-2223 monocore tapes have been measured at 77 K in low d.c. magnetic fields. Results have been obtained with the c -axis of the sample both parallel and perpendicular to the applied field, together with the force-free orientation. The current–voltage characteristics have been analysed in terms of their n -value ( V ∝ I n ), which characterizes the sharpness of the resistive transition and the second differential (d 2 V /d I 2 ), which shows the distribution in local critical current density values. The second differential has been fitted and analysed using the phenomenological Weibull distribution function, which is based on a weak-link model. The four parameters of this function have been used to investigate the microstructural nature of the sample, in particular, the factors influencing the critical current density and the transport current pathways. The results are consistent with the view that at low fields the critical current density is limited by weak-link grain boundaries and at high fields by intra-granular flux pinning.

AC transport losses in Ag sheathed (Bi,Pb) 2Sr 2Ca 2Cu 3O x tapes

The AC transport losses under self-fields have been investigated at 77 K on the Ag sheathed (Bi,Pb) 2Sr 2Ca 2Cu 3O x 19-filamentary untwisted and twisted (with a pitch of 13 mm) tapes, in comparison with the monofilamentary tape. The resistive voltages on the untwisted and twisted tapes show a dependence on the voltage-taps configuration, which is found in the monofilamentary tape. The AC transport losses per cycle for the tapes are nearly independent of frequency between 10 and 100 Hz, so that the main contribution to the losses comes from the hysteresis loss of superconductor in the same way as the monofilamentary tape. The twist of superconductor filaments causes no notable influence on the loss behaviors, and the results are explained by the theory of Norris for an elliptical superconductor with the same critical current, independent of whether or not the filaments are twisted. It is suggested that the distribution of transport currents in the filaments is not equalized in the twisted tape.

Current sharing and stability in a Bi-2223/Ag high temperature superconductor

This paper describes a full three-dimensional, numerical analysis of the transient fields of superconductor temperature, critical current density, electrical resistance and transport current distribution in a Bi-2223/Ag high temperature superconductor. The analysis is made for single (point-like) or extended thermal disturbances of finite duration. Current sharing between the superconducting material and the Ag jacket, intrinsic stability functions and maximum conductor temperatures are calculated for different ratios of transport to critical current, strength of disturbances, anisotropy ratio of thermal conductivity, and thermal resistances between superconductor and Ag jacket.

Interstrand coupling effect on losses and current distributions in superconducting cable conductors

We have successfully quantitatively estimated the interstrand coupling effect on fundamental electromagnetic properties such as losses and current distributions in a superconducting cable. A ·model conductor’, composed of test strands with various surface conditions, is used to obtain the value of the contact resistance between non-insulated strands in the conductor, a parameter which is closely related to the fundamental properties. The contact resistance is educed by comparing the measured transverse field loss in the model conductor with the numerically calculated value. The interstrand coupling effect increases the coupling loss of a cable-in-conduit conductor with a multi-stage twisting structure. This can theoretically be explained by an approximate analysis in which the specific structure is taken into account and the value of the contact resistance, as determined above, is used. Our measurement shows that the current distribution in a twisted conductor for a.c. uses is uniform among the strands. It can also be explained theoretically by considering the characteristic length necessary to make the transport current distribution uniform when the interstrand coupling effect is quantitatively taken into account. Extension of our estimates provides the basis for analysing the current sharing problems involving thermal effects caused when the conductors are quenched.

Magnetic field in the vicinity of BSCCO tapes carrying transport current

Measurements of magnetic field components B x,B y,B z, produced by both shielding and transport currents in BSCCO/Ag tapes are presented. The measured sample BSCCO 2223 was prepared by the powder in tube technique and the self field critical current densities were of the order of 10 4 A/cm 2 at 77 K. The shielding currents were induced by the external magnetic field perpendicular to the tape surface. The sample magnetic field was measured using 3 Hall probes with small active area (about 10 −2 mm 2) and sensitivity of the order of lOmV/T. They were fixed to a special holder so that they sense x,y and z component of the sample magnetic field at the distance ≥ 0.5mm from the sample surface. The measured profiles provide information on the critical current density inhomogeneities along the sample as well as on the spatial and time distribution of both transport and magnetization currents.

超電導撚線における高速常電導転移現象 Ｉ 発生機構

The mechanism of the so-called fast quench, i.e., an abnormal quench process with a high propagation velocity of normal zone in the multi-strand cables is discussed. Firstly, a set of basic equations were proposed so as to describe precisely the electromagnetic coupling among strands, propagation of normal zone and thermal interaction among strands. A good agreement of the theoretical value of transport current, at which the fast quench began to appear, with our observed data for a 2-strand cable supported strongly the availability of the proposed equations for a description of the fast quench. Then numerical calculations were also carried out for the commutation of transport current and temperature distribution inside a 2-strand cable when one strand was locally quenched. With the aid of the obtained results, a detailed mechanism for the occurrence of the fast quench was proposed.

Effect of anisotropy of current-carrying properties on transport current distribution in twisted superconducting wires

The author presents a problem for the solution of finding the AC transport current flowing in a wire with spiral anisotropy of superconducting properties. In the limiting instances, solutions describe the distribution of current and electromagnetic field in mono- and multifilamentary wires. The analysis shows that twisting affects the current redistribution more appreciably in wires with a low filament transverse-current-carrying ability. The work also discussed the effects on heat release, losses, and weakly expressed anisotropy.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Model calculations of the time-dependent flux-flow voltage in a thin-film type I superconductor

The temporal structure of the flux-flow voltage is calculated in the current-induced dissipative state of a thin-film type I superconductor. The instantaneous voltage generated by multiquantum flux tubes moving through a superconducting specimen depends upon both the instantaneous velocity and a geometry-dependent function of the position of the flux tube relative to the measuring circuit. The flux-tube velocity is strongly affected by the transport-current distribution, by the short-range edge-pinning effect, and by the repulsive interaction of simultaneously moving flux tubes. For a measuring circuit with leads kept far from the specimen surface, both nonuniform contributions due to the flux-flow velocity and the geometry-dependent function simultaneously cause the instantaneous voltage to peak when flux tubes nucleate and annihilate at the sample edges. Our model calculations reproduce reasonably well the experimental results obtained from signal averaging measurements.

On the distribution of a transport current inside a multifilamentary superconducting wire in a rapidly changing transverse magnetic field

For the current distribution inside a multifilamentary superconducting wire carring a dc transport current in a rapidly changing transverse magnetic field, inconsistencies with the existing models are shown by the following experimental evidence: when a transverse magnetic field is applied, the distribution of transport current is not unaltered but is forced to concentrate into the inner circular cross section region during a characteristic time constant τ c, called the coupling time constant. Secondly, the characteristic time constant for the transport current distribution inside the inner region to approach a uniform distribution is not τ c but a new time constant τ 1 called the ‘uniforming time constant’, though the variation in the distribution does not occur unless the external magnetic field changes with time. It is shown that the model of the current distribution based on the above experimental evidence exhibits a remarkable difference from the existing models, especially for the wires containing very fine superconducting filaments.

An experiment clarifying the distribution of transport currents in superconducting wires

The distribution of the transport current in a twisted multifilament superconductor is studied experimentally and theoretically for a sample configuration that simulates the windings of superconducting magnets. Theoretical analysis shows that the terminal voltage during a change of the current is directly related to the current distribution inside the composite conductor. A strongly non-uniform distribution due to the self-field effect has been found when the transport current is changed in a steady field. A time-varying transverse field tends to smear out this non-uniformity. The experimental results are in good agreement with calculations except for the high current region where the appearance of an anomalous resistive state peculiar to the filamented conductor must be taken into account. Time-varying external fields have been used to improve substantially the field stability of a small superconducting solenoid operating in the persistent mode.

An Experiment Clarifying the Distribution of Transport Currents in Superconducting Multifilamentary Composite Conductors

An Experiment Clarifying the Distribution of Transport Currents in Superconducting Multifilamentary Composite Conductors

Bending of flux lines by transport currents in type II superconductors measured by neutron diffraction

Neutron diffraction experiments have been performed on three Nb-13% Ta specimens in order to obtain information about the distribution of transport currents in type II superconductors. For sample one, which had the lowest pinning strength, it was concluded that most of the transport current was carried in a region near the surface. However, in sample two, in which the flux pinning was homogeneous, as indicated by ac flux penetration measurements, and in sample three, in which the pinning strength was highest, it was clearly demonstrated that a transport current was carried in the bulk and the experimental results could be explained by specific predictions of the critical state model.

The effect of localized surface changes on transport current distributions in plates of type I superconductors

The effect of localized surface changes on transport current distributions in plates of type I superconductors

Some experiments on the distribution of a direct transport current in sheets of type II superconductor

Some experiments on the distribution of a direct transport current in sheets of type II superconductor