Multifilamentary Superconductors Research Articles

Effect of strain on the critical current of Nb-Hf/Cu-Sn-Ga multifilamentary superconductors

The critical current of multifilamentary Nb-Hf/Cu-Sn-Ga has been determined as a function of both field and strain at fields from 4 to 19 T. The strain dependence of the critical current is significantly reduced compared with that of commercial multifilamentary Nb3Sn superconductors. This reduced strain sensitivity, coupled with an enhanced critical-current density above 12 T, make this a candidate material for economic high-field magnet construction. An analysis of the data in terms of the strain scaling law indicates that the Hf and Ga additives do not produce this improvement through a decrease in the strain sensitivity of the bulk upper critical field B*c2. Rather, they reduce the strain sensitivity of the critical current through a changed shape of the flux pinning curve and an enhancement in the strain-free maximum value of B*c2 (B*c2m = 25 T for Nb-Hf/Cu-Sn-Ga versus 21 T for Nb3Sn).

Hysteresis losses of multifilament superconductors in superimposed dc and ac magnetic fields

Abstract Theoretical investigations, magnetization and calorimetric ac loss measurements have been performed on the problem, how the superposition of dc- and ac-magnetic fields influences the hysteresis losses of filamentary superconductors. For a given ac-field amplitude the maximum loss value as a function of the dc field can easily be predicted without knowledge of the superconducting properties.

Review on AC losses in superconductors

Paper presents principal results obtained during the last years in the research of AC losses in superconductors with regard to the magnet technology. Special emphasis is given on these aspects which are important in demonstrating the technical feasibility and economical advantages of AC superconducting windings. In the domain of hysteresis losses paper reports mainly on the extension of the theory of losses in superconducting cylinder placed in transverse alternating and/or rotating magnetic field of arbitrary amplitude as well as on the problems related to the case when the field is not perpendicular to the cylinder axis and the anisotropy of pinning force is important. The progress in substantial reducing of the filament diameter by improving existing technologies of multifilamentary superconductors as well as by elaborating new ones /in situ/ and its impact on hysteresis losses is presented. Results obtained in the research of eddy current and coupling current losses at high amplitudes and rates of field variation, as well as at presence of DC and/or AC transport current in multifilamentary conductors in transverse alternating and/or rotating fields are discussed. Some problems arising in high current monolithic and/or multistage assembled cables with regard to AC losses are briefly reviewed. Resuming the mentioned theoretical and experimental results and using the practical experience of pulsed superconducting magnets for frequencies of about 1 Hz main lines of a program of developing AC superconducting windings for higher frequencies are outlined. The author is with Electrotechnical Institute of the Electro-Physical Research Center, Slovak Academy of Sciences, Dúbravská cesta, 809 32 Bratislava, Czechoslovakia

Nonequilibrium superconductivity characterized by electric charge

The nonequilibrium superconductivity in the presence of electric charge is investigated with use of temperature Green's function technique. The suppression of energy gap is found with the accumulation of charge. Some evidences are presented which reveal the effect of charge on the stability of current-carrying multi-filamentary composite superconductors.

Volume pinning force and upper critical field of irradiated Nb3Sn

Irradiation by neutrons and ions in A15 superconductors (Nb3Sn, V3Ga) exerts a stronger influence on the pinning behavior than in nonordered alloys (NbTi). In this work it is shown for deuteron irradiated Nb3/Sn wires prepared by the bronze process that the dose curve of the volume pinning force PV can be conveniently described by a sum of two terms, due to the grain boundary pinning and to the radiation pinning, respectively. After deduction of the contribution by the radiation-induced pinning centers, good agreement is obtained between the measured PV values and those calculated using the upper critical field Bc2 and the transition temperature Tc on the basis of the irradiation fluence. The use of a theoretical relationship between Bc2 and Tc is supported by measured values. Application to multifilamentary superconductors with high current carrying capabilities simplifies the calculation of PV, since the radiation induced volume pinning force can be neglected.

The effect of transport current and saturation on the losses of multifilamentary superconducting wires

A slab model is used to investigate the effects of saturation of the superconducing filaments on the losses of multifilamentary superconductors exposed to a uniformly changing field. It is necessary to consider not only the saturated region (where J = J c ), and the superconducting region, but also the region in which current is transferred to the copper. The volume of this region can be about double that of the saturated region. The losses are in qualitative agreement with previous models for round wires, but it is shown that these models do not use the correct current distributions. Some analytic features of the correct solution are derived but a complete solution requires numerical computation.

Further development of powder processed multifilamentary superconductors

Progress in the development of powder metallurgy (P/M) processing of multifilamentary superconductors is discussed for 4 approaches: 1) Cu-Nb-Sn wires were fabricated using internal microscopic diffusion of Sn produced by Sn powder or prealloyed (SnCu) powder; 2) Cu-NbTa-Sn was produced with prealloyed Nb 4 wt % Ta and Nb 7 wt % Ta powders and external Sn diffusion; 3) hot processing was demonstrated using sintering or hydrostatic extrusions with Cu and Nb, or Cu and NbTa powders with or without Al as a function of particle size; and 4) Nb-Al material was develop ed with a (P/M) process. All show high overall critical currents at high field, good strain tolerance, and promise for scaleup processing.

Variations in the characteristics of the superconducting wire used for the ISR superconducting high-luminosity insertion

A total of 30.000 meters of NbTi solid multifilamentary superconductor, <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">I_{c} \sim 2500</tex> A at 6 T, was tested. For each stretch of wire received, measurements were made to check if the wire met the specifications. The specification and the measuring set-up are described. Systematic measurements of the critical currents, Cu : SC ratio, Ti content and wire size are discussed. A strong variation in the critical current could be correlated to a strong variation in the Ti content.

Losses in superconducting magnets under fast ramp rate operation

AC losses in superconducting Pulse magnets have been investigated in order to establish a method which yields a quantitative calculation of the losses. The pulse magnets, I_{c} \simeq 1000 A and B_{\max} \simeq 5 T, wound from different kinds of Nb-Ti multifilament superconductors, were operated at the ramp rate over the range from 0.1T/sec to 40T/sec. The associated losses were measured by the calorimetric method. The conductor types employed in the present magnets were a monolith with Cu-matrix, a monolith with Cu-Ni barriers and a stranded cable. The transient losses in the magnet wound with the Cu-matrix monolithic conductor were found to be saturated at the ramp rate in excess of 0.25T/sec. In these experiments, no quench was observed. The results were discussed in terms of our loss model: The parameter which characterizes the loss is a dimensionless sweep rate \beta = \tau\dot{B}_{e}/B_{p} , where τ is the relaxation time for the coupling current, \dot{B}_{e} is the external field and B p is the full-penetration field for the composite. For \beta \geq 1 , the composite behaves like a solid conductor, so that the AC loss is expected to be saturated as observed.

A transmission electron microscopy investigation of multifilamentary and tape Nb3Sn superconductors at low temperatures

physica status solidi (a)Volume 62, Issue 2 p. K113-K117 Short Note A transmission electron microscopy investigation of multifilamentary and tape Nb3Sn superconductors at low temperatures C. D. Pitt, C. D. Pitt Department of Metallurgy and Materials Science, Imperial College of Science and Technology, London Search for more papers by this authorR. D. Rawlings, R. D. Rawlings Department of Metallurgy and Materials Science, Imperial College of Science and Technology, London Search for more papers by this author C. D. Pitt, C. D. Pitt Department of Metallurgy and Materials Science, Imperial College of Science and Technology, London Search for more papers by this authorR. D. Rawlings, R. D. Rawlings Department of Metallurgy and Materials Science, Imperial College of Science and Technology, London Search for more papers by this author First published: 16 December 1980 https://doi.org/10.1002/pssa.2210620248 Prince Consort Road, London SW7 2BP, England. AboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinkedInRedditWechat Volume62, Issue216 December 1980Pages K113-K117 RelatedInformation

An additional loss source in cabled multifilamentary superconductors

It is shown that irregularities in the twist rate of a multifilamentary superconducting strand, which may occur in the cabling or braiding process of a high current conductor, give rise to additional matrix currents and enhanced ac losses. Electrodynamic equations for matrix and filament currents in a strand with a sudden change in twist rate are derived and damped wave solutions are given for an external harmonic field as well as for a ramp field. Inder unfavourable conditions the associated loss enhancement may become comparable to the regular matrix losses. This is also verified experimentally by magnetization measurements. Some recommendations for cable design are given.

The effect of demagnetization on the eddy-current loss in a single-layered multifilamentary superconducting coil

The demagnetization effect of a single-layered superconducting coil made of a cylindrical multifilamentary wire on its eddy-current loss is studied for the case of ac magnetic fields parallel to the coil axis. An expression for an effective demagnetization coefficient of the wire is obtained by taking account of the interaction among turns of the coil. It is shown from this expression that the magnitude of the eddy-current loss increases remarkably with the distance between neighboring turns of the coil. The experimental loss values of the wires wound into various kinds of coils are within 20% of the theoretical values.

A pulsed dipole magnet made from a hollow composite superconductor with a circulatory refrigeration system

When a superconducting dipole magnetic field is limited by a value of about 2.5 T using a ‘window frame’ type dipole, the design of such a magnet can be essentially simplified, and the superconducting winding volume can be decreased. If the winding is made of hollow composite superconductor, the cryostat construction is simplified and it is easy to handle with superconducting magnets. In order to estimate the prospects of using ‘window frame’ type dipole magnets with a circulatory refrigeration system for the Nuclotron Accelerator project, a dipole magnet with a length of about 0.4 m, a 5.5 cm aperture and a magnetic field of up to 2.5 T has been constructed and tested at the High Energy Laboratory, JINR. The superconducting cable of the magnet consists of a cupro-nickel pipe with an outer diameter of 5 mm and a wall thickness of 0.5 mm on which multifilament superconductors are cabled. The magnet construction with a two-phase helium circulating refrigeration system is described. The dependences are presented of the critical current degradation and of ac losses on the magnetic field inhomogeneity and hydraulic resistance.

Pulsed magnetic field losses and critical current densities of V 3 Ga and Nb 3Sn multifilament superconductors

Pulsed magnetic field loss measurements have been performed on special V 3Ga filament superconductors, which are known to have very high critical current densities, and on a NB 3Sn filament superconductor. The dependence of the hysteresis losses on the diffusion heat treatment parameters for the V 3Ga layer formation is reported. To analyse the results, the hysteresis losses of hollow superconducting filaments in parallel and perpendicular magnetic fields were calculated. From E.J. Kramer's theory of flux pinning we postulate that, for a given material, upper and lower intrinsic limits of the hysteresis losses are due to the flux line shearing effect. A part of the results was presented at the ICEC 7.

Effect of magnetic properties of a composite superconductor on losses in variable magnetic field. Part 2: Experiment

The theory developed in Part 1 of this paper is compared with the results of experimental measurements of losses in multi-filamentary superconductors in a transverse field. The losses have been measured for different magnetic field variation patterns (sinusoidal, triangular, trapezoidal, single pulses). The numerical results obtained with the aid of the theory are in fairly good agreement with the experimental data. In the majority of the actual cases, analytical expressions for losses can be used.

Effects of magnesium addition to the CuSn matrix in the composite-processed Nb 3Sn superconductor

The effects of the addition of a small amount of magnesium to the CuSn matrix in the composite-processed (bronze-processed) Nb 3Sn super-conductor were investigated. It was found that the magnesium addition significantly suppresses the Nb 3Sn grain coarsening. The fine Nb 3Sn grain structure produced in the Nb/CuSnMg composite enhances the grain boundary diffusion of tin and consequently makes the Nb 3Sn growth rate nearly twice as much as that of the Nb/CuSn composite. The significant increase in superconducting critical current attained in the Nb/CuSnMg composite shows that the magnesium addition is promising for the improvement of the current-carrying capacities of multifilamentary Nb 3Sn superconductors.

Simple, repeatable method for soldering superconducting wire to cryogenic leads

A method is described for making a solder joint between a multifilamentary superconductor and a braided copper wire conductor.

Status of the development of high-field A15 superconductors

The status of the development of multifilamentary high-field A15 superconductors is reviewed briefly. Alternative techniques to circumvent the basic difficulty of fabricating a conductor with a brittle material are described and evaluated. The solid state diffusion or bronze route will remain the dominant technology for some years, however the “In Situ” technique is an attractive alternative with comparable J c and improved mechanical properties. Infiltration might find application in the development of materials for higher magnetic fields than possible with Nb 3Sn and V 3Ga. The future of powder technology is uncertain.

Asymmetry of normal zone propagation velocity with respect to current in a multifilamentary composite superconductor

We have observed the phenomenon of destruction of superconductivity by current in a high current multifilamentary composite superconductor. This bronze stabilized conductor had rectangular dimensions of 0.56 cm × 0.17 cm, contained 67,507 Nb 3 Sn filaments of about 5.8-μm diam and had a self-field critical current at 0 K of about 6.1 kA. The velocity of normal zone propagation was measured on a 56-cm-long sample of this conductor in a vacuum environment. Currents ranged from 200 A to 900 A and initial sample temperatures ranged from 14.5 K to 16 K. We observed a significant dependence of the velocity on the direction of the current. Because there is essentially zero heat transfer into a surrounding environment, the effect must relate to an asymmetry of the fundamental interactions between current and heat flow in the filaments.

Recovery capability of multifilamentary superconductors with non-uniform void distribution

The effects of several abnormal conditions resulting from a possible non-uniform void distribution in a multifilamentary and cabled superconductor on the cryogenic recovery capability of a triplex, a seven strand bundle and a 19 strand bundle were studied. For a triplex, if only one of the three strands was cooled, the maximum recoverable initial normal temperature is reduced by 1/3 of the value for the nominal case in which all the three strands are cooled equally. If the outer six strands of a seven strand bundle were cooled and the center one was not cooled, the recovery capability of this bundle is just slightly below that of the nominal triplex. If only the outer 12 strands of a 19 strand bundle were cooled, the bundle would not recover once it is driven normal. The effects of the cold and stagnant helium located in the space between the strands and of the insulation thickness are small.