Acoustic emission from nbti superconductors during flux jump

In experiments with NbTi superconductors, the emission of acoustic waves in the frequency range 0.1 to 0.3 MHz was recorded during a current sweep. The acoustic emission as the current increased started at about 30% of the critical current and continued as the current decreased. The emission was reduced in subsequent current cycles which is explained by reduced magnetic flux motion due to the previously trapped flux. The reduction of acoustic emission in subsequent cycles proves definitely it is due to flux motion and excludes other possible sources like mechanical deformation. Results of different NbTi compositions are presented and discussed qualitatively.     

NbTi超导线材的研究与发展

讨论了NbTi/Cu超导线材的应用范围及其特性,介绍了NbTi/Cu超导线材的传统制备工艺和人工钉扎中心制备工艺,以及这两种工艺生产的NbTi/Cu超导线材的临界电流密度和商业化生产NbTi/Cu超导线材对加工工艺的要求.NbTi/Cu超导线材是目前应用最广的一类超导材料,为了满足超导磁体工作的安全性要求,大铜比NbTi/Cu超导线材的制备工艺和6.5K下NbTi/Cu超导线材的性能将是NbTi/Cu超导线材今后的研究方向之一.     

Design,development and fabrication of indigenous 30 kA NbTi CICC for fusion relevant superconducting magnet

The fusion relevant superconducting magnet is under development in India using a cable-in-conduit-conductor (CICC) with operating current of 30 kA at 5.5 T and 4.5 K. The 30 kA NbTi based CICC is designed on the basis of desired critical design parameters as well as mechanical fabrication considerations. The 30 kA CICC has been designed having square cross-section (30 mm × 30 mm) consisting NbTi as superconducting cable, SS316LN as jacket material and SS304 foil as wrapping around the cabled strands. The design configuration of 30 kA NbTi CICC has been discussed in this paper. The NbTi base high current carrying strands have been fabricated indigenously using direct extrusion and cold drawing process. The 100 m long NbTi–Cu strands twisting, insertion of cabled strands into a circular conduit has been developed with pull through technology. The welding process qualification and effects of cold work on jacket material at room temperature have been elaborated in this paper. The manufacturing parameters and quality procedures for development of CICC have been successfully established and demonstrated with fabrication of 100 m NbTi based CICC without any technical difficulties.     

Superconducting properties of NbTi based multilayers

The transition temperature, the upper critical field and the transport critical current of Nb₆₅Ti₃₅/Ti, Nb₂₈Ti₇₂/Ti and Nb₆₅Ti₃₅/Nb artificial multilayer films are measured as a function of the structure modulation wavelength . The results are analyzed on the basis of the proximity effect. The observed enhancement of the parallel critical field and the relatively large critical current density make the NbTi/Ti system a hopeful candidate for application. On the other hand, the proximity effect damages superconducting characteristics of the NbTi/Nb system, limiting the possibility for use in application.     

磁体用NbTi超导体的研究进展

NbTi是典型的低温超导材料,广泛应用于核磁共振成像(MRI)、核磁共振(NMR)、高能物理(HEP)等领域.常用NbTi超导材料的临界温度Tc为9.6K,4.2K时的上临界场Hc2为11T,因此NbTi超导材料的应用都是在低场范围内,也是不可替代的低场超导材料.综述了NbTi超导材料的发展现状和未来的发展趋势.     

Magnetic field dependence of transition current of NbTi mechanical PCS

This paper deals with magnetic field dependence of transition current of NbTi mechanical persistent current switch (PCS), which realizes zero contact resistance and consists of two contact pieces made of NbTi bulk, in parallel or perpendicular magnetic field. The transition current is defined as the quench current with which the contact resistance abruptly generates. It is found that the transition current decreases with the field strength. Using a theoretical model to explain the formation mechanism of a superconducting connection at the contact of the mechanical PCS, it is confirmed that the transition current varies according to J_C–B characteristics of the contact piece material NbTi. These findings prove that NbTi mechanical PCS has superconductive connection between the contact pieces in the closed state and can be operated in persistent current mode as a superconducting PCS.     

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.     

Experimental hysteretic loss for a series of superconducting filamentary NbTi wires and a field dependent critical state model

Analytic expressions for hysteretic loss have been developed for half-cycle and full-cycle field sweeps for rod geometries for a critical state model with critical current inversely proportional to field. Bulk effects with surface-like character are discussed along with surface shielding fields and the demagnetizing factor. The loss expressions of various models are compared to experimental loss data made on a series of NbTi superconducting wires. Universal loss curves constructed from experimental loss curves by appropriate normalization are obtained and used to predict loss accurately. Bias field ripple losses are also reported.     

The temperature and magnetic field dependence of superconducting critical current densities of multifilamentary Nb3Sn and NbTi composite wires

Commercial NbTi and Nb3Sn multifilamentary superconducting wire is becoming increasing important for use in research and commercial magnet systems. In both materials the temperature dependence of Jcplays a major role in the determination of magnet system operating parameters and design stability margins. We report here critical current density measurements as a function of temperature from 4.2 to 19 K and of applied magnetic field upto 8 T for multifilamentary Nb3Sn wire and for 2 alloys of NbTi superconducting wire. From this data [partial J_{c}(H_{a})/partialT] and[partialH_{c2}/partialT]T=T_{c}can be obtained and stability criteria and other superconducting parameters of the wires may be extracted.     

Field and temperature dependencies of critical current on industrial Nb3Sn strands

The increasing need for high field magnetic devices has focused attention on filamentary Nb₃Sn conductors, whose critical data are superior to NbTi conductors. To choose the suitable operating parameters and to determine the stability margin of magnet systems, it is very important to know the effect of temperature and magnetic field on the superconducting properties, especially on the critical current. Up to now, for design calculation, the so-called “Summers model” was assessed theoretically on experimental data obtained by Spencer et al., (The temperature and magnetic field dependence of superconducting critical current densities of multiinflammatory Nb₃Sn and NbTi composite wires. IEEE Trans Mag, Mag-15 (1979) 76) and Suenaga et al., Superconducting critical-current densities of commercial multifilamentary Nb₃Sn(Ti) wires made by the bronze process. Cryogenics (1985) 25, 123). Apart these very useful preliminary experimental data, very little has been done on the very different industrial strands which are now produced in the industry. Industrial Nb₃Sn strands are generally tested and checked only at 4.2 K and their operating design temperature is often very different, sometimes around 6 K. It is now urgent to validate the model and to confirm that the data taken up to now in the design calculations are conservative.     

Boundary resistance in SC composite wires and cryogenic stability

The measurement of transverse resistivity of NbTi composite wires has shown already the existence of a resistive barrier between SC filaments and the copper matrix. The electric and thermal resistances of this barrier are respectively much higher than those of copper matrix and this barrier is expected to have an influence on the cryogenic stability of composite wires. The transverse and longitudinal resistivities are measured for NbTi composite wires which were heat-treated at different temperatures from 300°C to 600°C. These measurements show that the barrier grows with the heat-treatment temperature. From the experimental results, the effect of the barrier on cryogenic stability is estimated to be negligibly small for the composite wire which is heat-treated under the normal condition. As for Nb₃Sn composite wires, two different structures of composite wires, each of which has a tantalum or niobium diffusion barrier, are studied and the same measurements as on NbTi composite wires are carried out. The results obtained indicate that the transverse resistivity depends appreciably on the structure of composite wires and that the larger transverse resistivity reduces not only the cryogenic stability, but also requires a larger transfer length at a current lead junction.     

Irradiation and annealing effects of deuteron irradiated NbTi and V3Ga multifilamentary composite wires at low temperature

To study the effects of low temperature irradiation on technological type II-superconductors, NbTi and V3Ga multifilamentary composite wires, the critical current Icand the transition temperature Tcwere measured before and after irradiation with 50 MeV deuterons at 10 K and 15 K, respectively. While the irradiation effects on Icand Tcof NbTi are substantially unaffected, the V3Ga wires undergo a reduction in Icof about 50 % and Tcdecreases from 14.7 ± 0.1 K to 12.3 ± 0.1 K at a total deuteron flux of 2.7 × 10¹⁷cm^-2. Annealing experiments at room temperature and 100° C show only a small recovery of the superconducting properties up to 15 %. The field dependence of the volume pinning force densities Pvwas determined and the results are shown to be consistent with a qualitative dynamic pinning model.     

Critical current data of NbTi conductors at sub-4.2 K temperatures and high fields

Critical current data in the temperature and field ranges of 1.8 to 4.2 K and 8.5 to 12.5 T are presented for two size-groups of commercial NbTi conductors. The first group of conductors had critical currents at 4.2 K and 9 T of 6 to 77 A, while the second group's ranged from 1400 to 1570 A.     

Pinning in superconductors with anisotropic defects

By calculating the volume pinning force in superconductors with anisotropic pinning centers we have shown that the critical current densitiesj _c in NbTi by both current and magnetization measurements can be explained. The critical current density in fields parallel toj _c can also be included. We conclude therefore that in superconductors with strong pinning centers the critical state is always determined by the equilibrium condition between the Lorentz force and the volume pinning force and not by the instability of the force-free configuration. By detailed investigation of thej _c dependence on field direction, useful information can be obtained with respect to the volume and surface contributions to the pinning force.     

MgB2 wires with Ti and NbTi barrier made by IMD process

MgB₂ wires with Ti and NbTi barriers have been made by internal magnesium diffusion (IMD) into boron process. Critical currents, strain tolerances and AC loss of wires with Ti and NbTi barriers have been compared. It was shown that worse uniformity of NbTi barrier affects the creation of regular MgB₂ layer and consequently influences (reduces) also the current densities. Positive effects of NbTi barrier are in improved strain tolerance and reduced coupling losses. The maximum AC loss of not twisted wire with Ti barrier is measured at frequency 9 Hz, but it is shifted up to 60 Hz for NbTi due to considerably increased barrier resistance at 20 K.     

Selffield losses and selffield stability of superconducting wires with low conductivity matrix material

The influence of the normal material in a superconducting wire on the selffield losses has been investigated. The results show that the contribution of the normal material is considerable in the case of a highly conductive material like Cu. Measurements on wires with CuNi-material show no significant contribution of the normal material to the losses. In the latter case, however, special attention should be paid to stability. Measurements show that the obtainable maximum current under a.c. conditions obeys the adiabatic stability criterion rather well. The dependence of the maximum current amplitude on the critical current density and the diameter of the wire has been obtained. Wires with Al cores inside the NbTi filaments for better stabilization have also been investigated regarding maximum current and selffield losses.     

Numerical treatment of the quenching process in superconducting magnet systems

A computer program for testing coil and protection design of a composed magnet system is presented. Small high field magnets consisting of two uniaxial cylindrical coils of different superconducting materials (eg NbTi and V₃Ga or Nb₃Sn) are considered, each coil may be subdivided into several sections which are protected by parallel resistors. Quench propagation due to thermal conduction and also due to rapid current increase (important in inductively coupled systems) is taken into account by means of solving a one-dimensional thermodiffusion equation. Field and temperature dependence of the critical current for every layer of the coils, is taken into account. The program calculates the time dependence of currents, external and internal voltages, resistances of the sections and of the radial temperature distribution. Calculations are compared with experimental investigations of three different systems, the results agree with experimental values concerning current decay and propagation velocity.     

Multifilament niobium-tin conductors

A new Nb3Sn wire fabrication method has been developed, improving wire drawing workability and superconducting properties, such as stability and ac losses. A cross section of the single filament wire consists of a niobium tube with a copper sheathed tin rod inside and high conductivity copper tube outside. These constituents show scarcely any workhardening. Wires with 54 to 295 filaments were drawn down to 0.2 mm to 1.0 mm diameter. Heat-treatment conditions to obtain the highest critical current were clarified as a function of the tin content inside the niobium tube. The effect of bend strain in Nb3Sn on the critical current was also examined for samples with different wire diameters and Nb3Sn layer thickness. Losses were measured for twisted and non-twisted samples by means of magnetization experiments. Results were compared with calculated values. It was found that the effective resistivity between Nb3Sn filaments was one order of magnitude higher than that of pure copper. A coil was constructed using a 1 km long Nb3Sn composite having 258 filaments with 1 × 2 mm cross section. The maximum field obtained was 10.65T at 236A in the 6T backing field by NbTi solenoid.     

Magnetic field orientation dependence of critical current in industrial Nb3Sn strands

In usual superconducting devices such as magnets for NMR, the magnetic field is perpendicular to the superconducting strand axis. But in some special devices, such as magnets for the toroidal field system of fusion machines, the strands can experience any field orientation. For NbTi strands, the pinning force is dependent on the field orientation because of the drawing process (Takacs, S., Polak, M. and Krempasky, L., Critical currents of NbTi tapes with differently oriented anisotropic defects, Cryogenics, 1983, 23, 153–159). In the case of Nb₃Sn strands, the draw and react process suggests that the pinning force is isotropic. In fact, preliminary experiments have shown the contrary, which is why the magnetic field orientation dependence of the critical current for two types of industrial Nb₃Sn strands has been measured. These measurements have been performed for seven field orientations at field strengths up to 20 T. A clear anisotropic effect has been observed, which cannot be explained by Kramer's pinning law. The results are in very good agreement with an empirical law proposed in a recent study by Takayasu et al. (Takayasu, M., Montgomery, D.B. and Minervini, J.V., Effect of magnetic field direction on the critical current of twisted multifilamentary superconducting wires, Inst. of Phys. Conf. Ser., 1997, 158, 917–920). The parameters to be used in this law could be specific to the manufacturing process.     

Equal-Channel Multi-Angle Pressing Effect on the Properties of NbTi Alloy

An improvement of NbTi alloy functional properties by equal-channel multi-angle pressing (ECMAP) combined with hydrostatic extrusion, drawing and thermal treatment is revealed. The ECMAP method allows to increase the billet accumulated deformation with preserving its initial dimensions. The formed highly dispersed and homogeneous nanocrystalline structure with a more uniform distribution of α-phase precipitations as a result of ECMAP treatment improves the functional properties of the alloy. In the field of 5 T, the critical current density in wire samples produced with application of the ECMAP method is enhanced approximately by a factor of 2 in comparison with the values obtained for the samples produced without the ECMAP treatment.