Multifilamentary Superconductors Research Articles

Index number (n) below "critical" current in Nb-Ti superconductors

For multifilamentary superconductors such as Nb-Ti and Nb/sub 3/Sn, a phenomenological theory ("sausage model") predicts a monotonic increase in the index number n as operating current is decreased below a "critical" current. An experiment was conducted to test the prediction; preliminary experimental results appear to confirm the prediction. For multifilamentary Nb-Ti with a measured n value of 26 at 3.4 T and 4.2 K - deduced from the shape of a V versus I plot at the conductor's nominal critical current of 542 A (defined at E/sub c/=0.1, /spl mu/V/cm) - the experiment reported here finds n to increase to 40 at I/sub op//I/sub c/=0.8; this increase in n agrees quite well with that predicted by the theory. One important implication of this theory is that it is permissible to specify n at I/sub c/ to be smaller than the required value if the conductor's normalized operating current (I/sub op//I/sub c/) is to be smaller than 1. The theory also can quantify n at I/sub c/ for required operating values of n and I/sub op//I/sub c/.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

人工ピン型NbTi交流用超電導線材の開発とその応用

人工ピン型NbTi交流用超電導線材の開発とその応用

Recent advances in bismuth-based superconductors

Significant progress has been made in the development of high-critical-temperature (high-Tc) mono- and multifilament bismuth-based superconductors by the powder-in-tube (PIT) technique. High critical current density (Jc) has been achieved in both short- and long-length monofilament conductors. Jcs up to 1.2 × 104 A/cm2 were achieved in an 850 m long multifilament conductor. Pancake-shaped coils and test magnets, fabricated from long conductors, were characterized at various temperatures and applied magnetic fields. One such magnet containing 480 m of high-Tc tape generated a record-high field of 2.6 T at 4.2 K. Multifilament conductors appear to possess better mechanical properties and retain a higher percentage of their initial critical current under strain than monocore conductors, which is important for practical considerations. PIT processing of superconducting tapes with Ag-Al2O3 as the sheath material is perhaps another route to improve mechanical properties.

Current distribution in multifilamentary superconductors and their quench current degradation

When the thickness of the saturated region where the current density reaches the mean critical current density is larger than some threshold value, flux jump occurs. In multiply-twisted cables, such as ones for AC use, or in complicated magnet systems, such as ones for fusion devices, multifilamentary superconductors are exposed to the longitudinal magnetic field that is possible to increase the thickness of the saturated region and to degrade the quench current. The current distribution in multifilamentary superconductors is calculated with two models. With the first one, its temporal evolution and axial variation can be calculated. This is also useful to calculate AC losses. In the second model, the influence of the transverse magnetic field and the filament diameter as well as of the longitudinal magnetic field is taken into consideration. With this model, the transport current at the stability limit against the longitudinal magnetic field instability is estimated. The influences of the critical current density, filament diameter and conductor diameter on the stability limit are evaluated.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Advances in dilution refrigeration in high magnetic fields

Abstract 3 He/ 4 He dilution refrigerators have become a standard laboratory tool used to cool experiments to the milli-kelvin temperature range. Many of these applications involve the use of high magnetic fields. One system includes a 20.7 T superconducting magnet (the highest field ever achieved using multi filamentary superconducting wire in compact magnet form) and it can cool samples to 20 mK. Other systems have been designed for use with rapidly sweeping superconducting magnets, and even for pulsed liquid nitrogen cooled magnets, with maximum fields up to 60 T.

Critical current degradation of a NbTi-multifilament conductor due to heat treatment

In the framework of the development of superconducting coils for the stellarator Wendelstein 7-X, the reduction of the critical current due to thermal treatment was investigated. During production steps like the extrusion of the Al-jacket onto the superconducting cable and also during welding and soldering, heat flux into the NbTi-strands occurs. For optimized NbTi-strands a degradation of the critical current was expected. The results of measurements of the critical current of an annealed NbTi multifilamentary superconductor at 4.2 K in the field range from 2 T to 10 T are presented. The commercially available conductor has been annealed at various temperatures up to 600/spl deg/C and times up to 120 s in order to simulate the conditions during such production steps. Special attention is paid to the shape of the voltage-current transition which is characterized on the one hand by a power law and on the other hand by an exponential function. >

Normal domains in large multifilamentary composite superconductors

We study the dynamics of normal domains in a multifilamentary composite superconductor with a large amount of stabilizer outside the multifilamentary area. We consider the case when there is a transition layer with high thermal and electrical resistance between the superconductor and the stabilizer. We show the existence of localized and propagating normal domains in these composites. We derive an implicit expression for the propagation velocity of a normal domain.

An experimental study of the transport properties of a flexible high-Tc superconducting cable

The powder-in-tube method was used for fabrication of multifilamentary (up to 10 filaments) superconductors in an Ag sheath of 9 mm and 16 mm diameter and several metres in length; corrugated cable elements with Y and Bi ceramic conductors were placed in the wall of the tube. After annealing at 920 degrees C for 72 h in air the specimens of composition Y-123 had the critical temperature Tc=85 K and the Bi-2223 specimens annealed at 850 degrees C for 100-120 h had Tc=105 K. The superconducting transition width of the specimens was about 1 K. The critical current density at 77 K was 103 A cm-2 for all specimens in zero magnetic field.

Comparison of image analysis results on α-β microstructures in NbTi superconductors using BSE imaging in FESEM and STEM

The critical current density (Jc) of NbTi multifilamentary superconductors having α-β microstructures is affected by the volume fraction, size, and spacing of small oc-Ti precipitates dispersed in the β-matrix. These morphological parameters are controlled in processing by interspersed cold drawing and heat treatment. They can be readily quantified by image analysis techniques, provided that representative binary images can be obtained which include the entire population of small α-Ti precipitates (0.03-0.4 μm). We have obtained BSE images of these α-β microstructures that are suitable for conversion to binaries and subsequent image analysis using either a FESEM or a high voltage STEM. Here we report on BSE imaging at low voltage in a FESEM and compare results with those previously reported for BSE imaging in a 300 kV STEM.

Material science problems in manufacturing of superconducting wire for iter magnetic system winding

Abstract The paper discusses material science problems in producing a superconducting wire for ITER magnetic system winding. Results of investigation and development of methods for producing single superconductors containing 45 000 niobium filaments 2 mm diameter including made up of largely composite precursors up to 300 mm diameter, are presented. The problems are considered related to the manufacture of copper channel shapes with a resistive barrier. Data on the development of soldering schedules for winding wire elements and leaktight welding of a steel sheath containing soldered transposed multifilamentary superconductors and copper channel shapes are given. The need is indicated to develop non-standard equipment lines for manufacturing long pieces of a winding wire.

Some effects of matrix additions to Nb 3Sn and NbTi multifilamentary superconductors

Abstract The addition of alloying elements to the copper matrix separating the filaments inmultifilamentary superconductors can have beneficial effects in the efforts to improve current carrying capacity, Jc, without increasing losses, reducing stability or decreasing ductility and piece length. Examples of the benefits are given for both Nb3Sn and NbTi practical superconductors.

Time-variable resistive voltage in superconductors with imperfect Nb 3Sn structure

An unusual hysteresis was observed in the voltage-current characteristics of rapidly heat treated multifilamentary Nb 3 Sn superconductors. The hysteresis is characterized by a lower resistive voltage on increasing the transport current than on decreasing it. The reason for this trend is the time instability of the resistive voltage at constant current, whereby the actual magnetic field and temperature are kept constant. The phenomenon is explained by an unstable and changing current path system caused by a small temperature rise in the disintegrated structure of the incompletely reacted and non-stoichiometric Nb 3 Sn layer. The observed phenomenon in the low T c superconductor is reminiscent of the well known relaxation processes in high T c superconductors.

V3Si multifilamentary superconductor produced by a modified bronze process

A V3Si multifilamentary superconductor with high overall Jc has successfully been fabricated by a modified bronze process. A double-stacked Cu-8.5 at. %Si/Ta/(Cu-8.5 at. %Si/V) composite with fine V filaments (∼1 μm) was prepared, where the primary bundle is sheathed with a Ta diffusion barrier; thus only Si in the bronze inside of the Ta barrier is available for the subsequent diffusion reaction. The combination of adjusting the total proportion of V to Si in the composite to ∼3 and reducing V filaments to ∼1 μm allows the completion of reaction in short times to decompose the initially formed V5Si3 and produce mostly V3Si without grain growth. The overall Jc(4.2 K) values so far obtained are 1.3×109 A/m2 at 5 T and 1×108 A/m2 at 12.5 T, which are 10 times larger than conventional V3Si wires. These values are comparable to those of Nb3Sn multifilamentary wires produced by the bronze process for ac use.

Influence of longitudinal magnetic field on thermomagnetic instabilities in a.c. superconducting cables

Abstract As the critical current of a single a.c. superconducting wire cannot be more than several tens of amperes, such single wires (strands) are bundled and twisted to make cables with a large current capacity. As the result of the twisting and bundling, a longitudinal magnetic field parallel to a strand axis is produced in the cables and windings by the transport current. It is shown theoretically and experimentally in this paper that the a.c. longitudinal magnetic field influences the current distribution in multifilamentary superconductors with an a.c. transport current and can induce thermomagnetic instabilities in them. A longitudinal magnetic field of the order of 10 −1 T is easily produced in these cables. It is shown that thermomagnetic instabilities induced by the longitudinal magnetic field in the cables cause a.c. quench current degradation in multiply-twisted a.c. superconducting cables. A way of twisting to decrease these instabilities is also proposed.

Unexpected time dependence of ac losses in multifilamentary NbTi superconductors

Losses of NbTi multifilament superconductors in a copper matrix in alternating magnetic fields were measured using a specially developed sensitive calorimetric method. The investigated samples have filament diameters in the micrometer range and exhibit a strong proximity effect. The measured losses at fixed ac frequency and field amplitude showed an unexpected time dependence on a very long time scale of hours, compared to the ac field cycle time, which was in the millisecond range. This time dependence is probably a consequence of slowly decaying proximity currents.

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. >

The evolution of tooling, techniques, and quality control for accelerator dipole magnet cables

The author summarize the results of the Superconducting Super Collider (SSC) R&D program from 1984 to 1990. The SSC R&D program has led to a significant improvement in critical current, piece lengths, and overall manufacturing capability for multifilamentary NbTi superconductors. The requirements for current density, filament size, dimensional control, long lengths, and low current degradation are much more stringent for the present accelerators compared with the earlier Tevatron and HERA accelerators. Also, to achieve higher field strengths with efficient use of the superconductor, the new designs require wider cables with more strands. These requirements have stimulated an active research effort which has led to significant improvements in critical current density and conductor manufacturing. In addition, they have stimulated the development of new cabling techniques, improved tooling, and better measurement techniques. The need to produce over 20 million meters of cable has led to the development of high speed cabling machines and online quality assurance measurements. These new developments are discussed, and areas still requiring improvement are identified. >

Coupling losses of multifilamentary superconductors having several concentric regions and mixed matrix

A method is presented to calculate the coupling loss in multifilamentary conductors containing a mixed matrix. The conductor geometry considered has several concentric regions with either isotropic normal material, anisotropic normal material, or superconducting filaments. In anisotropic normal regions, containing for example Cu hexagons in a CuNi environment, the effects of anisotropy and twist have to be taken into account. An interesting similarity is observed between such regions with highly anisotropic normal material and regions with superconducting filaments. >

(Nb, Ti) 3Sn forced-flow-cooled superconductor

Abstract A low ac loss (Nb, Ti) 3 Sn bundle-type forced-flow-cooled multifilamentary superconductor was developed for application as a toroidal field coil (dc operation) and a poloidal field coil (pulsed operation) of the fusion reactor. The diameter of the filament was reduced to 1.42 μm (the finest possible in the present technical stage) and filaments in the bronze matrix and the stabilizing copper were segmented by the high resistive CuNi layer and Nb barriers for the reduction of ac losses. The ac losses of a superconducting strand measured by an improved boil-off method are lower than for an ac superconducting (NbTi/Cu/CuNi) wire. A small forced-flow-cooled superconducting coil manufactured with this conductor (rated current I d = 2.5 kA, B max = 10 T) showed a high stability margin in dc operation. Repeated pulse operation to the rated current with a high sweeping rate of 13 kA/s (0.85 T/s) was possible without quench in a backup field of 10 T.

複合多芯線内部の電流分布と磁気的不安定性に与える外部縦磁界・横磁界の影響

複合多芯線内部の電流分布と磁気的不安定性に与える外部縦磁界・横磁界の影響