Showing papers in "Superconductor Science and Technology in 2010"

Anion height dependence of Tc for the Fe-based superconductor

Abstract: We have established a plot of the anion height dependence of superconducting transition temperature Tc for the typical Fe-based superconductors. The plot showed a symmetric curve with a peak around 1.38??. Both data at ambient pressure and under high pressure obeyed the unique curve. This plot will be one of the key strategies for both understanding the mechanism of Fe-based superconductivity and searching for the new Fe-based superconductors with higher? Tc.

Artificial pinning center technology to enhance vortex pinning in YBCO coated conductors

Abstract: Crystalline defects on the nano-scale, which are called artificial pinning centers (APCs), were successfully introduced into high-temperature superconductors (HTS) by nanotechnology, in order to strongly pin the quantized vortices. The critical current densities, Jc, of the HTS films were dramatically improved by APCs. It is possible to form APCs in high-quality epitaxial films, keeping the desired dimensionality, volume fraction, spatial distribution and so on. The in-field Jc of HTS films at 77 K was improved by one order of magnitude compared with previous values using APCs. This technology can be applied to the coated conductor technology in progress, and a high Jc has already been reported. A current outline of the research is described in this review.

New Fe-based superconductors: properties relevant for applications

Abstract: Less than two years after the discovery of high temperature superconductivity in oxypnictide LaFeAs(O, F) several families of superconductors based on Fe layers (1111, 122, 11, 111) are available. They share several characteristics with cuprate superconductors that compromise easy applications, such as the layered structure, the small coherence length and unconventional pairing. On the other hand, the Fe-based superconductors have metallic parent compounds and their electronic anisotropy is generally smaller and does not strongly depend on the level of doping, and the supposed order parameter symmetry is s-wave, thus in principle not so detrimental to current transmission across grain boundaries. From the application point of view, the main efforts are still devoted to investigate the superconducting properties, to distinguish intrinsic from extrinsic behaviors and to compare the different families in order to identify which one is the fittest for the quest for better and more practical superconductors. The 1111 family shows the highest Tc, huge but also the most anisotropic upper critical field and in-field, fan-shaped resistive transitions reminiscent of those of cuprates. On the other hand, the 122 family is much less anisotropic with sharper resistive transitions as in low temperature superconductors, but with about half the Tc of the 1111 compounds. An overview of the main superconducting properties relevant to applications will be presented. Upper critical field, electronic anisotropy parameter, and intragranular and intergranular critical current density will be discussed and compared, where possible, across the Fe-based superconductor families.

Probing multiband superconductivity by point-contact spectroscopy

Abstract: Point-contact spectroscopy was originally developed for the determination of the electron–phonon spectral function in normal metals. However, in the past 20 years it has become an important tool in the investigation of superconductors. As a matter of fact, point contacts between a normal metal and a superconductor can provide information on the amplitude and symmetry of the energy gap that, in the superconducting state, opens up at the Fermi level. In this paper we review the experimental and theoretical aspects of point-contact spectroscopy in superconductors, and we give an experimental survey of the most recent applications of this technique to anisotropic and multiband superconductors.

Superconducting wind turbine generators

Abstract: We have examined the potential of 10?MW superconducting direct drive generators to enter the European offshore wind power market and estimated that the production of about 1200 superconducting turbines until 2030 would correspond to 10% of the EU offshore market. The expected properties of future offshore turbines of 8 and 10?MW have been determined from an up-scaling of an existing 5?MW turbine and the necessary properties of the superconducting drive train are discussed. We have found that the absence of the gear box is the main benefit and the reduced weight and size is secondary. However, the main challenge of the superconducting direct drive technology is to prove that the reliability is superior to the alternative drive trains based on gearboxes or permanent magnets. A strategy of successive testing of superconducting direct drive trains in real wind turbines of 10?kW, 100?kW, 1?MW and 10?MW is suggested to secure the accumulation of reliability experience. Finally, the quantities of high temperature superconducting tape needed for a 10?kW and an extreme high field 10?MW generator are found to be 7.5?km and 1500?km, respectively. A more realistic estimate is 200?300?km of tape per 10?MW generator and it is concluded that the present production capacity of coated conductors must be increased by a factor of 36 by 2020, resulting in a ten times lower price of the tape in order to reach a realistic price level for the superconducting drive train.

Advances in second generation high temperature superconducting wire manufacturing and R&D at American Superconductor Corporation

Abstract: The RABiTS?/MOD-YBCO (rolling assisted biaxially textured substrate/metal?organic deposition of YBa2Cu3O7??) route has been established as a low-cost manufacturing process for producing high performance second generation (2G) wire American Superconductor Corporation (AMSC) has used this approach to establish a production scale manufacturing line based on a wide-web manufacturing process This initial production line is currently capable of producing 2G wire in lengths to 500?m with critical currents exceeding 250? A?cmwidth?1 at 77?K, in the self-field The wide-web process, combined with slitting and lamination processes, allows customization of the 2G wire width and stabilizer composition to meet application specific wire requirements The production line is currently supplying 2G wire for multiple cable, fault current limiter and coil applications Ongoing R&D is focused on the development of thicker YBCO layers and improved flux pinning centers This paper reviews the history of 2G wire development at AMSC, summarizes the current capability of the 2G wire manufacturing at AMSC, and describes future R&D improvements

An overview of Boeing flywheel energy storage systems with high-temperature superconducting bearings

Abstract: An overview summary of recent Boeing work on high-temperature superconducting (HTS) bearings is presented. A design is presented for a small flywheel energy storage system that is deployable in a field installation. The flywheel is suspended by a HTS bearing whose stator is conduction cooled by connection to a cryocooler. At full speed, the flywheel has 5 kW h of kinetic energy, and it can deliver 3 kW of three-phase 208 V power to an electrical load. The entire system, which includes a containment structure, is compatible with transportation by forklift or crane. Laboratory measurements of the bearing loss are combined with the parasitic loads to estimate the efficiency of the system. Improvements in structural composites are expected to enable the operation of flywheels with very high rim velocities. Small versions of such flywheels will be capable of very high rotational rates and will likely require the low loss inherent in HTS bearings to achieve these speeds. We present results of experiments with small-diameter rotors that use HTS bearings for levitation and rotate in vacuum at kHz rates. Bearing losses are presented as a function of rotor speed.

A scalable control system for a superconducting adiabatic quantum optimization processor

Abstract: We have designed, fabricated and operated a scalable system for applying independently programmable time-independent, and limited time-dependent flux biases to control superconducting devices in an integrated circuit. Here we report on the operation of a system designed to supply 64 flux biases to devices in a circuit designed to be a unit cell for a superconducting adiabatic quantum optimization system. The system requires six digital address lines, two power lines, and a handful of global analog lines.

AC losses in coated conductors

Abstract: Future use of coated conductors in electric power applications like transmission cables, transformers or fault current limiters is sensitive to the amount of dissipation in the AC regime This paper analyses factors controlling AC loss of coated conductors in typical configurations: the self-field case when transport current generates the magnetic field, and the case of AC applied field where the orientation of magnetic field with respect to the superconducting layer plays a significant role We illustrate that a high-quality CC tape with non-magnetic substrate follows rather well the models developed for a thin strip However, to meet an excellent agreement between experiment and theoretical prediction a detailed knowledge of the superconductor properties is necessary and a numerical method must be involved In the case of a superconducting layer deposited on a ferromagnetic substrate theoretical predictions give only basic directions and one must rely on numerical simulations entirely We demonstrate that, with the help of a dedicated analysis of experimental data, very good AC loss prediction is also possible for superconductor?ferromagnetic composites Novel designs of coated conductor architectures can be developed in this way

An overview on iron based superconductors

Abstract: The discovery of superconductivity at relatively higher temperatures in a non-cuprate system, LnFeAsO1 − xFx (Ln = lanthanides) has created tremendous activity among the reseachers in this field. This review is an overview on the present status and the future scope for iron pnictides. The various structural categories of iron based superconductors, the structural aspects, different preparation techniques of the material and the necessity for its optimization are discussed. The highlighting features of iron pnictide, i.e. the very high upper critical field, moderate magneto-transport and thermal properties, are also included. The article gives a summary of the prevailing arguments of researchers to relate the material to cuprates and also the comparative features of classical and MgB2 superconductors. The existing challenges, such as optimizing synthesis methods for technological applications and clarifying the ambiguity in the superconducting mechanism and the flexibility of the material for any site substitution, will keep iron based superconductors on the frontiers of research for a long time, in parallel to HTS.

Improved flux pinning in YBa2Cu3O7 with nanorods of the double perovskite Ba2YNbO6

Abstract: We report significantly enhanced critical current densities (Jc) and flux pinning forces (Fp) in applied magnetic fields for YBa2Cu3O7 (YBCO) films with embedded Ba2YNbO6 (BYNO) nanorods. The films were grown by pulsed laser deposition with a target consisting of YBa2Cu3Oy with five molar per cent additions of BaNbOy and Y2O3. With this composition, deposited films were found to contain a high density of BYNO nanorods that frequently traversed the entire thickness of the film (up to 1? ?m), depending upon the deposition conditions. Enhanced Jc performance occurs primarily for applied field orientations near the c-axis of the YBCO, which is nominally along the growth direction of the BYNO nanorods. The threading nanorod density of one film of the present work was measured by plan-view transmission electron microscopy to be 710?850 nanorods??m ? 2. For approximately 1??m thick films, typical Jc(75.6?K, sf) and values were ~ 4.5?MA?cm ? 2 and 1.3?1.5?MA?cm ? 2, respectively. For a 0.5??m thick film, was achieved, and values of Fp in excess of 30 and 120?GN?m ? 3 were achieved at 75.5?K and 65?K, respectively.

Effect of strain, magnetic field and field angle on the critical current density of Y Ba2Cu3O7−δ coated conductors

Abstract: A large, magnetic-field-dependent, reversible reduction in critical current density with axial strain in Y Ba2Cu3O7−δ coated conductors at 75.9 K has been measured. This effect may have important implications for the performance of Y Ba2Cu3O7−δ coated conductors in applications where the conductor experiences large stresses in the presence of a magnetic field. Previous studies have been performed only under tensile strain and could provide only a limited understanding of the in-field strain effect. We now have constructed a device for measuring the critical current density as a function of axial compressive and tensile strain and applied magnetic field as well as magnetic field angle, in order to determine the magnitude of this effect and to create a better understanding of its origin. The reversible reduction in critical current density with strain becomes larger with increasing magnetic field at all field angles. At 76 K the critical current density is reduced by about 30% at − 0.5% strain when a magnetic field of 5 T is applied parallel to the c-axis of the conductor or 8 T is applied in the ab-plane, compared to a reduction of only 13% in self-field. Differences in the strain response of the critical current density at various magnetic field angles indicate that the pinning mechanisms in Y Ba2Cu3O7−δ coated conductors are uniquely affected by strain.

A new finite-element method simulation model for computing AC loss in roll assisted biaxially textured substrate YBCO tapes

Abstract: This paper presents a new finite-element simulation model for computing the electromagnetic properties and AC losses in systems of YBCO (yttrium barium copper oxide) conductors on roll assisted biaxially textured substrates (RABiTS). In this model, the magnetic field dependent permeability and ferromagnetic loss of the substrates in RABiTS YBCO tapes are taken into account. The simulations were employed to simulate the AC loss in stacks of two parallel connected YBCO tapes. The simulation results are compared with the experimental data to check the validity of the simulation model. The result reveals an effective way of significantly reducing AC loss in YBCO tapes by stacking two RABiTS YBCO coated conductors with the appropriate relative tape orientation.

Transport and magnetization ac losses of ROEBEL assembled coated conductor cables: measurements and calculations

Abstract: Many superconductor applications require cables with a high current capacity. This is not feasible with single-piece coated conductors because their ac losses are too large. Therefore, it is necessary to develop superconducting cables with a high current capacity and low ac losses. One promising solution is given by ROEBEL cables. We assembled three ROEBEL cables from commercial YBCO coated conductors. The cables have the same width but a different number of strands, which results in different aspect ratios and current capacities. We experimentally studied their ac losses under a transport current or a perpendicular magnetic field. In addition, we performed numerical calculations, which agree with the experiments, especially for the transport case. We found that in the cables there is good current sharing between the strands. We also found that stacking the strands reduces the magnetization losses. For a given critical current, thicker cables have lower magnetization ac losses. In addition, a conducting matrix is not required for a good current sharing between strands.

A scalable readout system for a superconducting adiabatic quantum optimization system

Abstract: We have designed, fabricated and tested an XY-addressable readout system that is specifically tailored for the reading of superconducting flux qubits in an integrated circuit that could enable adiabatic quantum optimization. In such a system, the flux qubits only need to be read at the end of an adiabatic evolution when quantum mechanical tunneling has been suppressed, thus simplifying many aspects of the readout process. The readout architecture for an N-qubit adiabatic quantum optimization system comprises N hysteretic dc SQUIDs and N rf SQUID latches controlled by bias lines. The latching elements are coupled to the qubits and the dc SQUIDs are then coupled to the latching elements. This readout scheme provides two key advantages: first, the latching elements provide exceptional flux sensitivity that significantly exceeds what may be achieved by directly coupling the flux qubits to the dc SQUIDs using a practical mutual inductance. Second, the states of the latching elements are robust against the influence of ac currents generated by the switching of the hysteretic dc SQUIDs, thus allowing one to interrogate the latching elements repeatedly so as to mitigate the effects of stochastic switching of the dc SQUIDs. We demonstrate that it is possible to achieve single-qubit read error rates of < 10 − 6 with this readout scheme. We have characterized the system level performance of a 128-qubit readout system and have measured a readout error probability of 8 × 10 − 5 in the presence of optimal latching element bias conditions.

Phase dynamics in a stack of inductively coupled intrinsic Josephson junctions and terahertz electromagnetic radiation

Abstract: The Josephson effect is a phenomenon of current flow across two weakly linked superconductors separated by a thin barrier, i.e.?a Josephson junction, associated with coherent quantum tunneling of Cooper pairs. Many novel phenomena appear due to the nonlinear property of the Josephson effect, such as Shapiro steps in dc current?voltage (IV) characteristics of a Josephson junction under microwave irradiation, which can be used as a voltage standard. The Josephson effect also provides a unique way to generate high-frequency electromagnetic (EM) radiation by dc bias voltage as the inverse process of the Shapiro step. The discovery of cuprate high-Tc superconductors accelerated the effort to develop novel sources of EM waves based on a stack of atomically densely packed intrinsic Josephson junctions (IJJs), since the large superconductivity gap covers the whole terahertz (THz) frequency band. Very recently, strong and coherent THz radiation has been successfully generated from a mesa structure of a Bi2Sr2CaCu2O8 + ? single crystal which works both as the source of energy gain and as the cavity for resonance. This experimental breakthrough posed a challenge to the theoretical study of the phase dynamics of stacked IJJs, since the phenomenon cannot be explained by the known solutions of the sine-Gordon equation so far. It is then found theoretically that, due to the huge inductive coupling of IJJs produced by the nanometer junction separation and the large London penetration depth of the order of micrometers of the material, a novel dynamic state is stabilized in the coupled sine-Gordon system, in which ? ? kinks are developed in the superconductivity phase difference responding to the standing wave of the Josephson plasma and are stacked alternately in the c axis. This novel solution of the inductively coupled sine-Gordon equations captures the important features of experimental observations. The theory predicts an optimal radiation power larger than the one observed in recent experiments by orders of magnitude, and thus suggests the technological relevance of the phenomena.

Angular dependence of J c for YBCO coated conductors at low temperature and very high magnetic fields

Abstract: We present very high field angle dependent critical current density (Jc) data for three recently obtained YBa2Cu3O7−x (YBCO) coated conductors used in the construction of high field solenoids. We find that strongly correlated pins, such as BaZrO3 (BZO) nanorods, while yielding strong c-axis peaks at 77 K, produce almost no measurable contribution at 4 K. Raising the field from <5 to 30 T at 4 K causes a marked transition from a Ginzburg‐Landau-like Jc(θ ) at low fields to a marked cusp-like behavior at high fields. Transmission electron micrographs show that all samples contain a high density of stacking faults which strengthen the plane correlated pinning parallel to the ab planes produced by the intrinsic ab-plane pinning of the Cu‐O charge reservoir layers. (Some figures in this article are in colour only in the electronic version)

YBCO step-edge junctions with highIcRn

Abstract: Step-edge junctions represent one type of grain boundary Josephson junction employed in high-temperature superconducting junction technology. To date, the majority of results published in the literature focus on [001]-tilt grain boundary junctions (GBJs) produced using bicrystal substrates. We investigate the step morphology and YBCO (yttrium barium copper oxide) film structure of YBCO-based step-edge junctions on MgO [001] substrates which structurally resemble [100]-tilt junctions. High-resolution electron microscopy reveals a clean GBJ interface of width ~ 1?nm and a single junction at the top edge. The dependence of the transport properties on the MgO step-edge and junction morphology is examined at 4.2?K, to enable direct comparison with results for other junction studies such as [001]-tilt and [100]-tilt junctions and building on previously published 77?K data. MgO step-edge junctions show a slower reduction in critical current density with step angle compared with [001]-tilt junctions. For optimized step parameters, transport measurements revealed large critical current and normal resistance (IcRN) products (~3?5?mV), comparable with the best results obtained in other kinds of [100]-tilt GBJs in YBCO at 4.2?K. Junction-based devices such as SQUIDs (superconducting quantum interference devices) and THz imagers show excellent performance when MgO-based step-edge junctions are used.

Filament to filament bridging and its influence on developing high critical current density in multifilamentary Bi2Sr2CaCu2Ox round wires

Abstract: Increasing the critical current density (Jc) of the multifilamentary round wire Ag/Bi2Sr2CaCu2Ox(2212) requires understanding its complicated microstructure, in which extensive bridges between filaments are prominent. In this first through-process quench study of 2212 round wire, we determined how its microstructure develops during a standard partial-melt process and how filament bridging occurs. We found that filaments can bond together in the melt state. As 2212 starts to grow on subsequent cooling, we observed that two types of 2212 bridges form. One type, which we call Type-A bridges, forms within filaments that bonded in the melt; Type-A bridges are single grains that span multiple bonded filaments. The other type, called Type-B bridges, form between discrete filaments through 2212 outgrowths that penetrate into the Ag matrix and intersect with other 2212 outgrowths from adjacent filaments. We believe the ability of these two types of bridges to carry inter-filament current is intrinsically different: Type-A bridges are high- Jc inter-filament paths whereas Type-B bridges contain high-angle grain boundaries and are typically weak linked. Slow cooling leads to more filament bonding, more Type-A bridges and a doubling of Jc without changing the flux pinning. We suggest that Type-A bridges create a 3D current flow that is vital to developing high Jc in multifilamentary 2212 round wire.

Superconductivity: its role, its success and its setbacks in the Large Hadron Collider of CERN

Abstract: The Large Hadron Collider (LHC), the particle accelerator at CERN, Geneva, is the largest and probably the most complex scientific instrument ever built. Superconductivity plays a key role because the accelerator is based on the reliable operation of almost 10?000 superconducting magnets cooled by 130 tonnes of helium at 1.9 and 4.2?K and containing a total stored magnetic energy of about 15?000?MJ (including detector magnets). The characteristics of the 1200 tonnes of high quality Nb?Ti cables have met the severe requests in terms of critical currents, magnetization and inter-strand resistance; the magnets are built with an unprecedented uniformity, about 0.01% of variation in field quality among the 1232 main dipoles, which are 15?m in length and 30 tonnes in weight. The results of this 20-year-long enterprise will be discussed together with problems faced during construction and commissioning and their remedies. Particular reference is made to the severe incident which occurred nine days after the spectacular start-up of the machine on 10 September 2008. The status of repair and the plan for the physics programme in 2010 are also presented.

Improvement of the critical temperature of superconducting NbTiN and NbN thin films using the AlN buffer layer

Abstract: Thin superconducting NbTiN and NbN films with a few nm thickness are used in various device applications including in hot electron bolometer mixers. Such thin films have lower critical temperature (Tc) and higher resistivity than corresponding bulk materials. In an effort to improve them, we have investigated an effect of the AlN buffer layer between the film and the substrate (quartz or soda lime glass). The AlN film is deposited by DC magnetron sputtering, and the process condition is optimized so that the x-ray diffraction intensity from the 002 surface of wurtzite AlN becomes the highest. By use of this well-characterized buffer layer, Tc and the resistivity of the NbTiN film with a few nm thickness are remarkably increased and decreased, respectively, in comparison with those without the buffer layer. More importantly, the AlN buffer layer is found to be effective for NbN. With the AlN buffer layer, Tc is increased from 7.3 to 10.5 K for the 8 nm NbN film. The improvement of Tc and the resistivity originates from the good lattice matching between the 002 surface of AlN and the 111 surface of NbTiN or NbN, which results in better crystallization of the NbTiN or NbN film. This is further confirmed by the x-ray diffraction measurement.

First commercial medium voltage superconducting fault-current limiters: production, test and installation

Abstract: In 2008/09 Nexans SuperConductors GmbH made the step from R&D activities to the production of the first non-publicly funded fault-current limiter units. In close cooperation with two customers, Applied Superconductor Limited (ASL, UK) and Vattenfall (Germany), Nexans was able to design, produce and deliver two resistive superconducting limiter devices. Both devices are designed for the medium voltage grid and were tested at the high voltage and high power lab IPH in Berlin. The superconducting components of both limiters, coils of bulk MCP BSCCO-2212, have been designed and produced by Nexans.

The reversible strain effect on critical current over a wide range of temperatures and magnetic fields for YBCO coated conductors

Abstract: The strain effect on critical current (Ic(?)) in Y Ba2Cu3O7 ? ? (YBCO) coated conductors was evaluated at temperatures in the range 20?83?K under magnetic fields parallel to the c-axis up to 10?T. The peaked reversible variation of Ic with applied uniaxial strain was confirmed in the self-field at all tested temperatures. The strain sensitivity increases with increasing temperature, resulting in a more pronounced reversible suppression with strain at higher temperature. Interestingly, it was found that the peak strain corresponding to the maximum of Ic shifts to the compressive side with decreasing temperature. Such a peak shift cannot be explained by a change in the thermal residual strain of the YBCO film, suggesting that the peak strain of the Ic(?) in YBCO coated conductors is not determined only by relaxation of the residual strain. The strain sensitivity of Ic(?) at 60?K becomes greater with increasing magnetic field, while the influence of the magnetic field is much less pronounced at 20?K. The in-field Ic(?), including the compressive strain region as well as the tensile region, shows a double peak behavior at low magnetic field at 77 and 83?K. The temperature and magnetic field effect on Ic(?) in YBCO coated conductors is discussed considering flux pinning within the grains and on grain boundaries.

Transport critical currents in the iron pnictide superconducting wires prepared by the ex situ PIT method

Abstract: The discovery of iron-based superconductors, the first non-cuprate family of superconductors with Tc above 40 K, has stimulated enormous interest in the field of superconductivity since last year. This remarkable discovery not only offers the opportunity to study the origin of superconductivity, but also opens up new possibilities of application. One of the most fascinating and useful properties of superconductors is the ability to carry electrical current with zero resistance. Here, we report the successful fabrication of dense Sr0.6K0.4Fe2As2 superconducting wires using the ex situ powder-in-tube (PIT) method and demonstrate a transport Jc of 3750 A cm − 2 at 4.2 K. The connectivity of grains was improved upon doping (Ag or Pb) and the transport property of Sr0.6K0.4Fe2As2 wires was enhanced for a lead-doped sample, especially in low fields, to a best Ic of 37.5 A. Our results suggest that grain boundary properties require much greater attention when looking for applications of the high-Tc iron-based superconductors.

Unified scaling law for flux pinning in practical superconductors: I. Separability postulate, raw scaling data and parameterization at moderate strains*

Abstract: The unified strain-and-temperature scaling law underlies the many pinning-force-model expressions proposed to parameterize the dependence of the critical current Ic on magnetic field B, temperature T and applied axial strain ? in superconductors for high-field magnet design. Increasingly, these expressions have been evaluated by use of multiparameter simultaneous fits, without the use of scaling. In this review, we reintroduce the unified scaling law on which the recent parameterizations are based, as well as the power of raw scaling data for parameter consistency, extrapolation capability and data-based model evaluation. The unified scaling law (USL) for the flux-pinning force per unit conductor length in practical high-field superconductors is expressed by where K(t, ?0) is a temperature-and-strain dependent prefactor. The scaling variables are: reduced magnetic field with Bc2*(t, ?0) an effective upper critical field; reduced temperature with Tc*(?0) an effective strain-dependent critical temperature; and intrinsic axial strain defined as zero at the strain ?m, where Ic is maximum. The scaling parameters?p and q are constants, which is a necessary (but not sufficient) condition for scaling. The strain dependences of Tc* and Bc2* are correlated as where and (bc2(?0) is also designated as s(?0) in the recent literature). It is shown that, when raw scaling data are used to determine the scaling parameter w, it has a constant value w?3.0 ? 0.1 in a wide range of Nb3Sn superconductors. This is significant, because w is the only scaling parameter that requires very large data matrices of Ic(B, T, ?) to determine its value. Unified scaling has been demonstrated in a number of different superconducting materials, including Nb3Sn, Nb3Al and more recently, the fundamentally distinct MgB2 and Bi-2223 material systems. The separable form of the USL is a great simplification in which the USL is parameterized in terms of separate functions of intrinsic strain ?0, reduced temperature t and reduced magnetic field b, given by with the following dimensionless notation: , and . C is a proportionality constant. With this separable form, the scaling law is broken down into five dimensionless scaling functions with values ranging from 0 and 1: bc2(?0), bc2(t), g(?0), h(t) and f(b). Each of these separate functions depends individually on strain, reduced temperature or reduced magnetic field, with no commingled variables. We show that most parameterizations recently proposed for the USL can be broken down into these five single-variable scaling functions, providing the ability to determine the free scaling parameters in small groups ( 0); temperature parameters Tc*(0) and ?; magnetic field parameters Bc2*(0, 0), p and q; and C is a proportionality constant. With this parameterization, the scaling parameters themselves are also separable, an important feature for practical engineering purposes, because the parameter values can be built up from separate strain and temperature measurements. The only non-separable parameter, w, is fixed at 3.0, as described above. The parameter ? is fixed at 0, 1 or 2, corresponding to the three parameterization models in present use for the temperature function h(t), all of which are effectively equivalent in fitting accuracy at T ? 4?K (the simplest being the original parameterization ? = 0). At high compressive strains (?0 < ? 0.5%), a consensus for the best parameterizations has not yet been achieved. In Part II of this review, raw scaling data will be used to assess the most commonly used parameterizations in this regime, especially bc2(?0) and g(?0) at high compressive strains, and h(t) over a wide temperature range.

Quench behavior of conduction-cooled Y Ba2Cu3O7−δ coated conductor pancake coils stabilized with brass or copper

Abstract: The quench behavior of conduction-cooled Y Ba2Cu3O7?? (YBCO) coated conductor pancake coils is reported. Two coils, one stabilized with copper and one with brass, are wound with 25?m of conductor and instrumented with a heater and a large number of voltage taps and thermocouples. The critical current, minimum quench energy (MQE) and two-dimensional normal zone propagation velocity (NZPV) are measured as a function of I/Ic, where I is the transport current and Ic is the critical current. Although the non-uniform temperature and self-field distributions within the coils result in a non-uniform Ic, the heater is able to induce quenches with energies above the MQE and both longitudinal and transverse propagation velocities are measured. In both coils, the longitudinal NZPV (10?40?mm?s ? 1) is about one order of magnitude larger than the transverse NZPV (1?2?mm?s ? 1). Moreover, a comparison between the Cu-stabilized coil and a short, straight Cu-stabilized sample shows that the one-dimensional longitudinal propagation in the short sample is significantly faster than the longitudinal propagation in the coil. This is due to transverse heat conduction (transverse propagation) which reduces the temperature gradients in the coil but also slows down the longitudinal propagation. Thus, designing a quench detection system based upon data from one-dimensional experiments may result in an unintended level of risk.

Development and characterization of the superconducting integrated receiver channel of the TELIS atmospheric sounder

Abstract: The balloon-borne instrument TELIS (TErahertz and submillimetre LImb Sounder) is a three-channel superconducting heterodyne spectrometer for atmospheric research use. It detects spectral emission lines of stratospheric trace gases that have their rotational transitions at THz frequencies. One of the channels is based on the superconducting integrated receiver (SIR) technology. We demonstrate for the first time the capabilities of the SIR technology for heterodyne spectroscopy in general, and atmospheric limb sounding in particular. We also show that the application of SIR technology is not limited to laboratory environments, but that it is well suited for remote operation under harsh environmental conditions. Within a SIR the main components needed for a superconducting heterodyne receiver such as a superconductor–insulator–superconductor (SIS) mixer with a quasi-optical antenna, a flux-flow oscillator (FFO) as the local oscillator, and a harmonic mixer to phase lock the FFO are integrated on a single chip. Light weight and low power consumption combined with broadband operation and nearly quantum limited sensitivity make the SIR a perfect candidate for use in future airborne and space-borne missions. The noise temperature of the SIR was measured to be as low as 120 K, with an intermediate frequency band of 4–8 GHz in double-sideband operation. The spectral resolution is well below 1 MHz, confirmed by our measurements. Remote control of the SIR under flight conditions has been demonstrated in a successful balloon flight in Kiruna, Sweden. The sensor and instrument design are presented, as well as the preliminary science results from the first flight. (Some figures in this article are in colour only in the electronic version)

Simulation of temperature and magnetic field distribution in superconducting bulk during pulsed field magnetization

Abstract: A theoretical simulation of electromagnetic and thermal fields was performed for a cryocooled superconducting bulk disc after applying a magnetic pulse. The results of the simulation qualitatively reproduced the experimental ones for the time and applied field dependences of the trapped field Bz and the temperature T on the bulk surface. For magnetic pulse application with a rise time of τ = 0.01 s, the magnetic flux propagation was about two orders of magnitude faster than the heat propagation because of the low thermal conductivity of the bulk. The results show that the intruding magnetic flux escaped because of the delayed temperature rise. For a longer magnetic pulse application with τ = 1 or 10 s, the flux propagation speed becomes slow and approaches the heat propagation speed. In this case, the magnetic flux escape attributable to the flux creep becomes small and a higher trapped field can be achieved. The method of exploring the enhancement of the trapped field using pulsed field magnetization is discussed.

Simulation of ac loss in Roebel coated conductor cables

Abstract: Roebel cables are a promising concept for high-current coated conductor cables with low ac loss. In order to optimize their design, simulations are necessary. In this paper we compute the field and current distribution and the ac loss due to either a transport current or an applied magnetic field. We distinguish between the coupled and uncoupled situations. These situations correspond to the limits of low and high resistance between strands in one transposition length, respectively. We perform the simulations with two different methods. One method assumes the critical-state model for the superconductor and calculates the current distribution by minimizing the magnetic energy variation; the other describes the superconductor with a power-law resistivity and uses the finite-element technique for solving the problem. We found that the results from both models agree with each other. Moreover, the coupling strongly influences the current distribution for all cases, as well as the magnetization loss. However, the effect on the transport loss is small. In conclusion, simulations are necessary in order to predict the loss reduction caused by the strand transposition in a Roebel cable. Our simulation methods are useful for these predictions.

WInd-and-react Bi-2212 coil development for accelerator magnets

Abstract: Sub-scale coils are being manufactured and tested at Lawrence Berkeley National Laboratory in order to develop wind-and-react Bi 2 Sr 2 CaCu 2 O x (Bi-2212) magnet technology for future graded accelerator magnet use. Previous Bi-2212 coils showed significant leakage of the conductors' core constituents to the environment, which can occur during the partial melt reaction around 890°C in pure oxygen. The main origin of the observed leakage is intrinsic leakage of the wires, and the issue is therefore being addressed at the wire manufacturing level. We report on further compatibility studies, and the performance of new sub-scale coils that were manufactured using improved conductors. These coils exhibit significantly reduced leakage, and carry currents that are about 70% of the witness wire critical current (l c ). The coils demonstrate, for the first time, the feasibility of round wire Bi-2212 conductors for accelerator magnet technology use. Successful high temperature superconductor coil technology will enable the manufacture of graded accelerator magnets that can surpass the, already closely approached, intrinsic magnetic field limitations of Nb-based superconducting magnets.