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High temperature superconductors, including Bi:2212, are being considered for high magnetic field magnets to be used in particle accelerators. A knowledge of HTS magnetization properties are needed for calculating field errors in such accelerator magnets. Here we present magnetic measurements of a 17 strand Bi:2212 Rutherford cable. The Bi:2212 strand was a Bruker OST PMM170123 non-twisted wire wound into a cable with dimensions 1.46 mm x 7.8 mm and cable pitch of 50.8 mm. M-H loop measurements were made with a Hall probe at 4.2 K. Two calibration methods were compared, (i) a flux exclusion approach, and (ii) a Ni replacement technique. The latter was found to be more reliable in this case. An effective filament diameter of 358 μm was calculated for the strand at 4.2 K and a perpendicular applied field of 3.33 T, significantly larger than the diameter of the filament bundle. Magnetization values at 1 T on the shielding branch were seen to be 100 kA/m when normalized to the total strand in the cable volume. The penetration field ≅ 0.4 T.

We present measurements on high-temperature superconducting (HTS) superconducting cables using a NbTi-wound, 3-T dipole ac magnetometer. Our goal is to measure the magnetization properties of the cables for use in accelerator magnet field error calculations. YBCO-wound CORCTM and Roebel cables were measured in pool boiling helium with no epoxy impregnation. The Roebel cable was a 9–5.6 style, while the CORCTM sample had 16 tapes, and was 3.21 mm OD. For measurement, the samples were inserted in a G-10 holder with pickup and compensation coils, and subjected to a ramping dipole field. The signals were read from a set of nanovoltmeters using data aquisition (DAQ) to monitor the pickup coils, and the data were imported into a LabVIEW software program. The Roebel and CORCTM cable measurements were compared to one another and to the tapes they were made with. Maximum cable magnetization was 1000 kA/m for the CORC and 2200 kA/m for the Roebel when normalizing to total cable volume. These values increased to 2860 and 2640 kA/m, respectively, when normalizing to tape volume.

High-temperature-superconducting (HTS) cables are being considered for use in particle accelerator beam steering magnets. Knowledge of the magnetic properties of these HTS cables is needed for accelerator magnet design. Here, we present the results from a Hall probe magnetometer made for measuring the M-μ 0 H of short sections of superconducting cables in applied magnetic fields up to 12 T. The M-μ 0 H of a CORC cable was measured with field sweeps of ± 4.5 T and also a -2- to 8-T loop. Relevant to a particle accelerator with an injection field of 1 T, the magnetization M inj of the CORC cable was ≅ 900 kA/m and the penetration field B p ≅ 1 T. The magnetization and penetration field of the cables are compared to tape from which the cables were wound, and the results are compared to NbTi and Nb 3 Sn conductors.

High-temperature superconductors (HTSs), including YBCO coated conductors, are being considered for high field dipole and quadrupole magnets for future particle accelerators. The quality of the magnetic field generated by the magnet and its temporal stability are important considerations. The persistent current magnetization of superconductors can degrade the field quality of the magnetic field generated by magnets made from them by causing error fields, and flux creep in the superconductor may lead to drift in the value of the magnetic field during magnetic field ramp holding sequences, including at particle injection. HTS composites possess larger persistent current magnetizations and larger flux creep than low-temperature superconductor composites, and therefore, the error fields and any temporal change of them are typically larger. In this paper, the 4.2 K magnetization of a small segment of coated conductor was measured in a vibrating sample magnetometer in perpendicular fields up to 14 T, and the decay was measured at 1 T. The magnetization and decay of a CORC cable were measured in a 12 T cryogen-free magnet. A Hall sensor, placed close to the sample, was used to measure the magnetization. The applied magnetic field was ramped through full flux penetration, and then, brought within one flux penetration depth of the target applied magnetic field before the decay measurement was started. Selective cycling of the magnetic field applied to the samples was explored as a technique to reduce the magnetization and its decay. Significant suppression of both the initial magnetization at 1 T and its decay were observed.

Filamentization has been shown to be an effective method in reducing magnetization ac loss of RE-Ba-Cu-O (REBCO) coated conductors. We have used a laser striation method followed by selective electroplating of copper to fabricate fully stabilized multifilamentary REBCO tapes with 12, 24, and 46 filaments. While expected levels of ac loss reduction has been confirmed in short segments of such multifilamentary tapes, electric coupling between the filaments of long tapes needs to be suppressed by transposition of the filaments. In this work, a technique was developed and implemented to reduce ac losses in REBCO multifilamentary coated conductors through a new design that allows us to transpose the filaments without any mechanical twisting. The process consists of patterning REBCO tapes by laser ablation followed by partial insulation and bonding. A twofold reduction of ac magnetization losses was achieved in the transposed multifilamentary REBCO tape when subject to an external perpendicular magnetic field at frequencies between 50 and 200 Hz and peak field values up to 88 mT. The filament-to-filament resistance and its effect on the coupling have also been investigated.

The ordered double perovskites Sr2CrOsO6 and Ca2CrOsO6 have been synthesized and characterized with neutron powder diffraction, electrical transport measurements, and high field magnetization experiments. As reported previously Sr2CrOsO6 crystallizes with R 3 ¯ symmetry due to a−a−a− octahedral tilting. A decrease in the tolerance factor leads to a−a−b+ octahedral tilting and P21/n space group symmetry for Ca2CrOsO6. Both materials are found to be ferrimagnetic insulators with saturation magnetizations near 0.2 μB. Sr2CrOsO6 orders at 660 K while Ca2CrOsO6 orders at 490 K. Variable temperature magnetization measurements suggest that the magnetization of the Cr3+ and Os3+ sublattices have different temperature dependences in Sr2CrOsO6. This leads to a non-monotonic temperature evolution of the magnetic moment. Similar behavior is not seen in Ca2CrOsO6. Both compounds have similar levels of Os/Cr antisite disorder, with order parameters of η=80.2(4)% for Sr2CrOsO6 and η=76.2(5)% for Ca2CrOsO6, where η=2θ−1 and θ is the occupancy of the osmium ion on the osmium-rich Wyckoff site.

MgB2 superconducting wires and bulks with nano-La2O3 addition have been studied. A series of MgB2 superconducting bulk samples with nano-La2O3 addition levels of 0, 5, 7, 18wt% were prepared. AC resistivity data showed slight increases of Bc2 and unchanged Birr for the bulk samples with doping levels lower than 7 wt% and decreased critical fields for the heavily doped (18 wt%) bulk. X-ray diffraction (XRD) showed the presence of LaB6 in the nano-La2O3 doped MgB2 bulk samples and decreased MgB2 grain size in nano-La2O3 doped bulks. Monocore powder-in-tube (PIT) MgB2 wires without and with 5 wt% nano-La2O3 addition (P-05) were prepared for transport property measurement. 2mol%C-doped Specialty Materials Inc. (SMI) boron powder was used for wire P-05 and previously prepared control wires (control wires were made without the addition of nano-La2O3 powder, W-00 and P2). Low field magnetic properties were obtained from magnetization loop (M–H), transport critical current density (Jc) was measured at 4.2 K for the nano-La2O3 doped PIT wire (P-05) and the control samples (P2 and W-00). The transport critical current density Jc (B) of P-05 at 4.2 K and 8 T (6.0 ×104 A/cm2) was twice that of the control wire. The critical magnetic fields (Bc2 and Birr ) of P-05 and the control sample P2 were compared. The critical fields of P-05 were slightly less than those of P2. Kramer-Dew-Hughes plots indicated a change from surface pinning to a mixture of volume pinning and surface pinning. It is shown that enhancement of P-05’s transport properties is due to additional flux pinning by the fine-size rare-earth borides rather than enhanced Bc2 or Birr.

Persistent magnetization currents are induced in superconducting filaments during the current ramping in magnets. The resulting perturbation to the design magnetic field leads to field quality degradation, particularly at low field, where the effect is stronger relative to the main field. The effects observed in NbTi accelerator magnets were reproduced well with the critical-state model. However, this approach becomes less accurate for the calculation of the persistent-current effects observed in Nb 3 Sn accelerator magnets. Here, a finite-element method based on the measured strand magnetization is validated using three state-of-the-art Nb 3 Sn accelerator magnets featuring different subelement diameters, conductor critical currents, magnet designs, and test temperatures. The temperature dependence of the persistent-current effects is reproduced. Based on the validated model, the impact of conductor design on the persistent-current effects is discussed. The strengths, limitations, and possible improvements of the approach are also discussed.

The coupling magnetization of a Rutherford cable is inversely proportional to an effective interstrand contact resistance R eff , a function of the crossing-strand resistance R c , and the adjacent strand resistance Ra. In cored cables, R eff continuously varies with W, the core width expressed as percent interstrand cover. For a series of un-heat-treated stabrite-coated NbTi LHC-inner cables with stainless-steel (SS, insulating) cores, R eff (W) decreased smoothly as W decreased from 100%, whereas for a set of research-wound SS-cored Nb 3 Sn cables, R eff plummeted abruptly and remained low over most of the range. The difference is due to the controlling influence of R c - 2.5 μΩ for the stabrite/NbTi and 0.26 μΩ for Nb 3 Sn. The experimental behavior was replicated in the R eff (W )'s calculated by the program CUDI, which (using the basic parameters of the QXF cable) went on to show in terms of decreasing W that: 1) in QXF-type Nb 3 Sn cables (R c = 0.26 μΩ), Reff dropped even more suddenly when the SS core, instead of being centered, was offset to one edge of the cable; 2) R eff decreased more gradually in cables with higher R c 's; and 3) a suitable R eff for a Nb 3 Sn cable can be achieved by inserting a suitably resistive core rather than an insulating (SS) one.

For many accelerator magnets field quality at the bore is a critical requirement for which reason it is necessary to fully characterize the persistent-current magnetization of strands of the kind under consideration for these magnets. The magnetization of a strand is generally measured in a magnetometer. However, certain effects can differentiate such measurements from the true magnetizations of strands in magnets. This report focuses on persistent-current magnetization: 1) measured by vibrating-sample magnetometer on segments of strand extracted from a section of heat treated Nb 3 Sn cable as functions of angle of the applied field, and 2) calculated as function of applied transport current. It is found that the magnetization of a strand in a cable increases by ~10% as the field applied to the cable is shifted from edge-on to face-on, and that the difference between the current-on and current-off magnetizations is not significant until close to the operational field of a magnet.

Suppression of magnetization and effective filament diameter deff with twisting was investigated for a series of recent Bi2212 strands manufactured by Oxford Superconducting Technologies. We measured magnetization as a function of field (out to 14 T), at 5.1 K, of twisted and nontwisted 37 × 18 double restack design strands. The samples were helical coils 5-6 mm in height and approximately 5 mm in diameter. The strand diameter was 0.8 mm. The magnetization of samples having twist pitches of 25.4, 12.7, and 6.35 mm were examined and compared to nontwisted samples of the same filament configuration. The critical state model was used to extract the 12-T d eff from magnetization data for comparison. Twisting the samples reduced d eff by a factor of 1.5-3. The d eff was shown to increase both with L and L p . Mathematical expressions, based upon the anisotropic continuum model, were fit to the data, and a parameter γ 2 , which quantifies the electrical connectivity perpendicular to the filament axis, was extracted. The bundle-to-bundle connectivity along the radial axis was found to be approximately 0.2%. The d eff was substantially reduced with L p . In addition, the importance of understanding sample length dependence for quantitative measurements is discussed.

In this paper, we report the properties of two tube type Ta doped Nb3Sn strands: one strand was additionally Ti doped by way of a Sn-Ti alloy core, and the other had high Cu/Sn ratio within the filaments. Higher irreversibility field (Birr) was obtained on the quaternary strand with respect to the (Nb-7.5 wt.%Ta)3Sn strand. High Cu/Sn ratio decreased the amount of coarse grain formation, but also degraded the layer Jc of the tube type strand by depressing the Sn content in the fine grain (FG) layer. A new type of strand, the subelement of which is composed of seven bare Cu-Sn cored Nb tube filaments, was designed with the aim to reduce the unreacted Nb area fractions. The test results of the first experimental strand are reported. The unreacted Nb ratio is reduced relative to normal tube type strands and the FG area fraction is improved. The unique structure of this strand makes it also possible to improve the stoichiometry of FG and reduce the effective diameter (deff) .

A dc transport current was applied to the strands of a Roebel cable at 77 K in liquid nitrogen bath. The inter-strand contact resistance was measured. It was modified either by applying a pressure on the cable at 77 K in liquid nitrogen bath or using different soldering patterns between the strands of the cable. Magnetization ac losses were measured in frequency range 50-200 Hz in applied magnetic field 4-70 mT perpendicular to the broader face of the cable to test the inter-strand contact resistance effect. High stability and very low level of coupling losses were observed in the cables even with the lowest inter-strand resistances.

One of the goals of the Large Hadron Collider Accelerator Research Program (LARP) is to demonstrate the feasibility of Nb3Sn technology for a proposed luminosity upgrade based on large aperture high gradient quadrupole (HQ) magnets. For such magnets, field quality at the bore is a critical requirement for which reason the parasitic magnetization of the windings must be reduced to manageable limits. In other words, it is necessary to minimize 1) the static intrastrand persistent-current magnetization of the cable and 2) the cable's coupling magnetization caused by coupling currents passing through interstrand contact resistance during field ramping. This report focuses on persistent-current magnetization as measured by vibrating-sample magnetometry on pieces of strand removed from a section of heat treated HQ cable.

Superconducting MgB2 nanopowder has been synthesized through chemical reaction between lithium borohydride and magnesium hydride at relatively low temperatures. From quantitative Rietveld analysis, the average crystallite size of MgB2 powder was evaluated to be 33 nm. The superconducting transition temperature of the MgB2 nanopowder was found to be 38.8–38.9 K from magnetization and DC susceptibility measurements. Powder morphology has been evaluated by scanning electron microscopy.

The magnetizations of eight high-gradient quadrupole cables designated HQ and QXF and a pair of strands, identical in architecture but with different effective strand diameters extracted from an HQ and a related QXF cable, were measured. In the service of field quality assessment, the cable magnetizations and losses were measured by pickup coil magnetometry at 4.2 K in face-on fields, B m , of ± 400 mT at frequencies, f, of up to 60 mHz. Based on the coupling component of loss, Q coup , the coupling magnetization M coup = Q coup /4B m was derived for a ramp rate of 7.5 mT/s. Persistent current (shielding) magnetization and loss (M sh and Q h,strand ) were measured on short pieces of extracted strand by vibrating sample magnetometry at 4.2 K. Unpenetrated M-B loops to ± 400 mT and fully penetrated loops to ± 14 T were obtained. M coup can be easily controlled and reduced to relatively small values by introducing cores and adjusting the preparation conditions. But in low fields near injection Nb3Sn's high J c and correspondingly high M sh,cable may call for magnetic compensation to preserve field quality. The suitably adjusted cable and strand fully penetrated M-B loops were in reasonable accord leading to the conclusion that strand magnetization is a useful measure of cable magnetization, and that when suitably manipulated can provide input to magnet field error calculations.

The ordered double perovskites Sr2CrOsO6 and Ca2CrOsO6 have been synthesized and characterized with neutron powder diffraction, electrical transport measurements, and high field magnetization experiments. As reported previously Sr2CrOsO6 crystallizes with R 3 ¯ symmetry due to a−a−a− octahedral tilting. A decrease in the tolerance factor leads to a−a−b+ octahedral tilting and P21/n space group symmetry for Ca2CrOsO6. Both materials are found to be ferrimagnetic insulators with saturation magnetizations near 0.2 μB. Sr2CrOsO6 orders at 660 K while Ca2CrOsO6 orders at 490 K. Variable temperature magnetization measurements suggest that the magnetization of the Cr3+ and Os3+ sublattices have different temperature dependences in Sr2CrOsO6. This leads to a non-monotonic temperature evolution of the magnetic moment. Similar behavior is not seen in Ca2CrOsO6. Both compounds have similar levels of Os/Cr antisite disorder, with order parameters of η=80.2(4)% for Sr2CrOsO6 and η=76.2(5)% for Ca2CrOsO6, where η=2θ−1 and θ is the occupancy of the osmium ion on the osmium-rich Wyckoff site.

Cu sheathed polycrystalline Ba0.6K0.4Fe2As2 superconducting wire was prepared by a two-step powderin-tube method. A pressure of 2 GPa was applied to a short sample before heat treatment. Magnetization and transport measurements were performed to investigate the effects of cold high pressure densification on the microstructure and superconductivity. The cold pressed sample shows an improved self-field transport critical current density of 2.8 10 4 A/cm 2 (Ic = 83 A) at 4.2 K, which is nearly as twice as the unpressed sample. However, both samples manifest pronounced weak-link behavior, suggesting the technique need to be further optimized. The comparison of properties between pressed and unpressed sample and related mechanism was discussed. 2012 Published by Elsevier B.V.

In-field ac losses were measured on a 2 G YBCO tape by three different methods; these consisted of an M(H) loop (inductive) based method as well as two different calibration-free approaches. For the M(H) loop-based method the sample was placed inside a pick-up coil connected in series with, and in opposition to, a compensation coil. The voltage developed in series across these two coils was directly proportional to the magnetic moment of the sample. AC loss was determined from the area of the hysteresis loop generated under an externally applied cyclic field. The two separate calibration-free methods used a lock-in amplifier-based technique. The first of these used a pick-up coil wound in parallel with the ac magnet winding. Two identical systems were used, each containing an ac magnet equipped with a measuring coil. One of the magnets contained the sample and the other one was left empty. The second calibration-free method measured components of the sample's magnetic moment in a transverse ac magnetic field. Here the sample was placed inside an ac magnet; two pick-up coils were placed outside the magnet. The ac magnet, which is rectangular in shape, has an aspect ratio such that a homogeneous field is produced in the sample space and almost no stray fields are present in the pick-up coils. To measure ac loss, a z-component of the magnetic moment of the tape is determined using a dipole approximation. The AC losses for a coated conductor in fields of amplitude up to 0.14 T applied perpendicular to the broad face of the tape were measured from 36 Hz-75 Hz in a liquid nitrogen bath using all three methods. These results were compared, and a satisfactory agreement between all three methods was obtained.

A tubular technique for economical production of Nb3Sn material with large numbers of subelements is being explored by Supergenics I LLC and Hyper Tech Research Inc. The number of subelements was increased to 919 (744 subelements plus 175 Cu filaments) by increasing the size at which restacking is carried out. The product exhibited no fabrication problems and was drawn down and tested at a wire diameter of 0.42 mm, where the subelements are 10 μm in diameter. Recently we increased the subelement number to 1387 (1248 subelements plus 139 Cu filaments), which gives a subelement size of 12 μm in 0.7 mm diameter wires. Heat treatment (HT) of different subelement restacks has been investigated, and the best results of critical current and stability are presented. The strain tolerance of the strands with 192 and 744 subelements was also tested, and the strand with fine subelement size showed a high intrinsic irreversible strain limit.

SmFeAsO1-xFx type oxypnictide bulks were made via a single-step synthesis. Polycrystalline bulk samples with nominal composition SmFeAsO1-xFx (x=0.35 and 0.4) were synthesized by conventional solid state reaction using the starting powders SmF3 , Sm, Fe, Fe2O3 , and high-purity As (specifically allotrope-gray arsenic). The weighed powders were thoroughly ground by hand with a mortar and pestle and pressed into pellets in an argon-filled glove box. The pressed pellets were sealed in an evacuated quartz ampoule and heat treated at 1100°C using different ramp rates to reach the reaction temperature. Magnetization vs. temperature measurements were made on the various specimens, where a maximum onset Tc of 55 K was achieved.

Multistrand cables may exhibit two classes of parasitic magnetization both of which can distort the bore-field of an accelerator magnet: (1) a static magnetization (“hysteretic”) resulting from intrastrand persistent currents, and (2) a dynamic magnetization produced by interstrand coupling currents generated during field ramping. The latter, which are moderated by the interstrand contact resistances (ICR), can be controlled by the presence of an insulating core inserted between the layers of the cable. Stainless steel ribbon (with its associated native oxide coating) is a frequently used core. Recently, however, MgO-paper tapes and woven s-glass ribbons have been suggested by LBNL (Lawrence Berkeley National Laboratory) as alternative core materials in the interests of improved flexibility and compatibility with the cabling process. This paper reports on the results of calorimetric AC loss (hence ICR) measurements on a set of four such cables and presents the results within the context of previously measured cored and uncored Nb3Sn cables. Also considered is a typical ramp-rate-induced coupling magnetization and its relationship to persistent-current magnetizations over the operating range of an accelerator magnet.

Modern high-Jc Nb3Sn strands and cables used in high field accelerator magnet models suffer from flux jump-related instabilities at low magnetic fields, these instabilities are exacerbated by enhanced magnetization and/or reduced RRR. Such instabilities and their response to deff and RRR in individual strands have been widely reported. In this work, we look at the influence of strand deformation during cabling on RRR and magnetization for strands extracted from Nb3Sn Rutherford cables. Seven separate cable types were used for this study, a number which had been previously used in both racetrack and quadrupole magnets. In the present experiments, longitudinal variations of RRR were estimated from multiple-tap measurements along the length of strands extracted from the various cables and then reacted in a special fixture. Magnetization measurements were then performed on cable sections to examine the effect of edge deformation. The implications of these measurements for stability are discussed.

Calorimetric and magnetic AC loss measurements were made on a pair of TQ (ldquotechnology quadrupolerdquo) class cables, one with and the other without a stainless steel core. The coupling losses are compared to the cables' hysteretic losses determined at the same time; the latter being compared to the magnetically measured hysteretic losses of individual strand segments. The outcomes of these measurements were: (i) the effective interstrand contact resistances of the uncored and cored cables, (ii) their hysteretic losses translated in terms of ldquoeffective, ICRs (interstrand contact resistances)rdquo for comparison with the above, and hence an evaluation of the real effectiveness of the core in a contemporary cable (iii) a comparison of the cable and strand hysteretic losses and hence an estimate of an effective filament diameter, , for the as-cabled strands.

The Nb3Sn cables being developed under the LARP program will need to have suitable, reproducible, and uniform interstrand contact resistances (ICR) in the interests of accelerator- field quality (low coupling magnetization) and stability (current- sharing between strands). In the first of a pair of studies ICR was measured in response to variation of strand surface condition. As a sequel to this the present set of measurements explores the influence of cable preparation conditions on ICR, determined as before using calorimetric and magnetic AC-loss measurement techniques in face-on and edge-on applied AC fields. Uncored Rutherford cables were wound at Fermilab from OST MJR-type strand and wrapped with S-glass tape impregnated in most cases with a ceramic binder (applied before reaction HT). The HT/pressuriza- tion sequence of the cables was made to mimic as closely as possible the expected LARP magnet-fabrication schedules. Derived from maxima in the loss-vs-frequency curves was an ICR "inho- mogeneity-two ICRs for each cable pack each associated with a particular fraction of the strands. Furthermore, both of the ICRs, at 0.4 and 4 muOmega, would in a magnet be too small to satisfy the generally accepted magnetization requirements.

High Performance internal-Sn-type Nb3Sn strands are only marginally stable. This has significant consequences for strand measurement at higher fields (12 T), and leads to magnetic flux jumping at lower fields (0-1 T). However, the most troublesome regimes of instability from a magnet maker's point of view are moderate fields where current transport and external magnetic fields interact to create an instability not evident from magnetization at current levels well below that predicted by higher field results. These effects can be described in terms of adiabatic and dynamic stability, using a model first developed by Wilson. Modifications are presented for a multifilamentary strand, with HEP-relevant values of deff, RRR, strand diameter, Jc, and local thermal environment. Model predictions are compared to experimental strand results to bring out the relative functional dependence of stability on RRR and deff.

Jelly-roll Nb-Al strands were rapidly heated and quenched (RHQ) to bcc phase from temperatures around 1900/spl deg/C. Transformation heat treatments were subsequently applied to convert the as-quenched bcc phase into superconducting A15 phase. These heat treatments consisted of a fast ramp (0.5 to 10 min) to 1000/spl deg/C followed by a long hold (10 h) at 800/spl deg/C. Magnetization measurements performed by vibrating sample magnetometry under fields up to 30 T were used to determine the irreversibility field, the upper critical field and the dependence of the critical current density on the field. These properties were all found to be higher for samples with high initial heating rate in the transformation heat treatment. Some variability of the critical current density was observed for samples prepared under apparently the same conditions. The scaling of the bulk pinning force density over temperatures ranging from 4.2 K to 8 K is analyzed.

The magnetization of various design internal-Sn type Nb/sub 3/Sn strands was analyzed in terms of effective filament diameter, d/sub eff/. Assuming circular geometry and ignoring demagnetization, values of d/sub eff/ have been calculated numerically for various configurations including unsplit subelement and split subelement cases with 2, 3, 4 and 6 splits. In the numerical calculation, the magnetization was treated as the summation of the magnetization of sections aligned perpendicular to the applied field. The angle-averaged d/sub eff/ value was calculated for every degree over the split regions. The d/sub eff/ of single restacked strands using subelements with various numbers of splits has been calculated and the results show the influence of the split design of strands on the magnetization in high fields. Furthermore, the numerical results were compared with the reported simplified analytical results, about 20% difference indicates the approximation of the simplification.

A simple investigation into the properties of weakly linked filaments in striated YBa2Cu3O7−d (YBCO) coated conductors has been performed. Such weak-link connections may arise in two different contexts within the coated conductors, the one case being the occurrence of unintentional connections. However, it may be desirable to intentionally induce the weak-link connections in order to allow a current sharing between the filaments. This allows the localized degradation of a given filament to be circumvented at a small cost in terms of the added losses and magnetization. This problem has been treated mathematically with a comparison of the superconductor weak linking in contrast to the normal-metal-induced current sharing. The estimates for target values of filament coupling are deduced, and the results in terms of magnetization are then determined. The YBCO samples created, using the pulsed laser deposition, are described that have these properties. The samples consist of one control and one striated sample...

Magnetization, AC loss, χ DC and d eff were measured for several designs of rod-in-tube based internal-Sn Nb 3 Sn type superconducting multifilamentary strands. Two kinds of subelement geometries were used in strand construction. The first had the standard annular Nb/Cu ring surrounding a Sn source; the second was similar but included an internal split intended to reduce magnetization and loss. Strands with 18 and 36 subelements were measured, at strand diameters of 0.5–0.8 mm. Optical, SEM, and EDS measurements were performed on these samples; average radii are reported and physical barrier integrity is found to be good. The magnetizations of these structures were analyzed in terms of a d eff parameter, in this case calculated for annular structures. Analytical and numerical results of these calculations are presented. It was found that in general annular structures should be expected to have d eff values somewhat larger than the subelement diameter; the value of this enhancement is reported. Also, the effect of subelement splitting on d eff and magnetization was calculated. The results of these calculations are compared to the experimentally measured results. Reductions in d eff due to subelement splitting are compared to direct, low-field susceptibility measurements. Magnetization values are seen to be nearly uniformly lower in the split subelement strands, and this leads in some but not all cases to significantly lower d eff values. Possible reasons for these discrepancies are discussed.

Magnetization vs. applied field measurements (M-H loops) were taken on YBCO thin films with filaments patterned into them. The YBCO was deposited onto LaAlO/sub 3/ substrates using PLD, and the filaments were formed by laser ablation. M-H loops were taken at 4.2 K in fields up to /spl plusmn/9 T using a vibrating sample magnetometer technique, the field applied perpendicular to the film width, d. The losses were seen to be greatly reduced by filament width reductions following the standard expression Q/sub h//H/sub m/ /spl prop/ dJ/sub c//10. The penetration field was also well described by the standard "high field" expression H/sub p/ = (0.4J/sub c/t)ln(d/t + 1), where t is the film thickness. The regimes of applicability of the loss expression were investigated, in particular near H/sub m/ /spl ap/ H/sub p/. A more general form of the loss equation was obtained and compared with the high-field approximation. The result was that although Q/sub h//H/sub m/ still increased in proportion to d, the rate of increase decreased as H/sub m/ approached H/sub p/.

The flux jump stability of Nb/sub 3/ Sn, internal-Sn strands was investigated using both experiment and calculation. An MgB/sub 2/ bulk sample was also used as a model system. M-H loops were measured in the temperature range of 4.2 K to 15 K for fields up to 1.7 T using a small vibrating sample magnetometer (VSM) system, while /spl plusmn/9 T loops were taken with a larger VSM. Measurement were made on samples up to 3 cm long in transverse fields, and in the form of spiral samples (in axial fields) containing up to 37 cm of sample length. Some strands displayed larger flux jumps at higher fields, and numerous small flux jumps at low fields. In one case, the outer Cu stabilizer was removed from the strand, and as a result the strand became less stable, with flux jumping occurring at higher fields (where the magnetization is lower). Additionally, the influence of ramp rate on flux jumping was investigated. This was done by direct experiment, the use of a model system (MgB/sub 2/), and the extension of existing theory. It was found that the maximum instability, which is related to the maxima in dJ/dT, could occur off-axis. Additionally, higher ramp rates led to decreased stability. This was seen to lead in some cases to a ramp rate dependence in the field position of the flux jump.

The tube filling/forming powder-in-tube (PIT) technique was employed at Hyper Tech Research for the continuous processing of MgB 2 /Fe composite strand. Demonstration lengths of up to 70 m of 1.2 mm diameter precursor strand––an Fe-clad Mg + B elemental powder mixture––have so far been produced. Short samples of this strand, heat treated for 3–30 min/800–950 °C, have yielded transport critical current densities ( J c ) of 2–3×10 4 A/cm 2 at 4 K/4 T, 4×10 4 A/cm 2 at 4 K/2 T, and at 4 K in self field an extrapolated J c of at least 2×10 5 A/cm 2 . Batch-processed PIT MgB 2 /Fe strands and tapes were made at the University of Wollongong’s Institute for Superconducting and Electronic Materials. These were the focus of magnetization- and magnetic-shielding studies. Depending on the thickness of the Fe sheath, the superconducting core could be completely shielded at low fields (e.g. below 2 kOe). Taking this into account, the magnetic J c is extracted, and found to be comparable to the extrapolated transport value.

Magnetization vs. applied field measurements ( M–H loops) were taken on short samples of YBa 2 Cu 3 O 7− δ (YBCO) thin films which were divided into narrow filaments. The YBCO was deposited using pulsed laser deposition onto single-crystal LaAlO 3 substrates, with a range of film thicknesses from 0.25 to 0.33 μm. Using a YAG laser, the thin films were patterned into linear striations by removing strips of the superconductor by laser ablation. The resulting striated filamentary structure serves to reduce the effective width of the YBCO films and hence the hysteresis loss in the superconducting samples. The magnetization measurements were taken over the temperature range of 4.2–77 K in applied fields of 0–17 kOe using a vibrating sample magnetometer. The measured hysteresis losses show a highly linear relationship between superconductor filament width and hysteresis loss as anticipated. However, the laser ablation process did result in the redeposition of YBCO along the edges of individual filaments. Degradation of T c and J c due to the ablation process is discussed.

Vibrating sample magnetization (VSM) measurements have been made on round Fe-clad monocore MgB 2 wire fabricated by a powder-in-Fe-tube technique. Magnetization, AC loss, and transport measurements are reported. The M – H and loss measurements were made with a field sweep amplitude of 17 kOe over a temperature range of 4–40 K. In a round monocore wire the superconducting core was completely shielded against an external field of ±2 kOe; at higher fields a fixed ΔH was observed (partial shielding). It was found that the signal picked up by the VSM was itself also partially shielded, i.e. reduced in magnitude during its ‘return passage’ through the sheath, leading to a low field dip in the magnetization. FEM calculations of this latter effect are presented. The fact that the field which reaches the inside of the conductor is less than the external field leads to a real loss suppression, even at moderate fields. The low field dip caused by a suppression of the signal emanating from the superconducting core leads to an apparent loss suppression, i.e., the magnetically measured loss is less than the real (e.g., calorimetric) loss. The difference between real and apparent loss is estimated by FEM calculation, and the field sweep amplitude dependence of these two components was computed neglecting the effects of partial flux penetration.

Vibrating sample magnetization (VSM) measurements have been made on Fe-clad monocore and multifilamentary MgB2 wire fabricated by the powder-in-Fe-tube technique. The measurements were made with a field sweep amplitude of 17 kOe over a temperature range of 4–40 K. Although the benefits of magnetic shielding to both high-Tc and low-Tc superconductors (SC) have been listed, and in the former case demonstrated, its applicability to the intermediate-Tc material MgB2 is new. Since Fe or low-alloy steels seem necessary primary cladding (sheath) materials for powderin-tube MgB2 both from mechanical- and chemical-compatibility standpoints, the resulting strands automatically become ideal candidates upon which to explore and exploit the principles of magnetic shielding. The effectiveness of the shield depends on its thickness, relative permeability (and hence strength of the applied field) and shape-dependent demagnetization. In the round monocore wire the SC core was completely shielded against an external field of � 2 kOe; at higher fields a fixed DH was observed (partial shielding). Finally, in considering AC loss in the partially shielded regime, allowance must be made for the fact that the signal picked up by the VSM is itself also partially shielded, i.e. reduced in magnitude during its ‘‘return passage’’ through the sheath. FEM calculations of this latter effect are presented. 2002 Published by Elsevier Science B.V.

Understanding the variation of the superconducting properties as a function of the maximum heat treatment temperature or heating time in jelly-roll Nb/sub 3/Al wire is important for the optimization of the rapid heating conditions. Jelly-roll Nb/sub 3/Al wires with a Nb matrix were ohmically-heated to maximum temperatures ranging from 1900 to 2300 /spl deg/C in vacuum in order to optimize the rapid heating conditions and to investigate the superconducting properties for this transformation method. The diameter of the wire was 0.94 mm. The sample was monitored using a noncontact measurement technique employing a photodiode to measure high temperatures (over 2000 /spl deg/C) during rapid heating. After the surface temperature reached a maximum, the sample was quenched in liquid gallium. The samples were annealed for 10 hours at 800 /spl deg/C after the rapid heating process to transform the bcc-phase to the A15 phase. The superconducting properties including DC magnetization and critical current were explored for various maximum heating temperatures and heating times.

Long dipole magnets guide the particle beams in synchrotron-type high energy accelerators. In principal Cu-wound DC-excited dipoles could be designed to deliver a very uniform transverse bore field, i.e. with small or negligible harmonic (multipolar) distortion. But if the Cu is replaced by (a) superconducting strand that is (b) wound into a Rutherford cable carrying a time-varying transport current, extra magnetizations present within the windings cause distortions of the otherwise uniform field. The static (persistent-current) strand magnetization can be reduced by reducing the filament diameter, and the residue compensated or corrected by strategically placed active or passive components. The cable’s interstrand coupling currents can be controlled by increasing the interstrand contact resistance by: adjusting the level of native oxidation of the strand, coating it, or by inserting a ribbon-like core into the cable itself. Methods of locally compensating the magnetization of NbTi and Nb3Sn strand and cable are discussed, progress in coupling-current suppression through the use of coatings and cores is reviewed, and a method of simultaneously reducing both the static and dynamic magnetizations of a NbTi cable by means of a thin Ni core is suggested.

Jelly-roll Nb/sub 3/Al wires with a Nb matrix were ohmically-heated to maximum temperatures ranging from 1800 to 2300/spl deg/C in vacuum in order to optimize the ohmic-heating conditions and to investigate the superconducting properties for this transformation method. The diameters of these wires were 0.80-1.34 mm. Surface temperature at the central point of the sample was measured by a photodiode during rapid ohmic-heating. After the surface temperature reached a maximum, the sample was quenched in liquid gallium. All the samples were annealed at 800/spl deg/C for 3-25 hours after the rapid heating process to transform the bcc-phase to the A15 phase. Critical currents were measured up to 23 T. The samples heated to 2000/spl deg/C showed a maximum critical current density of 64 A/mm/sup 2/ at 20 T. The critical current density decreased with increasing maximum temperature during rapid heating. This paper describes the superconducting properties, the rapid heating conditions and the achievement of high critical current density at high magnetic fields.

In hadron accelerators, between low energy particle injection and beam accumulation, the guiding dipoles are ramped at some rate dB/dt. Both at injection and during ramping the static and dynamic magnetizations of the magnet windings introduce multipolar distortions into the beam-line field. Dynamic magnetization, controllable by cable design, is estimated and used to provide a criterion against which to evaluate the allowable magnitude of static (persistent-current) magnetization, M, from a field-quality standpoint. The it is of NbTi and advanced Nb/sub 3/Sn conductors are compared and with regard to the latter the question of flux-jump stability is explored. A magnetization criterion for such stability is presented and compared to experiment. It is noted that since /spl Delta/M is proportional to critical current density, J/sub c/, and the strand's effective filament diameter, d/sub eff/, the latter has frequently been specified as a critical parameter, although it will need to be re-specified with every increase in J/sub c/. It is pointed out that although in the manufacture of MF Nb/sub 3/Sn composites d/sub eff/ will continue to be useful as a processing parameter in that it gauges the extent to which changes of processing conditions change the degree of interfilamentary bridging, and is measured by comparing the magnetic and transport-measured J/sub c/s, the most useful information is embodied in the results of the magnetization measurement alone.

Measurements of magnetization, M , vs. magnetic field strength, H , have been made on HTSC Bi:2223/Ag and LTSC NbTi/matrix multifilamentary tapes as a function of ramp rate. The integrated loss, Q tot , was extracted from these M – H loops, which had a maximum field amplitude of 17 kOe and ramp rates, d H /d t , ranging from 20 to 700 Oe/s. All measurements were taken at 4.2 K, with H directed normal and parallel, respectively, to the broad tape face. The samples were from 0.5 to 1.5 cm in length, and untwisted. The expressions needed to properly extract effective transverse matrix resistivities, ρ ⊥ , are discussed and applied. In general, these expressions indicate that transverse loss in untwisted strands grows linearly with the aspect ratio, in contrast to that in twisted strands, whose losses grow with the square of the aspect ratio. With low aspect-ratio LTSC tapes, our equations gave the expected values of ρ ⊥ . However, departures from the expected ρ ⊥ emerged with further flattening of the strands as the filamentary array itself became sparse and highly aspected, and the filaments themselves became flattened. A model attributing the anomalous ρ ⊥ to the effects of these features is proposed.

The problem of flux jumping in NbTi superconducting strands was solved 30 years ago by the introduction of suitably fine multifilamentary subdivision accompanied by twisting. But with recent advances in A15 superconductors we are entering a regime in which J/sub c/ may be increasing at a rate faster than effective filament diameter, d/sub eff/, can be reduced. This is particularly true for Nb-clad melt-processed Nb/sub 3/Al strands which at fields below H/sub c2,Nb/ have subelements which are superconductively coupled via the Nb matrix. Control of d/sub eff/, while still requiring attention, is less of a problem for the new very high J/sub c/ Nb/sub 3/Sn strands whose sub-elements, although fully bridged, are still able to be reduced in size. The results of some low field magnetization experiments are presented and interpreted in terms of a simple model.

A model which describes a low-field anomaly in the M–H loops of composite superconductors has been developed. Corresponding calculations were applied to the anomaly as it occurs in a composite system composed of NbTi filaments in a proximity-effect coupled matrix. The use of this system has allowed a very clear and unambiguous separation of the magnetization contributions of `strong' (S) and `weak' (W) superconducting elements. The low field anomaly, in the form of a difference between the shielding- and trapping-branch critical current densities ( J c ) for the W-phase, and hence an asymmetric magnetization loop, takes place when the magnetization currents in the W-phase become influenced by the density of trapped flux in the S-phase. Comparison of theory and experiment gives values for the W-region J c which are in general agreement with those derived from the results of more conventional measurements.

In generally well-connected high-temperature superconducting strands the overall measured transport critical current density (CCD), J cT,meas (`averaged' over grains and grain boundaries) is suppressed (or `throttled') to less than its maximum possible value, here referred to as the intrinsic CCD, J cT,intr , as a result, for example, of filament-diameter undulations (sausaging), path constrictions due to non-superconducting inclusions, cracking, and in extreme cases filament breakage. A realizable goal in the form of an estimated J cT,intr is a valuable adjunct to process optimization activities. It is shown that in fine-filament monocore or multifilamentary (MF) strands the magnetically measured CCD, J c,mag , although not necessarily equal to J cT,intr provides a useful lower limit to it. By the same token, in bridged MF strands the saturated bridged magnetization, Δ M br,sat , under ideal conditions could yield an actual value for J cT,intr , and in general will provide a lower limit. Based on these principles, analysis of the bridged magnetizations of a group of round MF Bi:2212/Ag strands yielded J ct,intr (4.2 K) lower limits of 3∼7×10 5 A/cm 2 .

Conventional magnetization measurements of proximity effect (PE) coupling in superconducting (S) multifilamentary (MF) strands have been made, and a previously developed technique has been used to quantify the coupling strength in terms of a PE coupling supercurrent, J/sub cn/. This quantity has been determined for a number of MF strand types processed under various conditions. All are NbTi/Cu MF composites in some of which the NbTi filaments were surrounded by Nb "barriers", while in the others they were not. The magnetic field dependence of J/sub cn/ was also explored. The measured differences in J/sub cn/ are attributed to differences in the properties of the filament/matrix (normal metal, N) interface treated as a S1-S2-N trilayer. These results, together with those of an earlier series of experiments confirm that PE coupling between the filament and the matrix, and hence the normal-state coherence length, /spl xi//sub N/, is enhanced by the presence of a Nb (S2) barrier; and furthermore that the field-dependent coupling strength, gauged in terms of J/sub cn/ or /spl xi//sub N/, depends on the thickness of that barrier. It is demonstrated that magnetic PE-coupling studies not only provide a technique for extracting /spl xi//sub N/, a quantity central to artificial pinning center (APC) design, but also may help in the investigation of S-N boundary conditions in the clean limit.

An existing model for bridging induced magnetization and hysteretic loss has been extended to take into account the inhomogeneous morphology of metallurgical bridges. After an appropriate way of calculating the magnetization of a strand with a distribution of filament-bunch sizes had been devised some reasonable conformity between the results of optical image analysis and magnetization measurement was obtained. The previously developed anisotropic continuum model of bridging in multifilamentary (MF) strands led to a description of bridging intensity in terms of a dimensionless parameter /spl gamma//sub 2/ representing the area-fraction of bridged matrix material perpendicular to the filamentary axis. Based on measurements of variously heat treated (HT) internal-Sn ITER strands /spl gamma//sub 2/ was found to have values of 0.2 to 0.7%. At the same time saturation values of the length- and twist-pitch dependent magnetizations of the bridged samples were 3-6 times greater than the estimated un-bridged values. Some attention is also given to the influence of HT time and ramp-rate on the extent of bridging.

Calorimetric and magnetic measurements of AC loss have been performed on stabrite-coated Rutherford cable conforming to the Large Hadron Collider (LHC) winding (28 strand) specification, with the field directed either perpendicular to the broad face (the face-on, FO, orientation) or parallel to it (edge-on, EO). It was found that the insertion of a thin metallic or insulating core into the cable suppressed the FO AC loss under typical conditions by a factor of 10 and rendered it practically insensitive to the application of cold uniaxial stress (of up to about 78 MPa). The FO loss having been suppressed, it could also be controlled (i.e. finely adjusted) by changing the level compaction (by varying core thickness at thickness) or external compaction (by changing the overall thickness of the cable). Of course the EO loss was much less sensitive to the presence of a core, the use of which therefore enabled the FO and EO losses to be independently adjusted.

The results of the 2nd VAMAS measurement intercomparison program on low-frequency (hysteretic) a.c. loss are presented and discussed. Two sets of multifilamentary NbTi strands (Set No. 1: copper matrix, fil. diams 0.5, 1, 3, and 12 μm; Set No. 2: cupronickel matrix, fil. diams 0.4, 0.5, and 1 μm) were subjected to interlaboratory testing. In an initial series of tests, samples in various forms (e.g. wire bundles, coils) were measured mostly by vibrating-sample- and SQUID magnetometry. Considerable scatter was noted especially in the small-filament-diameter a.c.-loss data. In a study of measurement accuracy, a supplementary series of tests compared the results of VSM measurement of a given pair of copper-matrix samples. In the light of all the results, factors contributing to a.c. loss error are discussed and recommendations are made concerning the specification of future a.c.-loss measurement intercomparisons.

In this work, the transport and magnetization properties of distributed-barrier Rod-in-Tube (RIT) strands and Tube Type strands are studied. While Tube Type strands had smaller magnetizations and thus better stabilities in the low field region, their 12 T non-Cu Jcs were somewhat smaller than those of the RIT strands. Microstructures were investigated in order to find out the reasons for the difference in non-Cu Jc values. Their grain size and stoichiometry were found to be comparable, leading to similar layer Jcs. Accordingly it was determined that the lower A15 area fraction rather than the quality of A15 layer was the cause of the discrepancy in non-Cu Jc. Subsequently, the area utilizations of subelements were investigated. While for a RIT strand the fine grain (FG) A15 area occupies ~60% of a subelement, for a Tube Type strand it is no more than 40%. Further analysis indicates that the low FG area fraction in a Tube Type strand is attributed to its much larger unreacted Nb area fraction. Finally, a simple change in strand architecture is proposed to reduce the unreacted Nb area fraction., 26 pages

A general treatment of the influence of metallurgical and electrical bridging and the associated interfilamentary coupling on the DC magnetic response of multifilamentary superconductors has been developed. Metallurgical bridging is emphasized, and is treated from the perspective of an anisotropic continuum; a comparison with the traditional d eff approach is made. Bridging-induced loss is seen to be determined by an intrinsic parameter γ 2 , describing the amount of electrical connectivity perpendicular to the filamentary axis. Expressions for magnetization and penetration fields for samples as a function of twist pitch and sample length are given. Transfer lengths are calculated, and the effects of weak connectivity along the filament axis are also discussed. The applicability of, and deviations from, the anisotropic continuum approximation are addressed. The expressions developed are then applied to experimental results for multifilamentary Nb 3 Sn and Bi:HTSC/Ag strands.