DC Transport Current Research Articles

Transient stability of AC multi-strand superconducting cables

AC application, it is necessary to estimate the stability of multi-strand superconducting cable. Therefore, we have been studying the transient stability of non-insulated multi-strand cable when one strand in a cable turns into the normal state locally. In the quench process, local temperature rise produced by current redistribution among strands is not desirable for stability. In a previous work, we discussed the effect of Cu matrix allocated to each strand on the transient stability and showed that the Cu matrix allocation can improve the stability of non-insulated multi-strand cable through mainly numerical simulations. In this paper, we carried out experiments on three kinds of non-insulated three-strand cables; one consists of NbTi/CuNi strands and the others consist of NbTi/Cu/CuNi strands having different cross-sectional arrangement. These sample strands have almost the same diameter, the same matrix to superconductor ratio and the same B– J characteristics to evaluate the effect of Cu allocation quantitatively. We choose to define the transient stability in terms of the minimum quench energy (MQE) at each DC transport current. We also investigated the transient stability of sample cables when quench is initiated in two or three (all) strands simultaneously.

Voltage spikes in superconducting Cable-In-Conduit Conductor under ramped magnetic fields. Part 2: Analysis of loop inductances and current variations associated with the spikes

A 27 strand hybrid superconducting Cable-In-Conduit Conductor (CICC) sample (so-called TPX-PF model sample) has been fabricated and tested in quickly ramped background magnetic fields. The voltage spikes that appeared in the sample's terminal voltages during magnetic field sweeps at DC transport current are analyzed using a model that calculates the magnitude of individual strand current drops and the strand to strand/cable inductances associated with each voltage spike. Dependencies of the strand inductances and current variations with consecutive voltage spike numbers, total transport current in the cable and background magnetic field are analyzed and discussed. The analysis confirms previously reported suggestions that voltage spikes and the corresponding rapid variations, or jumps, observed in the conductor's local magnetic field are indications of rapid redistribution of current from one of the cable's strands in which the current reached its critical level. It is shown that rapid current redistributions which are too small to initiate total cable quench lead to more uniform distribution of current among the strands in the CICC. Therefore, it may be possible to apply small disturbances to a CICC to improve its strand to strand current distribution in a cable and to stabilize its Ramp Rate Limitation behavior.

Self fields and current distribution due to DC transport currents on Ag-sheathed (Bi,Pb) 2Sr 2Ca 2Cu 3O x tapes

Using a scanning Hall sensor magnetometry with an active area 50 μm×50 μm, we investigate the self-field distribution at 77 K due to DC transport currents subjected to the programmed current sequence in zero external field on the surfaces of Ag-sheathed (Bi,Pb) 2Sr 2Ca 2Cu 3O x monofilamentary and multifilamentary tapes. We derive the field slope from the measured magnetic profile and relate it to the current distribution in the superconductor. The results for the monofilamentary tape are well explained by the Bean model on the assumption of an elliptical superconductor core, which claims that the current distribution is assigned to J c, − J c and/or 0 in the superconductor and the border of the current distribution forms concentric ellipsoids with the same aspect ratio of the sample. There is a resemblance in the self-field distribution and accordingly the current distribution between the monofilamentary tape and the multifilamentary tape. The multifilaments behave as a solid superconductor under DC transport currents so that the current distribution cannot be equalized in the filaments.

Effects of DC transport currents on AC losses by the magnetically induced currents in Ag sheathed Bi(2223) tape

AC losses at 60 Hz for a Ag sheathed Bi(2223) by a magnetically induced current technique were measured as a function of applied AC field and DC transport current. The results were qualitatively in agreement with the theory, which is based on Bean critical state model, by T. Ogasawara, et al. (Cryogenics, Vol.19, Dec. 1979, pp.236-740). Possible sources for some quantitative disagreements between the results and the theory are discussed.

Performance summary of a 4,000 a high temperature superconducting cable conductor prototype

Multifilamentary superconducting Bi-2223 tapes made with a powder-in-tube process have been used to assemble multistrand, multilayer cable conductor prototypes. The electrical performance of these conductors has been evaluated at 77 K in self-field with DC and AC transport currents. Results show that these conductor prototypes, carrying up to 4,200 A DC, approach the 2,000 A AC operation threshold required for the commercial application of HTS power transmission cables. Combined with recent improvements in tape mechanical performance, stability, and long-length manufacturability, these results show the potential for use of HTS tapes in power transmission cables.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Melt cast processed (MCP)-BSCCO 2212 tubes for power applications up to 10 kA

The Melt Cast process (MCP) is highly flexible to manufacture BSCCO 2212 bulk parts in a large variety of shapes and dimensions. MCP also allows the integration of current contacts during the fabrication process thus leading to HTS parts which can directly be integrated into electrotechnical devices, The ease of mechanical treatment of the material by machining methods offers additional options for the MCP bulk parts. Critical current densities of 2.5-4 kA/cm/sup 2/ are obtained in selffield, about 1 kA/cm/sup 2/ are measured in full rods. High DC (6.4 kA) and AC (10 kA) transport currents are obtained in large tubes (O 70 mm, L 200 mm) at 77 K. Different prototypes and a first application, each of low loss hybrid current leads, have been produced as well as functional models for current limiting and for shielding. >

An analysis of the transport losses measured on HTSC single-phase conductor prototypes

An analysis was made of published results of transport losses measured on state of the art HTSC single-phase conductor prototypes. Fitting the data to simple existing models was attempted and the agreement is reasonably acceptable for cable design purposes. The measured and estimated losses agree within a factor of two, in a range of peak transport current between 0.3 and 1.0 of the conductor DC critical current. The model used in this current range is one that considers the conductor as a solid SC tube in its own (purely tangential) self-field. The eddy losses are estimated as in a standard procedure for non-magnetic helical multilayer reinforcements and armours, and turn out to be negligible for this geometry. Losses higher than predicted are measured on two or more layer conductors in the low-current range, being typically for a peak transport current below 0.3 of the DC transport current of the conductor. In order to explain this, it is suggested that there might be a dephasing in the AC threshold voltages of the tapes, due to the variation of the in-field critical currents of the tapes across the conductor layers. Problems that are common to this type of measurement are discussed too. The simple model is not directly applicable to the more practical case of a non-coaxial trefoil configuration, because in this case a transverse field is also present, produced on each of the three conductors by the two others. Therefore, the losses need to be measured and modelled also on the trefoil configuration, to obtain reliable data in this practical case too.

AC susceptibility with superimposed DC magnetic field in a current biased granular high Tc superconductor

We have investigated the effect in AC magnetic susceptibility, ϰ ac = ϰ′+j ϰ″, of both DC transport current, J t , and DC magnetic field, H dc , in polycristalline HTSC. Two regimes are observed: at low DC fields critical state is modified by the current self field; but as H dc is increased, J t becomes dissipative ( V dc ≠0) and ϰ ac is determined by the samples's “resistance”.

Interaction between parallel magnetic fields and transport currents in YBCO superconductors

Magnetic AC susceptibility isotherms ( 80 K ≤ T ≤ 83 K) have been measured on an YBa 2Cu 3O 7−x ceramic superconducting rod in longitudinal DC transport currents and magnetic fields. The results are consistent with Critical State Theory predictions when transport and shielding currents are asummed in the same spatial region yielding lower field penetration and minimum magnetostatic energy.

Effect of applied magnetic field on the I–V characteristics of melt-textured YBa 2Cu 3O 7−x superconductors containing grain boundaries

The current-voltage ( I–V) characteristics of sintered and melt-textured YBCO superconductors containing grain boundaries have been studied in the presence of applied magnetic fields up to 1.5 T. The melt-textured samples contain grain boundaries of angles up to 16°. The measurements were performed at 77 K using DC transport current. In applied fields, sintered and melt-textured specimens are found to exhibit power-law I– V characteristics. In both cases, the power exponent n is found to decrease with increasing field and the corresponding decrease in J c. In sintered specimens, this behavior is interpreted in terms of network models where the n value is related to the width of the critical current distribution and the overall J c. It is also found that the network models can represent the I– V behavior in melt-textured specimens including those with grain boundaries. However, if these models do not apply to melt-textured specimens, the power-law I– V characteristics can be represented by flux motion models such as collective flux pinning or a vortex-glass transition.

Fabrication of Biaxially Oriented YBCO on (00l) Biaxially Oriented Yttria-Stabilized-Zirconia on Polycrystalline Substrates

AbstractIon-assisted, ion-beam sputter deposition is used to obtain (00l) biaxially oriented films of cubic yttria-stabilized-zirconia (YSZ) on polycrystalline metal substrates. Yttrium-barium-copper-oxide (YBCO) is then heteroepitaxially-pulse-laser deposited onto the YSZ. Phi scans of the films show the full-width-half maxima of the YSZ (202) and the YBCO (103) reflections to be 14° and 10° respectively. Our best dc transport critical current density measurement for the YBCO is 800,000 A/cm2 at 75K and 0T. At 75K, the total dc transport current in a 1 cm wide YBCO film is 23 A.

Current transfer in grain-boundary junctions of Bi(Pb)SrCaCuO thick films

This paper is devoted to the experimental study of the mechanism of energy dissipation caused by DC transport currents in superconducting Bi(Pb)SrCaCuO screen-printed films. Current-voltage characteristics were measured by the standard four-probe method, and their magnetic field and temperature dependence for different film widths was systematically investigated. The magnetic field and temperature ranged from 0 to 10 mT and from 4.5 to 120 K. Some peculiarities of the current-voltage characteristics were observed. Josephson-type vortex-antivortex pairs were generated at the edges of the film and they slip into the inner part of the film resulting in energy dissipation. This phenomenon appeared in the current-voltage characteristics as a linear part. The evaluation of the film width dependence of the critical current made it possible to calculate the Josephson penetration depth with the numerical result: λ j ∼50μm. At higher currents the voltage increased much faster than in the linear part and the characteristics became strongly nonlinear. A clear distinction could be made between the critical-current limitation caused by flux motion and that, at which the curve started to be nonlinear. The difference between these two dissipation mechanisms was demonstrated by quantum interference patterns observed in the nonlinear section of the current-voltage curves.

Evidence for unpinned magnetic flux vortices above theI ctransition in the 2223-phase high-T csuperconductor

A similarity in the dc voltage-current (V-I) curves for both direct and alternating transport currents is used to propose that unpinned flux vortices are generated above theI ctransition for dc transport currents, when Abrikosov flux vortices begin to penetrate the superconductor. Two methods can be used to give a dc voltage drop for an ac transport current: (1) if there is a slight dc offset voltage in the ac current which favors vortex loop collapse as in a traditional dcI ctest, or (2) if an asymmetric transverse magnetic field is present which favors vortex loop collapse for current in one direction over the reverse direction.

AC and DC transport current in melt-grown YBCO

It has been suggested that the transport J/sub c/ in multigrain samples of bulk YBCO is limited by the intergrain links. The authors report on preliminary measurements of intergrain currents. The intergrain critical currents in melt-grown YBCO do not appear to be as sensitive to the precise crystallographic alignment of adjacent grains as has been reported for thin films. The measured critical current of similar grain boundaries varies widely, between 15000 A/cm/sup 2/ and 200 A/cm/sup 2/ for adjacent boundaries in the same sample. Preliminary results support the conclusions of S.E. Babcock et al. (Nature, vol.347, p.167, 1990), who found weak-link-free behavior in some high-angle boundaries between bulk crystals.

Hall probe magnetometer for SSC magnet cables: effect of transport current on magnetization and flux creep

A Hall probe magnetometer has been constructed to measure the magnetization hysteresis loops of Superconducting Super Collider magnet cables. The instrument uses two Hall-effect field sensors to measure the applied field H and the magnetic induction B. Magnetization M is calculated from the difference of the two quantities. The Hall probes are centered coaxially in the bore of a superconducting solenoid with the B probe against the sample's broad surface. An alternative probe arrangement, in which M is measured directly, aligns the sample probe parallel to the field. M was measured as a function of H and field cycle rate both with and without a DC transport current. Flux creep as a function of current was measured from the dependence of AC loss on the cycling rate and from the decay of magnetization with time. Transport currents up to 20% of the critical current have a minimal effect on magnetization and flux creep.

AC loss measurements of Nb/sub 3/Sn wire carrying transport current

The AC losses of internal-tin Nb/sub 3/Sn wires have been measured by an isothermal calorimetric method in triangular-wave fields at ramp rates of up to 1.2 T/s and in small-amplitude ripple fields biased with DC fields of up to 9 T. Measurements were made both with and without transport currents, which were both AC and DC. Without transport currents, losses had near-parabolic profiles when plotted against bias field with constant ripple amplitude, against triangular-wave ramp rate, or against triangular-wave ramp amplitude. With transport currents of up to 90% of critical, losses for triangular-wave field were shown to increase up to 17% with increasing DC currents and up to 10% with increasing AC currents. With DC transport currents, the scaling of loss by (1+i/sup 2/), where i is the ratio of transport current to critical current, may be a good approximation.

Microwave absorption in the presence of dc transport current in polycrystalline YBCO

Low-field microwave absorption in polycrystalline YBa2Cu3O7−x was measured as a function of dc current applied through the sample at various temperatures below Tc. The peak-to-peak value of the magnetically modulated microwave absorption (MMMA) signal increases with dc current at a fixed temperature. Above a threshold value of the current, the absorption signal level rapidly drops to zero, indicating quenching of superconductivity. If this critical current is plotted as a function of temperature, the results can be explained on the basis of the flux creep model. The MMMA signal obeys a scaling rule as predicted by the flux creep model. We conclude that flux creep limits the critical current density in these materials; also, this presents a new way of measuring critical current densities while using only a two-contact method. It gives some insight into the relationship between microwave properties and dc transport phenomena in these superconductors.

Effects of dc transport current on low-field microwave absorption in ceramic superconducting YBCO samples

The effects of dc transport current on low-field microwave absorption have been investigated systematically on bulk ceramic YBa2Cu3O7−δ samples. At a fixed temperature T, the critical current Ic(T) at which the electron paramagnetic resonance (EPR) signal vanishes varies linearly with T. The EPR absorption characteristics obey a scaling rule in accordance with the flux creep model. A revised version of the flux creep model is also presented.

Simplified loss expressions for superconductors carrying DC transport current in an AC field

Simplified expressions are reported for the shielding and transport loss components as a function of the applied transverse field and the transport current. These expressions are based on a numerical solution but are approximated by analytic expressions in such a way that they are as simple to use as the equivalent expressions derived from the one-dimensional slab model. A comparison of the two models is presented.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

AC losses in ultra-fine filament NbTi superconductors

Loss measurements are presented on an ultrafine filament composite and on a regular AC conductor, having "thick" filaments. The wires have been subjected to an alternating transverse magnetic field. An AC as well as a DC transport current have been fed through the wires. The magnetic field and the AC transport current have been sinusoidal functions in phase. The magnetization losses and the transport current losses have been measured separately. The critical current - according to the 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-14</sup> Ωm criterium - and the quality factor n - according to V = V <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</inf> (I/I <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</inf> ) <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">n</sup> - have been determined.