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ABSTRACT: 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.
04/2013;
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ABSTRACT: Recent advances in MgB2 conductors are leading to a new level of performance.
Based on the use of proper powders, proper chemistry, and an architecture which
incorporates internal Mg diffusion (IMD), a dense MgB2 structure with not only
a high critical current density Jc, but also a high engineering critical
current density, Je, can be obtained. In this paper, a series of these advanced
(or second - generation, "2G") conductors has been prepared. Scanning electron
microscopy and associated energy dispersive X-ray spectroscopy were applied to
characterize the microstructures and compositions of the wires, and a dense
MgB2 layer structure was observed. The best layer Jc for our sample is 1.07x105
A/cm2 at 10 T, 4.2 K, and our best Je is seen to be 1.67x104 A/cm2 at 10 T, 4.2
K. Optimization of the transport properties of these advanced wires is
discussed in terms of B-powder choice, area fraction, and the MgB2 layer growth
mechanism.
06/2012;
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ABSTRACT: Monocore powder-in-tube MgB2 strands were cold-drawn and heat-treated at 600C
and 700C for times of up to 71 hours and structure-property relationships
examined. Drawing-induced elongation of the Mg particles led, after HT, to a
textured macrostructure consisting of elongated polycrystalline MgB2 fibers
separated by elongated pores. The superconducting Tc, Jc and Fp were correlated
with the macrostructure and grain size. Grain size increased with HT time at
both 600C and 700C. Jc and hence Fp decreased monotonically but not linearly
with grain size. Overall, it was observed that at 700C, the MgB2 reaction was
more or less complete after as little as 30 min; at 600C, full reaction
completion did not occur until 71 h. into the HT. Transport, Jct(B) was
measured in a perpendicular applied field, and the magnetic critical current
densities, Jcm\bot(B) and Jcm{\phi}(B), were measured in perpendicular and
parallel (axial) applied fields, respectively. Particularly noticeable was the
premature dropoff of Jcm\bot(B) at fields well below the irreversibility field
of Jct(B). This effect is attributed to the fibrous macrostructure and its
accompanying anisotropic connectivity. Magnetic measurements with the field
directed along the strand axis yielded a critical density, Jcm\bot(B), for
current flowing transversely to the strand axis that was less than and dropped
off more rapidly than Jct(B). In the conventional magnetic measurement, the
loop currents that support the magnetization are restricted by the lower of
Jct(B) and Jcm{\phi} (B). In the present case the latter, leading to the
premature dropoff of the measured Jcm(B) compared to Jct(B) with increasing
field. This result is supported by Kramer plots of the Jcm{\phi} (B) and Jct(B)
data which lead to an irreversibility field for transverse current that is very
much less than the usual transport-measured longitudinal one, Birr,t.
09/2011;
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ABSTRACT: The transport properties of a number of MgB2 strands have been investigated
in terms of their response to strand design, starting B powder choice, and the
approach to C doping used. The strands had various designs, specifically; (i)
several chemical barriers were introduced, viz: Fe and Nb, (ii) the strands
were encased in various outer-sheath materials, viz.: monel, Cu+monel,
monel+glidcop, Nb+monel, (iii) the filament counts were varied (1, 18, and 36),
and (iv) the final strand diameter was varied. In addition, for a subset of the
strand designs several B powder and C-dopant types were investigated.
Specifically, two types of amorphous B powder were used: (i) Moissan based
"Tangshan boron" (ii) "SMI-boron" which is produced in a plasma torch by the
reduction-by-hydrogen of BCl3. Two approaches to C doping were taken: (i)
"malic-acid treatment" in which C is introduced into the B powder precursor by
the moderate temperature drying out a slurry of B mixed in with a
malic-acid-toluene solution (during which the malic acid decomposes leaving C
as the only solid residue) before the Mg powder is mixed in; (ii) direct C
doping of the SMI-produced B by introducing a known percentage of CH4 into the
plasma flame. Critical current densities, Jc, were measured on 1.5 m long
samples at 4.2 K in fields of up to 14 T; of all the strands measured, that
doped with SMI-C at a nominal 4 mol% C (in relation B) yielded the highest Jc
values e.g 1.1x105 A/cm2 at 7 T, 4.5x104 at 10 T, and 2.2x104 A/cm2 at 12 T.
The n-values are given for all strands at 5 and 10 T, and for a certain set of
strands the magnetic field dependencies of the n-values and the influence of
C-doping is presented. Finally we demonstrate that, over a wide range of B,
log(Jc) decreases linearly with B with a slope -{\alpha} such that the Jc(B) of
any strand can be parameterized in terms of {\alpha} and its zero-field
intercept Jc(B=0).
09/2011;
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ABSTRACT: Transport measurements of critical current density, Jct, in monocore
powder-in-tube MgB2 strands have been carried out at temperatures, T, of from
4.2 K to 40 K, and in transverse fields, B, of up to 14 T. Processing methods
used were conventional continuous-tube-forming-filling (CTFF) and
internal-magnesium-diffusion (IMD). Strands with several powder compositions
were measured, including binary (undoped) MgB2, 2% carbon doped MgB2, and 3%
carbon doped MgB2. Magnetization loops (M-B) were also measured, and magnetic
critical current density, Jcm, values extracted from them. The transport,
Jct(B) and magnetic, Jcm(B), critical current densities were compared. Also
studied was the influence of doping on the resistively measured irreversibility
field, Birr and upper critical field Bc2. Critical current densities, Jct, and
n-values were extracted from transport measurements and were found to be
universally related (for all B and T) according to n \propto Jctm in which m =
0.52 \pm 0.11. Likewise n was found to be related to B according to n \propto
B-p with a T-dependent p in the range of about 0.08~0.21. Further analysis of
the field (B) and temperature (T) dependencies of n-value resulted in an
expression that enabled n(B,T), for all B and T, to be estimated for a given
strand based on the results of transport Jct(B) measurements made at one
arbitrarily chosen temperature.
08/2011;
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ABSTRACT: SmFeAsO<sub>1-x</sub>F<sub>x</sub> type oxypnictide bulks were made via a single-step synthesis. Polycrystalline bulk samples with nominal composition SmFeAsO<sub>1-x</sub>F<sub>x</sub> (x=0.35 and 0.4) were synthesized by conventional solid state reaction using the starting powders SmF<sub>3</sub> , Sm, Fe, Fe<sub>2</sub>O<sub>3</sub> , 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 T<sub>c</sub> of 55 K was achieved.
IEEE Transactions on Appiled Superconductivity 07/2011; · 1.04 Impact Factor
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ABSTRACT: 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 Nb<sub>3</sub>Sn 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.
IEEE Transactions on Appiled Superconductivity 07/2011; · 1.04 Impact Factor
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ABSTRACT: Superconducting undulators are being constructed to satisfy an increasing need within the synchrotron radiation community for higher and higher energy photon beams. The paper focuses attention on the design of a helical undulator for the ILC photon (hence positron) source. Finite element method (FEM) modeling was performed to extend the design of our existing 14 mm period undulator design to shorter periods with a view towards the final design, construction and testing of a 10 mm undulator. To launch this task we modeled the properties of 10 mm period undulators formed around bore tubes of the 6 mm and 8 mm OD with winding-pack cross-sections of 3 mm × 6 mm and 3 mm × 11 mm. These windings do allow a pole width of 2 mm and can take advantage of the corresponding about 0.2 T field increment due to it. In order to reach the target bore field of 1.1 T it is necessary to either reduce the bore size and tighten the associated tolerances, or to use a higher performing strand. In the present work we have chosen to optimize the undulator design basing it on the properties of an advanced tube-type strand keeping the bore tube diameter at 6-8 mm in order to see the needed performance to meet this undulator specification.
IEEE Transactions on Appiled Superconductivity 07/2011; · 1.04 Impact Factor
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ABSTRACT: 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.
IEEE Transactions on Appiled Superconductivity 07/2011; · 1.04 Impact Factor
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ABSTRACT: In this work, a new strand design for Bi-2212 conductors was studied. The new design had a single stack approach with randomly oriented but densely packed two-dimensional Bi-2212 filaments. This Bi-2212 Two-Dimensional Random-Oriented Single-Stack (2D-ROSS) Round Wire design led to high levels of grain texture and significant amounts of Ag-superconductor interface within the filaments, while maintaining a high strand fill factor. This new design was compared to a strand made with a conventional billet layout. Heat treatments were performed under 100% flowing oxygen, and the samples were melt processed at 884, 886, 888, and 89°C. The strands were studied via transport, magnetic, and electron optics techniques. The strands manufactured with the new process showed higher critical currents than the standard design strands in terms of engineering critical current density (J<sub>e</sub>) but slightly lower layer critical current density (J<sub>c</sub>). However both strand sets were limited by filament sizes which were not yet optimized and excessive C-content in the powders. J<sub>e</sub> values of 48 A/mm<sup>2</sup> for the new strand design, and 26 A/mm<sup>2</sup> for the conventional double stack design, at 12 T and 4.2 K were observed. An interesting property of the new strand method was that the strands were less sensitive to small heat treatment, HT, temperature variations in terms of their final J<sub>c</sub> properties. Magnetic and microstructure/phase assemblage studies observed less filament bridging in the new strand design as compared to the conventional one.
IEEE Transactions on Appiled Superconductivity 07/2011; · 1.04 Impact Factor
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ABSTRACT: Superconducting MgB2 strands with nanometer-scale SiC additions have been
investigated systematically using transport and magnetic measurements. A
comparative study of MgB2 strands with different nano-SiC addition levels has
shown C-doping-enhanced critical current density Jc through enhancements in the
upper critical field, Hc2, and decreased anisotropy. The critical current
density and flux pinning force density obtained from magnetic measurements were
found to greatly differ from the values obtained through transport
measurements, particularly with regards to magnetic field dependence. The
differences in magnetic and transport results are largely attributed to
connectivity related effects. On the other hand, based on the scaling behavior
of flux pinning force, there may be other effective pinning centers in MgB2
strands in addition to grain boundary pinning.
02/2011;
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ABSTRACT: Conventional doping methods that directly add C or a C-bearing species to Mg + B powder have the disadvantage of adding C inhomogeneously, yielding either under-reacted regions or, in some cases, secondary phases which may be either beneficial or detrimental. Alternatively, pre-doped B powder provides a more homogeneous distribution of the C dopant in MgB2. In this work, powders containing varying amounts of C were used to produce in situ MgB2 strands which showed high values of Bc2, Birr and transport Jc (104 A cm − 2 at 13.3 T). Compared to SiC-added and malic-acid-treated strands the pre-doped MgB2 showed high values of Birr primarily due to more efficient C substitution into the B sublattice and a concomitant increase in transport Jc.
Superconductor Science and Technology 12/2010; 24(1):012001. · 2.66 Impact Factor
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ABSTRACT: The decomposition of malic acid in the presence of Mg and B was studied using Differential Scanning Calorimetry (DSC) and Thermogravimetric Analysis (TGA) which revealed that malic acid reacted with Mg but not B. Also, the addition of toluene to dissolve malic acid followed by subsequent drying resulted in no reaction with Mg, indicating that the malic acid had decomposed during the dissolution/drying stage. The total carbon contributed by toluene versus a toluene/5 wt% malic acid mixture was measured using a LECO CS600 carbon analyzer. The toluene sample contained ~0.4 wt% C while the toluene/malic acid mixture had ~1.5 wt% C, demonstrating that the toluene contributed a significant amount of carbon to the final product. Resistivity measurements on powder-in-tube MgB2 monofilamentary wires established that the toluene/malic acid doped sample had the highest Bc2. However, the toluene-only sample had the highest transport Jc over most of the magnetic field range (0-9 T), equaled only by that of toluene/malic acid sample in fields above 9 T. Comment: 17 pages, 6 figures, 1 table
10/2010;
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ABSTRACT: Conventional doping methods that directly add C or a C-bearing species to Mg+B powder have the disadvantage of adding C inhomogeneously, yielding either under-reacted regions or blocking phases. Pre-doped B powder provides a more homogeneous distribution of the C dopant in MgB2. Powders containing varying amounts of C were used to produce in-situ MgB2 strands which showed high values of transport Jc (104 A/cm2 at 13.3T). Compared to SiC-added and malic acid-treated strands the pre-doped MgB2 showed both higher values of Birr and transport Jc, indicating that the pre-doping of B leads to more efficient C substitution into the B-sublattice. Comment: 12 pages, 3 figures submitted to Applied Physics Letters
08/2010;
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ABSTRACT: We have investigated the frequency dependent contributions to the magnetic ac loss in a 10 strand Roebel cable with 2 mm wide non-insulated strands and a transposition length of 90 mm. This cable is made from 40 mm wide YBCO coated conductor tape manufactured by AMSC and stabilized by electroplating 25 µm thick copper on either side prior to the mechanical punching of the cable strands. The measurements were carried out in both perpendicular and parallel field orientation, at frequencies in the range of 30–200 Hz. While the loss in the perpendicular orientation is predominantly hysteretic in nature, we observe some frequency dependence of the loss when the cable approaches full flux penetration at high field amplitudes. The magnitude is consistent with eddy current losses in the copper stabilization layer. This supports the fact that the inter-strand coupling loss is not significant in this frequency range. In the parallel field orientation, the hysteresis loss in the Ni–W alloy substrate dominates, but we see an unusually strong frequency dependent contribution to the loss which we attribute to intra-strand current loops.
Superconductor Science and Technology 07/2010; 23(8):085009. · 2.66 Impact Factor
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ABSTRACT: Multistrand cables may exhibit two classes of parasitic magnetization both of which can distort the bore-field of the host magnet. They are: (1) a dynamic magnetization that is produced by interstrand coupling currents generated by time-varying magnet excitation and moderated by the interstrand contact resistances (ICR), (2) a static magnetization (??hysteretic??) resulting from the intrastrand persistent currents. This paper (i) compares the ICRs of two sets of cables with and without stainless steel cores and subjected to three levels of compaction during cabling, (ii) presents the results within the context the previously measured ICRs of a series of similar cables with cores of various widths, and (iii) concludes by comparing the LHC-ramp-rate induced coupling magnetization of a typical Rutherford cable with its transport-current-moderated persistent-current magnetizations at low and high fields.
IEEE Transactions on Appiled Superconductivity 07/2010; · 1.04 Impact Factor
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M. Majoros,
M.D. Sumption,
M.A. Susner,
S. Bhartiya,
M. Mahmud, E.W. Collings,
M. Tomsic,
M. Rindfleisch,
J. Phillips,
D. Lyons,
J. Yue
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ABSTRACT: A model helical undulator 250 mm in length, with a period of 14 mm, was designed, fabricated and tested. The helical coil was wound with multifilamentary, internal-Sn type Nba<sub>3</sub>Sn strand. The 0.7 mm OD strand was insulated with S-glass, wound onto the former, and reacted, after which the coil underwent vacuum epoxy impregnation. The beam aperture was 7 mm, the winding bore diameter was 8 mm, and the OD of the complete winding was 18 mm. The helical poles were made from 1016 low carbon steel, and projected slightly above the coil pack, with an OD of 19.06 mm. The coil I<sub>c</sub> and the magnetic field in the bore were measured at 4.2 K in a liquid helium bath. The bore field was also measured as a function of position along the length of the undulator.
IEEE Transactions on Appiled Superconductivity 07/2010; · 1.04 Impact Factor
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ABSTRACT: Calorimetric and magnetic measurements of AC loss in Nb3Sn Rutherford cables were accompanied by magnetic measurements of persistent‐current (hysteretic) loss in strands extracted from those cables. From the cable‐loss measurements it is shown that the introduction of a stainless steel core increases the effective interstrand contact resistance (ICR), initially 0.3 μΩ, by more than two orders of magnitude. Uniaxial pressure applied to the cable during winding and subsequently, seemingly by reducing side‐by‐side contact, increased the ICR. In an edge‐on applied AC field the persistent‐current cable loss was fully accounted for in terms of the individual‐strand loss. The persistent‐current loss measured for the cable in a face‐on applied field was higher that that in an edge‐on applied field, this may have been due to demagnetization effects.
AIP Conference Proceedings. 04/2010; 1219(1):191-198.
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ABSTRACT: The thermal diffusion properties of several different kinds of YBCO insulations and the quench properties of pancake coils made using these insulations were studied. Insulations investigated include Nomex, Kapton, and Mylar, as well as insulations based on ZnO, Zn2GeO4, and ZnO-Cu. Initially, short stacks of YBCO conductors with interlayer insulation, epoxy, and a central heater strip were made and later measured for thermal conductivity in liquid nitrogen. Subsequently, three different pancake coils were made. The first two were smaller, each using one meter total of YBCO tape present as four turns around a G-10 former. One of these smaller coils used Mylar insulation co-wound with the YBCO tape, the other used YBCO tape onto which ZnO based insulation had been deposited. One larger coil was made which used 12 total meters of ZnO-insulated tape and had 45 turns. The results for all short sample and coil thermal conductivities were ~1-3 Wm-1K-1. Finally, quench propagation velocity measurements were performed on the coils (77 K, self field) by applying a DC current and then using a heater pulse to initiate a quench. Normal zone propagation velocity (NZP) values were obtained for the coils both in the radial direction and in the azimuthal direction. Radial NZP values (0.05-0.7 mm/s) were two orders of magnitude lower than axial values (~14-17 mm/s). Nevertheless, the quenches were generally seen to propagate radially within the coils, in the sense that any given layer in the coil is driven normal by the layer underneath it. Comment: 58 pages, 5 tables, 16 figs
01/2010;
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ABSTRACT: The anisotropy of the critical current density (Jc) and its influence on measurement of irreversibility field (Birr) has been investigated for high quality, in-situ MgB2 strands. Comparison of transport and magnetization measurements has revealed the onset of a regime where large differences exist between transport and magnetically measured values of the critical current density and Birr. These effects, initially unexpected due to the lack of crystalline texture in these in-situ processed strands, appear to be due to a fibrous microstructure, connected with the details of the wire fabrication and MgB2 formation reactions. Scanning electron micrographs of in-situ-processed MgB2 monocore strands have revealed a fibrous microstructure. Grains (~100 nm) are randomly oriented, and there is no apparent local texture of the grains. However, this randomly oriented polycrystalline material has a fibrous texture at a larger length scale, with stringers of MgB2 (~ 60 {\mu}m long and ~5 {\mu}m in diameter) partially separated by elongated pores -- the spaces previously occupied by stringers of elemental Mg. This leads to an interpretation of the differences observed in transport and magnetically determined critical currents, in particular a large deviation between the two at higher fields, in terms of different transverse and longitudinal connectivities within the strand. The different values of connectivity also lead to different resistive transition widths, and thus irreversibility field values, as measured by transport and magnetic techniques. Finally, these considerations are seen to influence estimated pinning potentials for the strands. Comment: 43 Pages, 11 Figures, accepted by Supercon. Sci. Tech
01/2010;
