Technical Superconductors Research Articles

Instabilities in Technical Superconductors upon AC Input

Instabilities in Technical Superconductors upon AC Input

Instabilities in Technical Superconductors

Instabilities in Technical Superconductors

Magnetic field sensors for detection of trapped flux in superconducting radio frequency cavities.

Superconducting radio frequency (SRF) cavities are fundamental building blocks of modern particle accelerators. They operate at liquid helium temperatures (2-4 K) to achieve very high quality factors (1010-1011). Trapping of magnetic flux within the superconductor is a significant contribution to the residual RF losses, which limit the achievable quality factor. Suitable diagnostic tools are in high demand to understand the mechanisms of flux trapping in technical superconductors, and the fundamental components of such diagnostic tools are magnetic field sensors. We have studied the performance of commercially available Hall probes, anisotropic magnetoresistive sensors, and flux-gate magnetometers with respect to their sensitivity and capability to detect localized, low magnetic flux amplitudes, of the order of a few tens of magnetic flux quantum at liquid helium temperatures. Although Hall probes have the lowest magnetic field sensitivity (∼96 nV/μT at 2K), their physical dimensions are such that they have the ability to detect the lowest number of trapped vortices among the three types of sensors. Hall probes and anisotropic magnetoresistive sensors have been selected to be used in a setup to map regions of trapped flux on the surface of a single-cell SRF cavity.

In-Field Thermal Conductivity of Ag and Cu/Ag Ba1-xKxFe2As2 Composite Conductors

This work reports the longitudinal thermal conductivity, κ, of Ag- and Cu/Ag-sheathed Ba <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1-x</sub> K <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</sub> Fe <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> As <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> conductors. Our investigation has been carried out between 4 K and 80 K with magnetic fields up to 19 T. In the case of Ag-sheathed tapes, the thermal conductivity reaches maximum values of ~80 Wm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup> K <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup> and is weakly dependent on the field. The use of Cu/Ag in the place of Ag allows increasing κ up to ~250 Wm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup> K <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup> . Zero-field κ values are about one order of magnitude lower than those achieved in other technical superconductors that employ a silver matrix, as Bi <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> Sr <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> CaCu <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">8+x</sub> (Bi2212) and Bi <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2-x</sub> Pb <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</sub> Sr <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> Ca <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> Cu <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">10-x</sub> (Bi2223). This is a consequence of the contamination of the metal sheaths because of elemental diffusion occurring during the tape sintering. The differences in κ between the Ba <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1-x</sub> K <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</sub> Fe <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> As <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> tapes and the other technical superconductors with an Ag matrix become smaller at high field. We suggest that performing the sintering at high pressure could represent a way to reduce the elemental contamination and thus improving the thermal conductivity.

Texture of Nb Filaments and Nb3Sn Phase in Technical Superconductors Fabricated by Bronze and Internal-Tin-Source Techniques

The texture of niobium in composite Nb3Sn superconductors prepared by internal-tin-source and bronze techniques, and in a subelement, which is required for fabricating the composite using the internal-tin-source technique, is studied in this work. It is shown that prior to the reaction heat treatment, the structural components of Nb have the (110) drawing texture; after the reaction heat treatment, no preferred orientation of Nb3Sn superconducting compound grains is observed.

Study of the Specific Features of Kinetics Formation and Structure of the Superconducting Nb3Sn Phase in Technical Superconductors

The growth kinetics of a layer of intermetallic compound Nb3Sn in bronze-processed multifilamentary superconductors of different designs and diameters, developed and fabricated at the A.A. Bochvar High-Technology Research Institute of Inorganic Materials (JSC “VNIINM”), has been studied. The influence of diffusion annealing conditions in a wide range of temperatures and exposure times on the formation of Nb3Sn phase in superconductors has been investigated by methods of metallographic analysis using optical microscopy, electron microscopy, and microanalysis. The structural features of Nb3Sn layers in the initial formation stage and after long-term diffusion annealing are analyzed, their thickness is measured, and the grain structure and composition are studied. Recommendations on improvement of the diffusion annealing conditions for Nb3Sn superconductors of different designs, aimed at obtaining high current-carrying ability, are elaborated.

Very-high thermal and electrical conductivity in overpressure-processed Bi2Sr2CaCu2O8+x wires

The residual-resistivity ratio (RRR) of the normal-metal matrix is a key parameter for the electrical and thermal stability of technical superconductors. In Bi2Sr2CaCu2 (Bi-2212) round wires, the precursor powders are embedded in a Ag matrix without any diffusion barrier, and elemental diffusion from the superconducting filaments into the Ag might be expected to contaminate the matrix during the melt processing required for high critical current density development. This work shows that the overpressure processing, which is adopted to enhance the critical current performance, improves the thermal and electrical conductivities of the conductor, too. In the case of wires reacted with a standard processing performed in 1 bar O2, the RRR of the Ag matrix is about 90, in spite of the simple conductor design that does not include diffusion barriers. Increasing the total reaction pressure to 100 bar improves the RRR to about 200. The differences in RRR reflect on the thermal conductivity of the whole conductor, which has been investigated in magnetic fields up to 19 T.

Nonlinear effects of the thermal stabilization theory of applied superconducting materials

The stability of dissipative states of a Bi2Sr2CaCu2O8 applied superconducting composite cooled by liquid helium or hydrogen with continuously increasing current-voltage characteristics described by a power equation has been studied. It has been shown that, under intensive cooling conditions, special conditions of thermal stabilization can exist for applied superconducting materials (technical superconductors) if the nonlinear temperature dependences of critical current density and specific resistance of the stabilizing matrix are taken into account. First, the minimum currents of existence and propagation of a normal zone can be absent. Second, the intensive cooling of a technical superconductor substantially increases the range of stable currents in the diapason of supercritical currents. Third, during the irreversible propagation of the thermal instability, the temperature of a technical superconductor can increase under conditions close to adiabatic, despite cooling by liquid coolant. These effects should be taken into account upon determining the conditions for overheating a technical superconductor. The numerical experiments have been compared with the results that follow from the thermal stabilization theory of combined superconductors, which assumes a linear temperature dependence of critical current density of a superconductor and a step-wise transition from the superconducting to normal state.

Thermal electrodynamic mechanisms of a rise of current-voltage characteristic (IVC) of technical superconductors at magnetic flux creep

The processes of the formation of the macroscopic states of a superconducting tape induced by the transport current at magnetic flux creep have been studied. It has been shown that there are characteristic values of electric field intensity that are affected by a rate of current injection, properties of a superconductor, cooling conditions, and properties of a stabilizing matrix. These values are the basis of thermal electrodynamic mechanism, which determines the slope of a rise of IVC of technical superconductors. The conditions of formation of current instabilities have been studied taking into account a nonuniform temperature distribution over the cross section of a technical superconductor. The conditions of the existence of the IVC of technical superconductors have been formulated. These conditions allow for the stable heating of a superconductor as high as the critical temperature. The results of the carried out studies should be taken into account when measuring the IVC of superconducting materials and determining their critical parameters and the current of instability occurrence.

Two-Dimensional and Three-Dimensional Mechanical Analyses of the Superconducting Outsert of the LNCMI Hybrid Magnet

Current technical superconductors, such as Nb3Sn, are limited to fields around 20 T by their intrinsic material properties. Generating significantly stronger magnetic fields can only be done by using resistive magnets, or by combining superconducting and resistive magnets (hybrid magnets), at the expense of large power consumption and operating cost. A CEA–CNRS French collaboration is currently developing a new hybrid magnet, which combines a resistive insert composed of Bitter and polyhelix coils and a new large-bore superconductor outsert to produce an overall continuous magnetic field of more than 42 T in a 34-mm warm aperture. Based on the novel development of a NbTi/Rutherford cable on conduit (RCOCC) cooled down to 1.8 K by means of the bath of superfluid helium at atmospheric pressure, the superconducting coil aims to produce a continuous magnetic field of 8.5 T in a 1.1-m cold bore diameter. This paper summarizes the results of the mechanical behavior study of the cold mass cooled from room to operating temperature and Lorentz forces at operating temperature. Computations have been performed in 2-D axisymmetry and 3-D, to take into account the axisymmetric discontinuous distribution of the tie rods.

Application potential of Fe-based superconductors

In this paper we report basic properties of iron-based superconductors and review the latest achievements in the fabrication of conductors based on these materials. We compare state-of-the-art results with performances obtained with low-Tc and high-Tc technical superconductors, evidencing in particular the most significant differences with respect to high-Tc cuprate coated conductors. Although the optimization of preparation procedures is yet to be established, a potential range of applications for iron-based superconductors in the high field low temperature regime can be envisaged, where they may become competitors to RE-123 coated conductors.

Targets for R&amp;D on Nb3Sn Conductor for High Energy Physics

High Energy Physics has been consistently pushing the performance of technical superconductors, for the benefit of high field magnet technology. So far the workhorse for particle accelerators has been Nb-Ti, but the practical performance limit has been attained with the LHC. Calls for higher beam luminosity (e.g., HL-LHC), and higher beam energy (e.g., FCC), demand a transition from Nb-Ti to Nb3Sn, presently the only practical candidate material offering the required high field performance. This paper provides a summary of desirable properties and performance targets for Nb3Sn to satisfy the challenging magnet specifications for upgrades of existing and future HEP accelerators.

Computation of Losses in HTS Under the Action of Varying Magnetic Fields and Currents

Numerical modeling of superconductors is widely recognized as a powerful tool for interpreting experimental results, understanding physical mechanisms, and predicting the performance of high-temperature-superconductor (HTS) tapes, wires, and devices. This is particularly true for ac loss calculation since a sufficiently low ac loss value is imperative to make these materials attractive for commercialization. In recent years, a large variety of numerical models, which are based on different techniques and implementations, has been proposed by researchers around the world, with the purpose of being able to estimate ac losses in HTSs quickly and accurately. This paper presents a literature review of the methods for computing ac losses in HTS tapes, wires, and devices. Technical superconductors have a relatively complex geometry (filaments, which might be twisted or transposed, or layers) and consist of different materials. As a result, different loss contributions exist. In this paper, we describe the ways of computing such loss contributions, which include hysteresis losses, eddy-current losses, coupling losses, and losses in ferromagnetic materials. We also provide an estimation of the losses occurring in a variety of power applications.

Maximum Entropy Distributions Describing Critical Currents in Superconductors

Maximum entropy inference can be used to find equations for the critical currents (Jc) in a type II superconductor as a function of temperature, applied magnetic field, and angle of the applied field, θ or φ . This approach provides an understanding of how the macroscopic critical currents arise from averaging over different sources of vortex pinning. The dependence of critical currents on temperature and magnetic field can be derived with logarithmic constraints and accord with expressions which have been widely used with empirical justification since the first development of technical superconductors. In this paper we provide a physical interpretation of the constraints leading to the distributions for Jc(T) and Jc(B), and discuss the implications for experimental data analysis. We expand the maximum entropy analysis of angular Jc data to encompass samples which have correlated defects at arbitrary angles to the crystal axes giving both symmetric and asymmetric peaks and samples which show vortex channeling behavior. The distributions for angular data are derived using combinations of first, second or fourth order constraints on cot θ or cot φ . We discuss why these distributions apply whether or not correlated defects are aligned with the crystal axes and thereby provide a unified description of critical currents in superconductors. For J//B we discuss what the maximum entropy equations imply about the vortex geometry.

Microstructure of the elements of a superconducting Alloy Nb-Ti cable

The formation of a micro- and macrostructure in the Nb-Ti-based technical superconductors that are intended for the current-carrying elements in the magnetic system of the International Thermonu-clear Experimental Reactor and are subjected to deformation by drawing is considered. The deformation structure and microstructure of cable samples are studied by optical microscopy, atomic force microscopy, and scanning electron microscopy. The specific features of plastic flow localization at the sites of fracture of a multiconductor cable are revealed.

Thermal Conductivity of Industrial $\hbox{Nb}_{3} \hbox{Sn}$ Wires Fabricated by Various Techniques

We have developed a new experimental setup specifically designed for measuring thermal conductivity on technical superconductors in the range of temperatures 3-330 K in magnetic fields up to 21 T. Three Nb <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Sn wires, produced by the powder-in-tube technique, the bronze route, and the internal tin restacked-rod process, respectively, have been investigated. We show that, due to the complexity of the architecture of these wires, direct measurement of thermal conductivity is required for a correct estimation of thermal stability in superconducting magnets.

Upper critical fields and critical current densities of Fe-based superconductors as compared to those of other technical superconductors

Three years since the discovery by the Hosono’s group of Fe-based superconductors, an enormous number of compounds, belonging to several different families have been discovered and fundamental properties have been deeply investigated in order to clarify the interplay between magnetisms and superconductivity in these compounds. Indeed, the actual potential of these compounds for practical applications remains still unclear.Fe-based superconductors are midway between high temperature superconductors (HTSCs) and MgB2. In Fe-based superconductors the critical current is rather independent of the field, similarly to HTSCs, as a consequence of the exceptionally high upper critical field and strong pinning associated with nm-scale local modulations of the order parameter. They exhibit low anisotropy of the critical current with respect to the crystalline directions, as in the case of MgB2, which allows current flow along the c-axis. However, Fe-based superconductor polycrystalline materials currently available still exhibit electromagnetic granularity, like the HTSCs, which suppresses superconducting current flow over long length. Whether the nature of such granularity is extrinsic, as due to spurious phases or cracks between grains or intrinsic, as related to misalignment of adjacent grains, is under debate. These aspects will be reviewed in the light of the recent literature.

100 Years of Superconductivity and 50 Years of Superconducting Magnets

This review celebrates the centenary of Kamerlingh Onnes' discovery of superconductivity and also the 50th anniversary of the first conference to discuss superconducting magnets. Our growing understanding of superconductivity and its relationship with magnetic fields is outlined, together with the technical high field superconductors, which first made magnets possible. Engineering problems in utilizing these new materials and the applications which they made possible are described. Just 75 years after Kamerlingh Onnes' discovery, high temperature superconductivity HTS was discovered, opening up a new range of fields and temperatures for magnets. The difficulties of making these new materials into practical magnet conductors are outlined. Some applications of HTS are described and speculations are offered about those areas where it is most likely to succeed in future.

Current Sharing and Critical Current Distribution in Bi-2223 Tapes

For the applications of technical superconductors, for example in superconducting high-field or NMR magnets, a detailed knowledge of the superconducting-to-normal transition is essential. Usually, this transition is described by a power law leading to the critical current, I <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">C</sub> , and the n-value. I <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">C</sub> and n are global variables which ignore the microstructure of a superconductor. In contrast, the assumption of a distribution of local critical currents provides a statistical approach to the superconducting-to-normal transition taking into account the non-uniformity of the material. In this paper, the model of local critical currents is applied to describe the measured voltage-current characteristics of the superconducting transition for several types of Bi-2223 tapes. The probability density of the distribution is evaluated and in addition the current sharing occurring in the transition region between the superconducting phase and the matrix material of the composite is discussed.

Model for the angular dependence of critical currents in technical superconductors

We propose a model for the effect of anisotropic pinning on the criticalcurrents in superconductor wires as a function of the direction of an externalmagnetic field. The model emphasizes the statistical distribution of the pinningencountered by a vortex as determining the functional forms observed. Ifwe assume anisotropic defect populations with normal distributions of pinningthen we can derive exact functions for the critical current as a function of angle:Jc(θ). The model predicts that the combination of extended defects in thec-axisand ab-plane directions can lead to peaks at intermediate angles. The predicted forms agree wellwith experiments on superconductors with layered structures including cases wherepeaks occur at intermediate angles. The results suggest that common features ofJc(θ) can be identified with combinations of defects. We discuss some implications of thestatistical model including a possible sample thickness dependence of criticalcurrents.