A 12 T 80 mm bore superconducting V3Ga NbTi magnet system

A V₃Ga-NbTi magnet system for a field of 12 T in 80 mm clear bore is described. The V₃Ga and NbTi magnets are connected in series to one power supply. The differences in the materials (NbTi multifilamentary wire and V₃Ga bronze processed stranded cable) require special measures for protection during a quench. In the V₃Ga magnet no training or degradation was observed. The short sample critical current value was reached in the V₃Ga magnet with a reversible active voltage which made it possible to reduce the current without quench.     

Coupling-current loss in a multifilamentary superconducting wire with a normal-metal core

Coupling-current losses in a multifilamentary superconducting wire with a normal metal core were calculated for the case of transverse magnetic fields. The results showed that the dependence of the coupling-current loss of such a wire on the rate of field variation is markedly different from that of a wire without the core, the former being characterized by two time constants while the latter by one. The theoretical result was confirmed experimentally. It was pointed out that in the case when a wire with Cu/CuNi matrices is to be used in a rapidly changing magnet field, Cu for a stabilization should be arranged at the centre of the wire rather than at the surface of the wire in order to reduce the loss.     

玻璃与陶瓷高温超导体制取工艺的进展

简略介绍以玻璃与陶瓷为基体的高温超导材料的各种制取工艺。     

YBa_2Cu_（3－x）Sn_xO_y超导体系的正电子寿命谱研究

研究了Ｓｎ作Ｃｕ位元素替代的ＹＢａ２Ｃｕ３－ｘＳｎｘＯｙ超导体系（ｘ＝０－１．５）的正电子寿命谱，计算了局域电子密度和缺陷浓度的变化；发现了存在于高浓度替代区域（ｘ＝０．８－１．０）的正电子湮没特征；讨论了对超导电性的影响以及相应的物理机制。     

Response of superconductors to fast heat pulses

We show that for magnets operating close to their short sample limit, the energy density in a fast heat pulse, applied at the edge of the conductor, that is just sufficient to quench the magnet, is typically a small fraction of the reserve enthalpy/cm of the conductor. The minimum energy density needed to quench can be expressed as a simple, and universal function of the physical parameters of the conductor and the operating temperature.     

Properties of niobium-based Josephson tunnel junctions with a lead-indium alloy top electrode

A fabrication process for niobium-based Josephson superconductive tunnel junctions with a Pb/In alloy top electrode which showed very good mechanical and electrical properties and high oxide uniformity is described. The dependence of the junction parameters on the In-percentage in the alloy is also reported, and the results, in connection with data on the barrier composition, obtained via Auger electron-spectroscopy analysis, are discussed.     

Continuous determination of the cross section area of the components of a niobium-titanium superconducting conductor

Multifilamentary superconducting conductors, which are usually manufactured by hot extrusion and cold drawing, consist of bundles of small diameter superconducting filaments embedded in a normal conducting matrix. It is most desirable for the magnet designer as well as for the conductor manufacturer to control the uniformity of properties along the length of conductors which will be used in large magnets. In this paper we present a continuous monitoring of the superconducting and normal matrix areas through a non-destructive sampling procedure over the whole length of a superconducting conductor.     

The mechanism of frictional motion and its effects at 4.2 K in superconducting magnet winding models

Frictional sliding occurs on both a microscopic and a macroscopic scale. Sliding on a microscopic scale appears as discrete events called microslips. Microslips are inherent in all sliding events and are quite different from macroscopic instabilities such as stick-slips. It is thought that the training effect observed in quench current data from a superconducting braid may be caused by microslips.The mechanisms of sliding motion and its effects at 4.2 K were studied in detail for a number of metal/insulator pairs that model superconducting magnet windings; the results impact the performance of superconducting magnets. Organic surface coating materials are generally effective in eliminating macroscopic instabilities. Instrumentation used in these experiments includes a high-resolution extensometer and an acoustic emission sensor, both with sensitivities capable of detecting microslips (～ 1 μm).     

The use of nanophase powders for high Jc superconductor tape production

Advances in processing of superconductors has demonstrated that ceramic powders with exact composition, super-homogeneity, and extremely fine particle size can be prepared by chemical processing to achieve high density and uniform microstructure. The chemical processing routes have great advantage in preparing powder with high reactivity thus promoting solid state reactions required to produce ‘purer’ superconducting phase. We have studied extensively the coprecipitation technique for the preparation of superconducting ceramic powders. Chemical solution methods have been used in order to model several superconducting systems and to predict optimum conditions for obtaining exact composition. Several improvements on the control of precipitation reaction have been implemented in order to obtain powders of few nanometers in size. This has been done by carrying out the reaction in the bulk of a solution or in microemlusion systems. The corecipitated precursor powder has been used for production of 20− 30 meter long Ag-clad tapes. These tapes require considerably reduced processing times. By controlled heat treatment conditions, these tapes have been processed to carry transport critical current density, J_c <4 × 10⁴ A/cm² at 77K in zero applied field.     

History effects in low-field magnetoresistance of BPSSCO polycrystals

Low-field (H<40 G) magnetoresistance measurements have been made on Bi_1·6Pb_0·4Sr₂Ca₂Cu₃O₁₀ polycrystals at several temperatures between 80 and 105 K. Considerable hysteresis in ρ(H) is found in a zero-field-cooled sample when the applied field is increased from 0 to a maximum value and then lowered back to 0 at all temperatures. The observation of hysteresis is taken as an evidence for field trapping in the grains. We show that the hysteresis in ρ(H) occurs for applied fields much lower than that at whichdρ(H)/dH exhibits a discontinuity. In addition, we find that when the applied magnetic field (H _a) is lowered from a maximum field, the effective intergranular field,H _eff, becomes zero forH _c>0, which gives rise to a minimum in ρ(H).