Superconducting gap function and upper critical field in the presence of inhomogeneous magnetic order

Rare earth ternary superconductors are known to exhibit oscillatory magnetic orders below their superconducting transition temperatures. The study of behaviour of superconducting electrons in an inhomogeneous magnetic field is therefore important for such systems. We report here the results of our theoretical study of superconducting gap function Δ(T) and upper critical fieldH _c2 (T). The results are applied to analyse and explain the variation of Δ(T) andH _c2 (T) in case of NdRh₄B₄.     

Coupling losses in cables in spatially changing ac fields

The coupling losses between strands of multifilamentary superconductors can be important in alternating and pulsing magnetic fields. Up to now, mainly spatially homogeneous applied fields were considered.In this paper, the coupling losses in cables of the EURATOM-LCT-conductor type are calculated for transverse ac fields periodically changing along the cable. Sinusoidal, triangular and trapezoidal field distributions are taken into account as special cases. The losses are enhanced by some orders of magnitude for the sinusoidal field, when the ‘wave length’ of the field I_m comes close to the cabling length I_o. This enhancement is expressive also for triangular and trapezoidal fields, if the wave length of the Fourier components is close to I_o.Some consequences of this effect are pointed out, important for designing larger magnets (eg for the fusion, for generators, etc.)     

Current Inducting Techniques for Superconducting Rings

A procedure has been developed to generate a persistent current in superconducting rings. The basis of the method is a magnetic circuit made of a permanent neodymium-iron-boron magnet and an iron core; the current induced in the superconductor comes only from the energy of the permanent magnet avoiding the power supply. This device would allow the generation of current densities ≥10⁵ A/cm². Furthermore, an inductive method to reduce the persistent current circulating on the samples is also shown, the utilization of a simple iron core being its base. This method avoids possible damages when the critical temperature is exceeded with a high persistent current circulating.     

磁流体在交变磁场中的热效应研究

制备了平均粒径分别为4、6 和8nm 的Fe3O4 粒子,将其分散在不同的介质中,在63kHz、7kA/m的交变磁场中研究其热效应。实验结果显示,粒径较大的Fe3O4 粒子具有较高的产热比功率(SAR),Fe3O4 粒子经过表面处理后SAR显著增加,而且SAR随表面活性剂不同而不同,还显示SAR与磁场强度的平方成正比。     

Generation of 1 T, 0.5 Hz alternating magnetic field in room temperature bore of cryocooler-cooled Bi-2212 superconducting magnet

Alternating magnetic fields have been stably generated at frequencies up to 0.5 Hz in a room temperature (RT) bore of a Bi-2212 superconducting magnet cooled by a Gifford-McMahon (G-M) cryocooler. The magnet has generated alternating magnetic fields with peak magnetic fields up to 1.0 T. The present Bi-2212 superconducting coil heat-treated in an oxygen partial pressure (pO₂) of 1 atm has shown an alternating current (a.c.) loss characteristic, reducing the intergranular hysteresis loss compared with the heat treatment in pO₂ of 0.2 atm.     

On the interaction of magnetic fields and magnetic shields

Some general mathematical expressions have been derived for the induced self and mutual inductances of the wires inside a cylindrical ferromagnetic shell. An application of these results is given. It is shown that the effect of the ferromagnetic shell on the inductances of the multiconducting wires is strongly geometry dependent; the closer the wires to the magnetic shell, the stronger the effect. For wires in the center of the cylinder, no significant result will be caused by the presence of the magnetic shell.     

Numerical simulation of chainlike cluster movement of feeble magnetic particles by induced magnetic dipole moment under high magnetic fields

Abstract

In this paper, the motion of a chainlike cluster of feeble magnetic particles induced by high magnetic field is discussed on the basis of the results of numerical simulations. The simulations were performed on glass particles with a diameter of 0.8 mm; and the viscosity, applied magnetic field and magnetic properties of the surrounding medium were changed. In addition to the magnetic field and the difference in magnetic susceptibility between the particles and the surrounding medium, the obtained results indicate that the viscosity is an essential factor for the formation of the chainlike alignment of feeble magnetic particles. We also carried out simulations using glass particles with a smaller diameter of 0.1 mm. Chainlike clusters were produced similar to those of ferromagnetic particles formed in a ferromagnetic fluid.     

Numerical simulation of chainlike cluster movement of feeble magnetic particles by induced magnetic dipole moment under high magnetic fields

In this paper, the motion of a chainlike cluster of feeble magnetic particles induced by high magnetic field is discussed on the basis of the results of numerical simulations. The simulations were performed on glass particles with a diameter of 0.8 mm; and the viscosity, applied magnetic field and magnetic properties of the surrounding medium were changed. In addition to the magnetic field and the difference in magnetic susceptibility between the particles and the surrounding medium, the obtained results indicate that the viscosity is an essential factor for the formation of the chainlike alignment of feeble magnetic particles. We also carried out simulations using glass particles with a smaller diameter of 0.1 mm. Chainlike clusters were produced similar to those of ferromagnetic particles formed in a ferromagnetic fluid.     

Losses in a multifilamentary superconducting wire caused by a simultaneous sweep of current and magnetic field

A theoretical discussion is presented on the energy loss in a multi-filamentary superconducting wire when an applied transport current and an external transverse magnetic field are varied simultaneously with a repeating pulsive wave form. In the present calculation, the effects of the ‘uniforming time constant’ which has been introduced by the authors as a characteristic time constant for the change in the transport-current distribution inside the wire is taken into account, together with the field dependence of the critical current density of superconducting filaments.Thus the present analytic expression for the energy loss of multi-filamentary wire is available to the whole range of the external magnetic field. It is shown that the contribution of the dynamic resistance loss to the total loss is strongly dependent on the position of the wire inside a coil.     

Using magnetic field pulses with a slow sweep rate to produce uniform current distribution in multifilamentary superconducting wire

A detailed discussion is presented on the change in current distribution which takes place in a twisted multifilamentary superconducting wire induced by successive magnetic field pulses with a slow sweep-rate. The number of the field pulses required for a localized current-distribution to become uniform is estimated. Some modification of existing theories is necessary to describe the phenomenon. The results obtained are confirmed by systematic measurements of the net flux penetration into the wire during each cycle of field pulses.     

高温超导输电电缆的交流损耗

介绍了高温超导输电电缆的交流损耗试验系统,电缆交流损耗理论模型以及1米长高温超导输电电缆的交流损耗试验结果。试验结果表明该电缆在设计电流为工频50Hz,600Arms时的交流损耗值为0．22W,相当于0．368W／kA.m。     

A superconducting magnetic levitation device for the transport of light payloads

A device for the frictionless transport of light payloads, e.g., silicon wafers, using superconducting magnetic levitation has been demonstrated. The device consists of an array of rigidly connected carrier magnets levitating above a corresponding array of superconducting discs. Silicon wafers placed on the carrier have been linearly transported with velocities up to 50 cm/s. The configuration provides for excellent lateral stability. Studies of the height and lateral friction effects (caused by flux pinning) were measured as a function of payload mass. Future applications may include the frictionless transport of silicon wafers in vacuum environments.     

A temperature-variable sample rotating cryostat in high magnetic fields

A temperature-variable sample rotating cryostat has been developed in order to measure the angular and temperature dependence of the upper critical field in several single crystal Chevrel phase superconductors. In the cryostat, the temperature of the sample can be varied from 2 to 15 K within an accuracy of ±10 mK. The sample can be rotated in an adiabatic vacuum can around a horizontal axis at the centre of our superconducting magnets up to 16.5 T within an accuracy of ±1°.     

Superconducting transition widths in applied magnetic fields

The width of superconducting-normal transitions for inhomogeneous ellipsoidal superconductors in an applied magnetic field is calculated. A simple model is used for the variation in transition temperature with position and the range of this variation is assumed to be larger than the Landau-Ginzburg coherence length. It is found that the decrease in transition width with increasing applied magnetic fields at low fields occurs because the boundaries between superconducting and normal regions begin to be determined less by the inhomogeneities and more by the loss of condensation energy between domains, and by sample surface effects. At higher fields, the transition width increases with applied field because of the finite demagnetization coefficient.Contribution of the National Bureau of Standards. Not subject to copyright.     

Superconducting spherical magnetic shields on the basis of niobium and its compounds

The method of fabrication of superconducting spherical magnetic shields on the basis of super-conducting electrolytic coatings made of niobium, Nb₃Sn, and NbC is developed. These shields can be used for weakening the external magnetic fields with strength up to 800 kA/m and obtaining a magnetic vacuum lower than 0.8 × 10⁻⁵ A/m.     

二硼化镁超导线带材及磁体应用研究进展

由于具有超导转变温度（39K）较高,晶体结构简单,原材料成本低廉以及长线制备容易等一系列特点,金属间化合物二硼化镁（ MgB2）超导体自2001年被日本科学家发现以来,引起人们广泛的关注,被认为是目前最有可能首先实现大规模工业应用的超导材料。尤其在制冷机工作温度（15～20 K）、较低磁场（1～2 T）条件下的医疗核磁共振成像仪（ MRI）超导磁体应用上有着广泛的前景。本文主要围绕实用化 MgB2超导长线（带）制备研究而展开,重点回顾了近年来粉末套管法、连续粉末装管成型法及中心镁扩散法等MgB2超导线（带）材制备及加工方面的最新研究进展;同时综述了在 MgB2超导线带材工程临界电流密度性能改进方面的最新研究工作;最后,对近几年来 MgB2超导磁体及线圈等应用研究进展进行了回顾。     

On the distribution of a transport current inside a multifilamentary superconducting wire in a rapidly changing transverse magnetic field

For the current distribution inside a multifilamentary superconducting wire carring a dc transport current in a rapidly changing transverse magnetic field, inconsistencies with the existing models are shown by the following experimental evidence: when a transverse magnetic field is applied, the distribution of transport current is not unaltered but is forced to concentrate into the inner circular cross section region during a characteristic time constant τ_c, called the coupling time constant. Secondly, the characteristic time constant for the transport current distribution inside the inner region to approach a uniform distribution is not τ_c but a new time constant τ₁ called the ‘uniforming time constant’, though the variation in the distribution does not occur unless the external magnetic field changes with time.It is shown that the model of the current distribution based on the above experimental evidence exhibits a remarkable difference from the existing models, especially for the wires containing very fine superconducting filaments.     

A simple heat switch for use at millikelvin temperatures

A simple technique is described for fabricating a high conductivity silver-aluminium joint for use in the construction of an aluminium superconducting heat switch which can be operated in the millikelvin temperature range.