NbTi Wires Research Articles

Fluctuation Effects on a Strongly Pinned Vortex Lattice in a Thin Type-II Superconducting Wire.

We report on transport properties of the vortex lattice in fine, round NbTi wires fabricated by a novel wire-drawing process. The voltage onset in the isothermal $I\ensuremath{-}V$ characteristic is sharp for thick wires ($d\ensuremath{\ge}1$ \ensuremath{\mu}m) at 4.2 K and 6 T, but is rounded for thinner wires. The thinnest wires also display an Ohmic resistivity at low current densities. This linear resistivity decreases exponentially with temperature near ${T}_{c}(H)$ and approaches a constant value at low temperatures. We explain our results as finite size effects of the thermal and quantum collective creep of vortex lines.

A new definition of the critical current in low T c superconductors based on the Ambegaokar-Halperin model

We report on a new way to determine I c for low temperature superconductors. We have analyzed a series of I–V curves taken at 4.2 K in magnetic fields from 0 to 10 Tesla. Our results on multifilamentary NbTi wire in an Al matrix are used to illustrate this technique. The I–V curves are analyzed using a model based on the Ambegaokar-Halperin equation for a single Josephson junction. The critical current defined in this fashion is larger than that defined from the traditional E-field criterion.

Experimental investigation of copper matrix longitudinal resistance in a composite Nb-Ti wire

Copper-matrix longitudinal resistance was studied as a function of wire diameter on specimens of multifilamentary Nb-Ti wire with filaments coated by a Nb layer. Experimental results are compared with the Sondheimer calculations (1952) for a monofilament conductor as well as with the Cavalloni calculations (1982) for an ideal wire with hexagonally located filaments. It has been shown that the best fit with the experiment is provided by the Sondheimer approximation. Comparison of the results of this work with other authors' data obtained for the specimens with no niobium barrier, allows us to single out the influence of a pure size effect and diffusion of Ti on the resistivity.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Flux dynamics in submicron superconducting NbTi wires

We find that the diameter of a superconducting filament plays a major role in the pinning and depinning of flux lines as the diameter is reduced to the submicron scale. We attribute this behavior to a finite-size effect of flux pinning.

Factors affecting the upper critical field of Nb [sbnd]Ti wires

The intercomparison on the upper critical field measurement in the Nb wire(NST SRM 1457) has been performed through the VAMAS activity. The homogeneity in the Hc 2 of the wire was good enough, and the coefficient of scatter in the Hc 2 intercomparison was about 0.60 %. Effects of several factors on the HC of the Nb Ti wire are reported in the present paper. The coiled specimen shows slightly lower Hc 2 than the straight sample. The Hc 2 of the wire decreases by annealing, and recovers by wire drawing, The HC 2 of the wire shows an appreciable anisotropy with respect to the direction of applied field. The anisotropy in HC 2 is eliminated by annealing, and reintroduced by the subsequent wire drawing.

Indirect measurement of nbti wire temperature field response to local heat input

Indirect measurement of temperature field profile on the NbTi multifilament wire locally heated by steady or transient thermal disturbance is presented. Contrary to point like measurements by thermometers glued upon the wire surface it can be applied for continual mapping of the T(x) profile. Especially useful may be the method in the case of tiny conductors where influence of gauge thermal conductivity and capacity may result in distortion of axial temperature field profile as well as its dynamics. Based upon experimental T c(B) data it can be applied both for steady local heating in I c(B,T) measurements or in monitoring of T(x,t) dynamics during superconductor recovery after transient heat input.

D.c. magnetization study of fine-filamentary NbTi superconductors

Measurements of d.c. magnetization M( H) and magnetic susceptibility χ( T) in perpendicular and parallel magnetic fields were performed by a SQUID magnetometer on four multifilamentary NbTi wires with a Cu matrix and a superconducting filament diameter ranging from 11.8 μm up to 0.54 μm (these samples belong to the VAMAS reference samples). Anomalies in the χ( T) and initial M( H) curves were observed in samples having lower filament diameters in the perpendicular magnetic field. The initial linear M(H) slope of these samples corresponds to perfect diamagnetism of their strands, indicating proximity effect coupling of the NbTi filaments. At higher fields, a second linear slope of M( H) was observed, corresponding to diamagnetism of individual NbTi filaments. From this slope, using the M( H) relation for the Meissner state of a cylinder in a perpendicular magnetic field, the London penetration depth λ was determined. Hysteretic effects were observed for samples having lower filament diameters in fields corresponding to the second linear M( H) slope, indicating a complex proximity effect pinning structure within the strands. Deviation from linearity of the two linear M( H) parts allows us to evaluate H c1 of the strands and NbTi filaments. Estimated H c1 values from the Ginzburg-Landau theory were lower than the measured values. Measurements in the parallel magnetic field did not indicate any proximity effect coupling within strands, with the exception of the sample with a filament diameter of 0.54 μm, where H c1 values were quite close to those calculated by the Ginzburg-Landau theory.

NbTi split magnet directly cooled by cryocooler

The experiment on a single coil directly cooled by a G-M cryocooler has been done to develop a superconducting magnet with several tesla without using liquid helium. The size of the coil was the outer diameter of 140mm, the inner diameter of 30mm and thickness of 39mm. A multifilamentary wire of NbTi wire was wound on a copper bobbin and immersed by wax. The cooldown process and operating conditions were optimized. As a result, the maximum magnetic field of 6.3 T was obtained at 6 K. Then the stored energy and the overall current was 2.9kJ and 286A/mm2, respectively. The NbTi wire used was made full use of its performance as a superconducting conductor. It was indicated that the critical state of the superconductor at the operated temperature was realized satisfactory.

Critical current density in superconducting Nb-Ti under proximity effect

It has been found that the critical current density J c in a multifilamentary superconducting Nb-Ti wire first increases and then reaches a peak, while the critical temperature decreases monotonically as the superconducting filament diameter is decreased by drawing to the submicrometre range. This behaviour originates from a variation in the morphology of the normal α-Ti phase precipitated during heat treatment, coupled with the proximity effect between the superconducting matrix and normal α-Ti. In this paper the elementary pinning force f p of α-Ti ribbons is theoretically estimated for idealized multilayers of superconducting and normal regions, and J c is calculated from a statistical summation of the elementary pinning interactions. The theoretical result predicts that J c increases monotonically with drawing, owing to the increase in density of pin points, in spite of the decrease in f p. The reason for the observed degradation of J c is discussed.

AC loss time constant measurements on Nb/sub 3/Al multifilamentary superconductors

AC loss time constants have been measured by an inductive method developed at the Japan Atomic Energy Research Institute. Measurements were performed on four multifilamentary Nb/sub 3/Al test wires, manufactured by the jelly-roll method and one multifilamentary NbTi test wire. The theoretical AC loss time constants were calculated and compared with the measured values. The time constants of the Nb/sub 3/Al wires were found to vary as the square of the twist pitch as predicted. The Nb/sub 3/Al wires have time constants that are higher than the lower limit predicted by theory, suggesting that the contact resistance between the copper matrix and superconducting filaments is lower than that in the NbTi wire. >

Material and electro-magnetic aspects of newly developed Nb-Ti wires for AC use with Cu-Si alloy matrix

The material characteristics of Cu-Si alloy as a new matrix material for AC superconducting wires are presented. The successful trial fabrication of submicron-diameter Nb-Ti multifilamentary wires with Cu-2.5 wt.% Si alloy matrix is described. The wires, which had filament diameters down to 0.1 mu m, showed high enough critical current density and low AC loss compared to Nb-Ti AC superconductors with a Cu-Ni alloy matrix. >

Concept for a high-resolution thermometer utilizing the temperature dependence of the magnetic penetration depth

A thermometer using the temperature dependence of the magnetic penetration depth in superconductors is described which has the potential for temperature resolution, when using a DC superconducting quantum interference device (SQUID) readout, on the order of 1 pK. One such device has been fabricated and characterized to demonstrate proof of concept. It consists of primary and secondary coils of NbTi wire wound on a copper toroidal core on which a thin ( approximately 15-nm) layer of In (T/sub c/=3.4 K) has been deposited. The temperature dependence of the mutual inductance, M(T), or self-inductance, is used to detect changes in temperature. Measurements of M(T) have been made with an AC excitation of the primary for various frequencies and peak magnetic field strengths. Estimates of ultimate temperature resolution are given.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

AC susceptibility and filament coupling of multifilamentary superconducting wires

The authors have studied proximity effect coupling on multifilamentary wires by measuring AC susceptibility. AC susceptibilities were measured on three kinds of samples. One was a Nb-multifilamentary wire with a Cu matrix, and the others were NbTi wires differing in the matrix, Cu or CuNi. A peak of the proximity effect coupling loss was seen as a function of temperature. It was foud that the peak temperature depends strongly on the measuring field and filament spacing. It decreased consistently with an increase in the applied field and the filament spacing. The coupling depends on the matrix, and no coupling peak was observed for the CuNi matrix wire at temperatures above 4.2 K.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

History effect of critical current density in superconducting multifilamentary Nb-Ti wire

DC magnetization and four-probe resistive measurements were carried out for Nb-Ti multifilamentary wires, and critical current densities in the filament and matrix were separated. The Campbell method was also used for measuring the effect of magnetic field history on critical current density in the matrix. The history dependence was found only for the current density induced in the matrix due to the proximity effect. This close correlation between the proximity effect and history effect shows a similarity to the phenomenon in high-T/sub c/ superconductors. The possible reason for the history effect in critical current density is discussed. >

Extended measurements of the resistive critical current transition

The resistive critical current transition has been measured in a variety of Nb-Ti wires over several decades in voltage. The measurements were made as a function of magnetic field for wires with a variety of processing conditions, leading to both intrinsic and extrinsic limitations to the critical current (I/sub c/). Measurements of the wire geometry by image analysis have been used to assess the extent of extrinsic limitation of I/sub c/ and to see if any correlation exists between the V(I) transition and the filament geometry. A weak correlation between the linear variation in the cross-sectional area of the filaments and the I/sub c/ distribution of the samples was observed. >

Ac low-frequency magnetic measurement of the proximity effect between fine filaments of superconducting NbTi wires

The magnetic weak coupling between superconducting fine filaments of multifilamentary NbTi wires for high-energy-physics applications (dipole magnets for large hadron collider at CERN) is studied through ac magnetic measurements down to very low frequencies (0.5 Hz). The shielded and trapped magnetic fluxes are measured versus the temperature at fixed ac field superimposed on a constant dc field. The measured curves look like measurements on high-Tc superconductors with the characteristic two maximum values of the trapped magnetic flux. One of these two peaks can be related to the superconducting transition of NbTi filaments, the other one to the weak coupling between the filaments. The measurements of the field and the temperature dependence of the latter peak allow the critical current of the weak coupling versus the field to be calculated giving interesting information about the proximity effect in the superconducting-normal-superconducting array.

ＮｂＴｉ極細多芯超伝導線の臨界電流密度とピンニング特性 Ｉ． 熱処理線

Critical current density Jc and its temperature dependence in multifilamentary NbTi wires were measured. Further their pinning behavior was examined. Used samples which have been fabricated by a double stacking method are divided into two groups. One group was heat-treated in the fabrication process in order to produce the precipitates of α-Ti and the filament diameter df of NbTi wires ranges from 0.39 to 1.53μm. For the other group, such a heat-treatment was not done and the df of some samples was reduced to the level of below 0.1μm. In this paper, the experimental results on the former group, namely heat-treated (HT) samples with Cu-CuNi mixed matrix or CuNi matrix, are given. The Jc of samples is affected by df. In case of 4.2K, these samples show a slight difference in Jc at high fields. As the temperature rises, the effect of df on Jc becomes remarkable and the Jc-B curves are very different among samples. Normally, samples with small df show low Jc values and reveal a rapid decrease in Jc with increasing magnetic field. An anomalous change in Jc at low fields of below 0.1T was seen at temperatures close to Tc for samples with sub-micron sized filaments. The pinning force Fp vs. reduced field b(=B/Bc2) curves at 4.2K for different samples coincide with each other except for the sample with df=0.39μm. The reduced pinning curves (Fp/Fp, max vs. b) obtained at various temperatures have similar shapes for each sample. The bm where the maximum of Fp takes place is around 0.35 except for the sample with df=0.39μm. The pinning properties of these samples are discussed in terms of their upper critical field Bc2 and transition temperature Tc.

Nonlinear acoustic properties of vibrating wires at very low temperatures

We have investigated the acoustic properties of superconducting NbTi and Ta wires in vacuum at 1 mK≤T≤1 K and at a few kHz. The temperature dependence of the acoustic properties of the wires is similar to that found in amorphous materials: a maximum in the sound velocity and a plateau in the dissipation at temperatures above it. In addition, we have observed a strong influence of the acoustic power on the measured properties in agreement with recent measurements on amorphous SiO 2 . This strain dependence can be interpreted by a modification of the tunneling model as a change of populationS of the two-level system energy states in non-crystalline materials. We can explain the strain-dependent anomalies as the superposition of three nonlinear effects: the change of population of the tunneling systems energy states, self-heating of the wire, and a nonlinear restoring force. These effects have a strong influence on the lineshape of the resonance curve. In addition, we have investigated the behavior of vibrating wires in liquid 3 He and liquid 3 He- 4 He solutions atT<100 mK. We can show that in superfluid 3 He-B atT<0.2 mK or in solutions of 3 He in 4 He with a small concentration of 3 He it is impossible to use a vibrating wire as a viscometer without having exact information about its intrinsic properties.

Ultrafine multifilamentary Nb  Ti wires with Cu  Si alloy matrix

The Cu  Si alloy has been proposed as a new matrix material for filamentary Nb  Ti wires in a.c. use. The Cu  Si alloy shows appropriate mechanical and electrical properties, and is economically more favourable than the Cu  Ni alloy matrix used currently. Moreover, the addition of Si to Cu prevents the formation of intermetallic compounds around the filaments. After extensive investigations on CuSi alloy as a matrix material, an ultrafine multifilamentary Nb  Ti/Cu − 2.5 wt% Si composite wire has been successfully fabricated. The d.c. superconducting properties of the wire were adequate for use in electrical apparatus. A preliminary study has revealed that the a.c. loss of the new wire is equivalent to that of a typical ultrafine multifilamentary Nb  Ti/Cu  Ni composite wire.

Conceptual design and development of a superconducting bus-line for the Large Helical Device

A superconducting bus-line is proposed and preliminary tested as an electrical feeder between the superconducting coils of the Large Helical Device (LHD) and their electrical power supply. The bus-line consists of a superconductor and its cryogenic transfer-line. The superconductor is a specially developed aluminum stabilized NbTi wire, which is installed in the innermost channel of the transfer-line. The vacuum insulated transfer-line consists of four corrugated tubes assembled coaxially. Liquid helium flows through the innermost channel and shield gas flows through another annular channel in the line. We are completing the conceptual design of the bus-line and the installation plan for the LHD experimental hall and are carrying out development of wires, including an investigation of their mechanical properties and electrical insulation. This report describes the conceptual design of the superconducting bus-line for the LHD, and the results we obtained recently during the design and development of a full-scale demonstration facility.