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The development of coils that can survive a quench is crucial for demonstrating the viability of MgB(2)-based main magnet coils used in MRI systems. Here we have studied the performance and quench properties of a large (outer diameter: 901 mm; winding pack: 44 mm thick × 50.6 mm high) conduction-cooled, react-and-wind (R&W), MgB(2) superconducting coil. Minimum quench energy (MQE) values were measured at several coil operating currents (I(op)), and distinguished from the minimum energy needed to generate a normal zone (MGE). During these measurements, normal zone propagation velocities (NZPV) were also determined using multiple voltage taps placed around the heater zone. The conduction cooled coil obtained a critical current (I(c)) of 186 A at 15 K. As the operating currents (I(op)) varied from 80 A to 175 A, MQE ranged from 152 J to 10 J, and NZPV increased from 1.3 to 5.5 cm/s. Two kinds of heater were involved in this study: (1) a localized heater (“test heater”) used to initiate the quench, and (2) a larger “protection heater” used to protect the coil by distributing the normal zone after a quench was detected. The protection heater was placed on the outside surface of the coil winding. The test heater was also placed on the outside surface of the coil at a small opening made in the protection heater. As part of this work, we also developed and tested an active protection scheme for the coil. Such active protection schemes are of great interest for MgB(2)-based MRIs because they permit exploitation of the relatively large MQE values of MgB(2) to enable the use of higher J(e) values which in turn lead to competitive MgB(2) MRI designs. Finally, the ability to use a quench detection voltage to fire a protection heater as part of an active protection scheme was also demonstrated.

Bulk samples of magnesium diboride (MgB2) doped with 0.5 wt% of the rare earth oxides (REOs) Nd2O3 and Dy2O3 (named B-ND and B-DY) prepared by standard powder processing, and wires of MgB2 doped with 0.5 wt% Dy2O3 (named W-DY) prepared by a commercial powder-in-tube processing were studied. Investigations included x-ray diffractometry, scanning- and transmission electron microscopy, magnetic measurement of superconducting transition temperature (T c), magnetic and resistive measurements of upper critical field (B c2) and irreversibility field (B irr), as well as magnetic and transport measurements of critical current densities versus applied field (J cm(B) and J c(B), respectively). It was found that although the products of REO doping did not substitute into the MgB2 lattice, REO-based inclusions resided within grains and at grain boundaries. Curves of bulk pinning force density (F p) versus reduced field (b = B/B irr) showed that flux pinning was by predominantly by grain boundaries, not point defects. At all temperatures the F p(b) of W-DY experienced enhancement by inclusion-induced grain boundary refinement but at higher temperatures F p(b) was still further increased by a Dy2O3 additive-induced increase in B irr of about 1 T at all temperatures up to 20 K (and beyond). It is noted that Dy2O3 increases B irr and that it does so, not just at 4 K, but in the higher temperature regime. This important property, shared by a number of REOs and other oxides promises to extend the applications range of MgB2 conductors.

Superconducting winding are enabling for the development of the highest power density motors and generators for aircraft use. It is presently estimated that motors with standard Cu wire windings can reach a future limit of at best 20 kW/kg. On the other hand, motors with superconducting windings are estimated to be able to produce more than double this, roughly 45 kW/kg. Superconducting windings carry very large current densities (1 kA/mm2 winding J e and more) and can generate much higher winding fields (even as high as 3–8 T in some designs), but do generate losses in the windings which must be removed at cryogenic temperatures. Here we focus on comparison of calculated and measured losses for one of the most promising superconducting materials for low loss motor and generator application, MgB 2 . MgB 2 is available in the form of wires (about 0.3-1 mm OD), and as triplet or 6 x 1 cables of these wires. Here we measure the hysteretic, coupling, and eddy current components of loss for a set of low-loss, multifilamentary MgB 2 wires which have been recently developed. It is shown that present day MgB 2 conductors are capable of 1–5 W/cm3 in the 100–200 Hz range for field amplitudes of about 0.4 T, which makes them usable for motors and generators with sufficient attention to cooling design. Specific loss values are very dependent on frequency, field amplitude, and nature of the applied field (related to motor rotational speed, number of poles, and design) as well as conductor design. However, we compare low frequency results measured at 20 K and 0–3 T to theoretical expectation, and we then make extrapolations to frequencies up to 200 Hz (and give explicit expressions for such extrapolation). We in particular include here also the effect of maximum field amplitude variation on the average J c , allowing us to include for the conductors a semi-bean approximation which allows for more realistic loss modelling results.

The critical current densities ( J c ) and stabilities of Tube Type Nb 3 Sn conductors have been measured. The strands had superconducting subelement counts ranging from 192 to 744, and flat-to-flat filament sizes (for 0.7 mm OD wire) of from 35 μm down to 15 μm. These Tube Type conductors had a very simple structure: prior to heat treatment the filaments consist of a Sn core surrounded by a thin Cu tube, itself surrounded by a Nb or Nb alloy tube. Eight different strand types were investigated using various techniques including SEM, residual resistance ratio (RRR), transport J c , and stability measurement. Most strands were studied at 0.7 mm OD, with one representative at 0.42 mm. The transport measurements were made at 4.2 K in fields up to 14 T. Numerous heat treatment schedules were investigated, with reaction temperatures ranging from 615 °C to 650 °C, and times ranging from 36–500 h. The highest J c s were seen for the lowest reaction temperatures, with 12 T transport J c values as high as 2450 A/mm 2 observed. The RRRs were lower for longer time and higher temperature reactions and ranged from 4 to 180. Strand stability was a strong function of the effective filament diameter, d eff , and RRR. The most stable strands showed stability currents, J s , of 8700 A/mm 2 and 15,300 A/mm 2 for 0.7 mm OD and 0.42 mm OD conductors, respectively.

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.

A react-and-wind MgB2 coil segment for a conduction-cooled magnetic resonance imaging (MRI) machine has been fabricated and tested. The coil was developed as part of a collaborative effort on a conduction-cooled, MgB2-based, whole-body MRI image guided radiation therapy device. This study focuses on the fabrication, winding, instrumentation, cooling, and initial critical current (I c ) testing of this near-full-size MgB2 segment coil. The coil was 0.9 m in diameter; the winding pack, 44.0 mm wide × 50.6 mm high, used 1.7 km of an 18 filament MRI-style conductor with Nb chemical barriers, Cu interfilamentary matrices, and an outer monel sheath. The conductor was insulated and reacted before winding onto a stainless steel former. The coil was instrumented with Cernox and E-type thermocouple temperature sensors, strain sensors, and voltage taps. The conduction-cooled coil was mounted in a cryostat capable of accepting coils of up to 0.9 m in diameter and 0.5 m in height. Critical current measurements were performed as a function of temperature during a controlled heating of the coil. The operational target was I = 200 A at 13 K. The full magnet was designed to produce 0.75 T in the imaging area (at I = 200 A), with a maximum field of 1.93 T in the winding. The single segment coil measured here exceeded this operation specification, with an I c of 280 A at 15 K and a maximum field 1.93 T in the winding. The coil was modeled using a finite element method, and a load line plot showed that 100% of short sample was reached at 21.5 K and above. These measurements demonstrate the viability of conduction-cooled MgB2 background coils as replacements for liquid helium cooled NbTi background coils in future MRI devices.

The effect of wire diameter reduction on the critical current density of pristine MgB2 wire was studied. Wires were treated by a hot isostatic pressing method at 570 °C and at pressures of up to 1.1 GPa. It was found that the wire diameter reduction induces an increase of up to 70% in the mass density of the superconducting cores. This feature leads to increases in critical current, critical current density, and pinning force density. The magnitude and field dependence of the critical current density are related to both grain connectivity and structural defects, which act as effective pinning centers. High field transport properties were obtained without doping of the MgB2 phase. A critical current density jc of 3500 A mm−2 was reached at 4 K, 6 T for the best sample, which was a five-fold increase compared to MgB2 samples synthesized at ambient pressure.

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 Nba3Sn 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 Ic 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.

MgB2 strands with various dopants and various types of B were characterized using transport Jc. In particular, amorphous, nanocrystalline, and plasma spray processed B powder based samples were compared. In a number of cases, various nanoparticle additions were then made to improve the properties with the given strand and B powder type. SiC, TiC, Ag nano ceramics were purchased from a commercial source and were then mixed into the wire during the milling stage. In one case, Malic acid was the dopant, and added at this milling stage. The doped powders were then dispensed into monofilamentary wires in which a Nb-lined monel sheath construction was used. Transport Jc measurements were performed on all samples under a variety of magnetic fields and in some cases temperatures. These results were then compared to our previous results with and without dopants.

The tubular technique for economical production of Nb3Sn material with large numbers of subelements is being explored further by Supergenics I LLC and Hyper Tech Research Inc. The number of subelements has been raised to 271 (246 + 25) by increasing the size at which the 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 18 mum in diameter. Heat treatment of a 271 subelement restack has been varied and a pre heat treatment of 48 h at 575degC followed by long times at 635degC has been shown to be advantageous. Restacks with 217 subelements have been subjected to both compositional and heat treatment changes and the best results, closely approaching those found on PIT materials, have been found on samples given 500 h at 625degC.

The superconductivity of carbon nanotube (CNT) doped MgB2 sintered in pulsed magnetic field (PMF) was investigated with Raman scattering measurements and Raman spectral fit analysis. Although the carbon (C) substitution for the boron (B) is increased for the sample sintered in PMF, its superconductivity is advanced comparing with the sample sintered without PMF. The improved critical transition temperature, Tc, is attributed to the strengthening of the electron-phonon coupling (EPC) in MgB2, as reflected by the broadened E 2g mode in the Raman spectra, because the carbon atoms are homogeneously distributed in the boron planes.

MgB2/Fe wire samples were prepared by using ball-milled 96% boron (B) powder with strong semi-crystalline phase. We observed samples that contained ball-milled 96% B in comparison with one made from as-supplied commercial 96% B, with the results showing a significant enhancement in the high field transport critical current density (J ct) due to small grain size and better reactivity. However, the inter-grain connectivity became worse, which could lead to poor J ct in low field and an increased level of disorder.

AC applied magnetic field loss measurements have been performed on MgB2 superconducting strands. We measured strands with six untwisted filaments made via an in-situ route with Nb chemical barriers, a Cu-inter-filamentary matrix, and with either non-magnetic (glidcop) or magnetic (monel) outer sheaths. AC losses in magnetic fields applied perpendicular to the wire axis have been measured in fields up to 140 mT (peak) in the frequency range 50-200 Hz in a liquid helium bath at 4.2 K. For samples with no ferromagnetic elements present in their matrix there is a reasonable agreement of the losses with the critical state model in low frequency region. At higher frequencies the losses are dominated by eddy currents in Cu matrix. The samples containing a weakly ferromagnetic matrix showed complex hysteresis loops and AC loss dependencies.

We have systematically studied the flux pinning behavior of MgB2 bulks synthesized by direct diffusion of Mg into pressed pellets of high purity crystalline B powder, with and without mixing with C and SiC nanoparticles, at a reaction temperature of 850degC for 10 hrs. All of the samples showed very high purity and high density, but their microstructure and flux pinning behavior showed significant differences. It was found that the pure MgB2 agrees with the deltaTc pinning model, nano-C doped MgB2 agrees with the deltal pinning model, while the SiC+MgB2 composite agrees with the deltaepsiv pinning model (stress/strain field pinning), since the dominant micro-defects that influence the flux pinning in these three samples are different.

Multifilamentary, tube-type composites consisting of subelements of Nb with a simple Cu/Sn binary metal inserts were studied in an attempt to enhance the performance boundaries of these conductors. Transport J c values for these multifilamentary strands were found to be as high as 2440 A/mm2 at 12 T, 4.2 K for 217 strand conductors. We focused on correlating the composition and morphology of the intermetallic A15 to the transport and magnetic properties for varying heat treatments. In particular, lower temperature HTs were studied, specifically 625degC and 635degC as a function of time. The extent of A15, the ratio of the coarse/fine grain areas, and amount of the untransformed 6:5 were then observed as a function of time-temperature at these lower temperatures where the ternary phase diagram three-phase region boundaries have shifted from their 675degC positions. Compositional analysis was carried out on the A15 phase to study the variation in stoichiometry as a function of temperature. Fractography was also performed to investigate the effect of temperature and alloying on the morphology and grain size of the fine grain A15.

A short model helical undulator, 250 mm long, with a period of 14 mm was designed, fabricated and then subsequently tested in a liquid helium bath. The helical coil was wound with stabilized multi-filamentary MgB2 strand, 0.5 mm in diameter, in a wind and react mode with S-glass insulation followed by vacuum epoxy impregnation. Beam aperture of the coil was 7 mm, winding bore diameter 8 mm, coil winding OD=18 mm and the 1010 Fe pole (flange) has an OD=19.05 mm. The coil was measured for I c and magnetic field in the bore at 4.2 K in a liquid helium bath. The bore field as a function of depth was also determined near the critical current at 4.2 K. The load line of the coil was compared to short sample I c values.

Tube type composites containing up to 167 subelements were studied in terms of their reaction systematics in an attempt to enhance their properties. These conductors had transport Jc values in excess of 2200 A/mm2 at 12 T and 4.2 K. Samples were heat treated at temperatures between 650-800degC for varying amounts of time. The phase formations as well as the growth kinetics of the A15 layer during reactive diffusion between the Nb and a liquid Sn-rich bronze alloy were experimentally investigated at 650degC and 725degC. The A15 layer growth exponent 'n', proportionality constant 'k', and the rate controlling process were determined at these temperatures. Compositional analysis was performed using EDS on samples with various initial Cu-Sn ratios. The variation of grain size and morphology with temperature and composition were studied in detail. The activation energy for grain growth in the fine and coarse grain A15 regions were determined.

In this study, preparation, structure, morphology and thermal properties of ceramic insulation coating on Monel/Fe/MgB2 wires, which were fabricated by Hyper Tech Research Inc., using Continuous Tube Forming and Filling (CTFF) process were studied for coil development. Ceramic insulation coatings were prepared by reel-to-reel sol-gel method using solutions of Y and Zr based organometallic compounds. The coating thickness was controlled by number of coating, withdrawal speed and solution chemistry. 29 meter of Monel/Fe/MgB2 wire was insulated by Y2O3-ZrO2 using sol-gel method. Monel/Fe/MgB2 coil was layer wounded and impregnated with stycast. Sister samples of coil were tested between 4.2 K and 30 K with a Variable Temperature Insert (VTI) in a 20 T superconducting magnet using the analog DC pulse four-wire method with 1 (muV/cm) criterion. The sample had a Jc of 0.16 times 105 (A/cm2) at 4 T at 21 K. The surface morphologies and microstructure of sister samples of coil were characterized by ESEM, EDS, DTA, TGA and XRD. Resistance was measured by using HP 4339A High Resistance Meter.

Niobium tin was one of the first superconductors to be made into relatively small high field magnets forty-six years ago. Conductors made from it are widely used for a range of different applications and development efforts are still being carried out to improve properties and reduce costs. The paper describes some of the work being carried out by the authors to make such improved conductors for fusion, high-energy physics and other applications. Three different types of niobium-tin conductors are described: 1. Low loss material with modest current density for fusion magnets. 2. High current density, higher loss material for high field high energy physics magnets. 3. Tubular type, low cost material with intermediate current density and small subelement size that could have application in a wide range of different magnets. Some of the advantages and drawbacks of each type of conductor are described together with illustrative properties.

We fabricated MgB2/Fe wire by a powder-in-tube (PIT) technique, using an in situ process. All wire samples were sintered for 30 min at different sintering temperatures ranging from 650 to 1000 °C. We found strong correlations among crystallinity, critical current density (Jc), irreversibility field (Hirr), upper critical field (Hc2), and microstructures for all MgB2/Fe wires. We observed that the sample with the lowest sintering temperature, ~ 650 °C, had a larger lattice strain, Jc, change in resistivity Δρ(ρ300 K–ρ40 K), Hirr, and Hc2, but a lower density and residual resistivity ratio (RRR). Based on the relationships between all these superconducting and microstructure parameters, grain boundaries are likely to be acting as the predominant pinning centers for MgB2, so grain growth of MgB2 corresponds to a reduction of effective pinning. It should be noted that changes in the MgO fraction within the MgB2 matrix were almost independent of the sintering temperature. This indicates that most MgO may be coming from the starting material.

In a continuation of a series of reports on the fabrication and properties of several demonstration coils and solenoids wound with MgB 2 strand this paper describes the winding, testing, and load-line comparisons for a racetrack coil (R-4) and two solenoids (S-1 and S-2). Racetrack R-4 was wound with 19-filament strand, S-1 and S-2 with 7-filament strand. The filaments were clad in Nb-lined Cu and the restack bundle was enclosed in a monel sheath; some Cu stabilization was included. Transport J c measurements were performed initially in liquid He, and then as a function of temperature up to 30 K. Racetrack R-4 achieved 240 A in self-field at 7.7 K corresponding to a J c of 240 kA/cm 2 , leading to a strand J e of 43 kA/cm 2 , and a winding J e,w of 22 kA/cm 2 . At 4 K, S-1 (310 m of strand) achieved an I c of 138 A and a bore field of 3.4 T; S-2 (742 m of strand) an I c of 85 A and a bore field of 3.9 T. The magnetic field profiles for all coils were calculated using a finite element approach in conjunction with the Biot–Savart equation. The windings were treated as a series of small segments, each tangential to the winding, numerical errors were estimated. These calculations were used to generate load-line comparisons for the coils.

Improvements in B c2 , B irr , and high field transport J c s are crucial for many MgB 2 applications. As transport J c is inextricably linked with microstructure, a knowledge of the detailed influence of two of the most practical variables – Mg/B ratios and SiC additions – on microstructure, B c2 / B irr , and high field J c , is of interest. In this work, the influence of Mg/B ratios on microstructure, B irr , and transport J c was investigated in MgB 2 strands with and without SiC doping. The binary compositions chosen were Mg x B 2 where x = 0.85, 0.90, 1.0, 1.10, and 1.15 in monofilamentary strands. In general, high Mg molar percentages were seen to increase transport J c , especially at higher fields. At 8 T, for example, heat treatments at 700 °C for 40 min yielded J c s at 4.2 K which ranged from 5.8 × 10 3 A/cm 2 for x = 0.85 to 2.73 × 10 4 A/cm 2 at x = 1.15. Microstructures evolved from powdery (B-rich) to web-like (Mg-rich). In a second set of samples, Mg-rich stoichiometries were investigated with and without SiC. High field transport J c s increased, and the microstructure appeared to densify. Both excess Mg and SiC doping significantly increased high field J c . The highest transport result was seen for SiC dopant (5 mol%) added to 15% excess Mg strands, which yielded a transport J c at 4.2 K and 8 T of 5 × 10 4 A/cm 2 . Variable temperature analysis was performed on the 15% excess Mg strands with and without SiC in fields of 0–15 T. At temperatures below 20 K, excess Mg + SiC doping performed best, while above 20 K, excess Mg without SiC had better transport properties.

The fundamental reaction of Mg + B to MgB2 formation was investigated in order to improve the connectivity of the reacted strands, dopant diffusion, and ultimately the transport properties. Initially, differential scanning calorimetry (DSC), studies were performed to determine the thermodynamics of the solid-state reaction. It was consistently evident from the DSC scans that the formation of the MgB2 phase was completed below the Mg melting point (~ 655degC). Efforts were made to characterize and understand the differences between the microstructures resulting from the high temperature (above 655degC) and the low temperature (below 655degC) heat-treatments. Transport properties (4.2 K, mostly) of the MgB2 strands, synthesized by the in-situ reaction between mixed Mg and B powders with 5% or 10% of SiC, both above and below the Mg melting point were measured. The results were correlated with the reaction temperatures. Transport Jcs of the order of 4.8 times 104 A/cm2 at 8 T and a Bc2 of 22 T (both at 4.2 K) were obtained for the lower temperature HTed samples, these Jcs are higher than those measured on samples reacted above the Mg melting temperature.

We have investigated magnesium diboride wires as an element in a resistive superconducting fault current limiter using a pulsed system, which could deliver a fixed number of AC cycles to avoid burnout due to thermal instabilities. Experiments have been carried out in both liquid helium and at 27 K on magnesium diboride mono-core wires in stainless steel tubes, in a CuNi sheath with an Fe barrier and also on multifilamentary wires with Nb barriers. Experiments showed good current limiting in the first cycle and no damage after six cycles. Also wires with a number of voltage contacts spaced at about 1 cm intervals to assess the effect of inhomogeneity were tested. Relatively small differences in local Jc led to large differences in the local temperature rise although in no case did this lead to failure of the conductor. The results were compared with those on Bi-2223/Ag tapes and YBCO coated conductors measured in liquid nitrogen.

Magnesium diboride multifilamentary strands consisting of 7, 19, 37, and 61 filaments were fabricated using a Nb chemical barrier, 101 Cu as an internal stabilizer, and either monel or oxide-dispersion-strengthened Cu (ODS Cu) as an outer sheath. The various strands were given single-step heat treatments of 700degC for 20-40 minutes. Transport critical current density, Jc, measurements were performed at 4.2 K on 1-meter lengths of strand on ITER barrels in fields of up to 15 T. Higher temperature transport values at 0-12 T were subsequently measured on short samples. Typical Jc values were at 1times105 A cm-2 at 5 T and 4.2 K and similar values at 2 T and 20 K. The use of ODS Cu as the outer sheath material improved the ductility and stability of the strand without much sacrifice in transport Jc. Optimization of transport properties through variation of the Mg/B stoichiometry was also studied. Pinning force curves at various temperatures were analysed.

The diffusion of Sn through the interfilamentary matrix within a subelement and the formation the of associated Cu-Sn phases was observed experimentally for several different Nb3Sn internal-Sn type strand subelements during the pre-reaction part of a heat treatment. Two different pre-reaction heat treatment schedules were investigated (10degC/hr ramp up, pausing at 185degC/24 hr + 25degC/hr ramp up, pausing at 340degC/48 hr as well as 10degC/hr ramp up, pausing at 210degC/48 hr + 25degC/hr ramp up, pausing at 400degC/48 hr). Four subelement types were measured, one without Ti, the others with Ti added either as NbTi rod replacements for selected Nb(Ta) filaments, or as wraps around each individual Nb(Ta) filament. In all but one case the majority of the filaments were a Nb-7.5 wt%Ta. A simple model was developed to determine the time and temperature dependence of Sn-diffusion through the Cu matrix of the subelements. The output of the model, in the form of radial positions of eta, epsiv, and a phases as a function of time during the pre-reaction heat treatment process, was compared to the experimental results.

Two standard internal-Sn type rod-in-tube (RIT) and two newly developed tube type conductors were investigated. Both types of strand were doped with Ta. A series of heat-treatments with varying temperature and duration time were performed to study their influence on the microstructure and properties. In this work, the fully reacted strands were analysed. Non-Cu Jc was measured and Bc2 values were estimated via Kramer plot extrapolation. Bc2 values were also measured by resistive transitions at high magnetic fields, and compared with the values from Kramer plot extrapolations. High resolution scanning electron microscopy (SEM) was used to observe the cross-section and the composition across the strands' cross-section was measured by using energy dispersive spectroscopy (EDS). scanning transmission electron microscopy (STEM) was used to measure the composition variation in the individual filaments in the RIT strands. Superconducting properties were correlated to strand composition.

A process designed to produce Nb3Sn conductors by simply restacking Nb 7.5 wt.%Ta tubes with Cu-clad Sn cores is being explored. Previous work has shown that, in this tubular process, the A-15 compound forms in a somewhat different manner from that in the conventional internal-tin process. It was necessary therefore to determine if this had a significant effect on the properties of the A-15 formed. This was done with subelements containing different percentages of Nb 7.5 wt.%Ta, Sn and Cu. The initial work on single subelements has been extended to include restacks with relatively large numbers of subelements. The critical current density (Jc) of the various designs is reported and some of the factors involved in optimizing the properties are discussed.

The influences of strand twisting and bending (applied at room temperature) on the critical current densities, Jc , and n-values of MgB2 multifilamentary strands were evaluated at 4.2 K as function of applied field strength, B. Three types of MgB2 strand were evaluated: (i) advanced internal magnesium infiltration (AIMI)-processed strands with 18 filaments (AIMI-18), (ii) powder-in-tube (PIT) strands processed using a continuous tube forming and filling (CTFF) technique with 36 filaments (PIT-36) and (iii) CTFF processed PIT strands with 54 filaments (PIT-54). Transport measurements of Jc(B) and n-value at 4.2 K in fields of up to 10 T were made on: (i) PIT-54 after it was twisted (at room temperature) to twist pitch values, Lp , of 10-100 mm. Transport measurements of Jc(B) and n-value were performed at 4.2 K; (ii) PIT-36 and AIMI-18 after applying bending strains up to 0.6% at room temperature. PIT-54 twisted to pitches of 100 mm down to 10 mm exhibited no degradation in Jc(B) and only small changes in n-value. Both the Jc(B) and n-value of PIT-36 were seen to be tolerant to bending strain of up to 0.4%. On the other hand, AIMI-18 showed ±10% changes in Jc(B) and significant scatter in n-value over the bending strain range of 0-0.6%.

We studied the effects of MgB 2 with embedded multiwall carbon nanotubes (MWCNTs) on the crystallinity, lattice parameters, critical current density ( J c ), upper critical field ( H c2 ), irreversibility field ( H irr ), and microstructure of MgB 2 . Fe sheathed un-doped and MWCNT doped MgB 2 wires were fabricated by the powder-in-tube (PIT) method and sintered at the high sintering temperatures of 900 °C. We observed that for the MWCNT doped sample high temperature sintering resulted in depressed crystallinity, shrinkage of the a -lattice parameter, higher J c up to 12 T, and lower critical temperature ( T c ) values. Specifically, MWCNT doped samples sintered at 900 °C exhibited excellent J c , ∼10 4 A cm −2 up to 9 T at 4.2 K. This can be explained by lattice distortion and poor crystallinity due to carbon (C) substitution from the MWCNT.

The U.S. Dept. of Energy's goal for multifilamentary (MF) strand intended for accelerator applications includes current densities, Jc, of up to 1200 A/mm2 in fields of 12-16 T. Although MgB2 films possess Bc2s at 0 K as high as 50 T, and Jcs (self field) as high as 107 A/cm2 have been reported, it may be several years before the above goal is attained in round MF MgB2 wire. But when its electrical properties become suitable MgB2's critical temperature of 39 K will be an added advantage in the accelerator magnets' high radiation environment. In the meantime, some near-term applications have been identified. Continually improving properties may enable MgB2 to be used in the windings of: (i) undulator magnet upgrades and replacements, (ii) wiggler magnets, (iii) light source bending magnet replacements, or (iv) some of the lower field solenoids of a proposed muon collider. Active research-and-development programs are yielding MgB2 strands with some remarkable properties: critical fields Bc2 and Birr in the ranges 28-33 T and 24-29 T, respectively, and Jc (4.2 K)s in the range 1.1-5.5times105 A/cm2, 5-2 T, respectively. Recent superconducting undulator (SCU) magnets are calling for Jc, BW (winding-field), B0 (beam-line field) combinations such as 1.1times105 A/cm2, 3.3 T, 1.0 T. These specifications, in the light of its existing performance levels, indicate MgB's potential suitability for the windings of SCU's. A model MgB2 coil-set is described

The transport critical current density (Jc) was measured at 4.2 K for MgB2 monofilamentary and 7-, 19-, and 37-stack multifilamentary strands. Simple, one-step heat treatments (HT) were used, with temperatures of 675 and 700 °C, and times from 10–40 min. Most measurements were performed on 1 m segments of strands wound onto barrel holders. Transport properties of monofilament, 7-, 19-, and 37-stack strands were compared, and the influence of CuNi and monel outer sheaths was investigated. HT optimization studies were performed on various strands. Transport Jcs of 0.8 mm OD strands reached 2 × 105 A cm−2 at 4 T and 4.2 K (1 µV cm−1), and 1.15 × 106 A cm−2 at 4.2 K and zero field. Smaller 10-filament wires with ODs as small as 0.25 mm (40 µm filaments) exhibited good performance in some cases. The temperature and field dependences of the transport Jc were also measured; a typical example was 2 × 104 A cm−2 at 4 K, 20 T.

A racetrack coil using MgB/sub 2//Cu strand has been fabricated and tested for transport current density at 4.2 K in self field. The monofilamentary strand was 1.0 mm OD and insulated with S-glass braid. Eighty turns of strand (42 m) were wound onto a stainless steel former with outer dimensions 25 cm/spl times/10 cm/spl times/0.86 cm. The resulting racetrack coil was heat treated at 675/spl deg/C for 30 minutes in flowing Ar. The strand, with a superconducting fraction of 26%, occupied 49% of the total coil pack cross sectional area. The coil I/sub c/ at 4.2 K and self field was 120 A (using a 1 /spl mu/V/cm criterion). This led to a J/sub c,sc/ (across the whole coil) of 6.12/spl times/10/sup 4/ A/cm/sup 2/, a J/sub e/ in the wire of 1.59/spl times/10/sup 4/ A/cm/sup 2/, and an overall winding J/sub e/ of 7.9/spl times/10/sup 3/ A/cm/sup 2/ at 4.2 K in self field. The n-values ranged from 17 to 56.

Nb/sub 3/Sn superconductor wires were investigated in terms of Nb/sub 3/Sn phase formation and Ti diffusion. Two series of subelement strands (HP and EG) with Ta, Ti addition were fabricated. For comparison, a special model sample consisting of an annular Nb region containing Sn and surrounded by Cu was fabricated. All samples were given a standard initial up-ramp-and-hold schedule (185/spl deg/C/24 h (ramp rate 10/spl deg/ C/hour) + 340/spl deg/C/ 48 h (ramp rate 25/spl deg/C/hour) + 700/spl deg/C/up to 150 h (ramp rate 10/spl deg/ C/hour)). Reaction heat treatments at 700/spl deg/C were performed for various times on these subelements. The diffusion of Sn and Ti was studied by SEM, EDS, EPMA and STEM analysis. The Nb/sub 3/Sn reaction rate was determined for strands with and without Ti, and Ti was seen to double the rate of Nb/sub 3/Sn formation for these strands. EDS analysis showed that within the filament annular shell, an even distribution of Ti existed with a concentration of 0.4-1.0 at. %. However, for the highest Ti bearing sample, it had a Ti Nausite at the filament boundaries which may reduce the strands superconductor property.

The advantages of titanium as a third element in Nb/sub 3/Sn have been known for many years. It increases the upper critical field and lowers the critical current density at lower fields, thus reducing the losses, which are usually measured in this field region. Several methods for the introduction of titanium are listed and discussed briefly. The two methods discussed in more detail are both introduction by way of Nb47wt.%Ti. One is by wrapping foils around the filaments and the other by introducing rods into the filament array. A relatively uniform distribution of Ti in the filaments, results from both methods. Scanning Electron Microscopy (SEM) was used to illustrate the various phases, produced under different heat treatment conditions. Energy Dispersive Spectroscopy, (EDS) and Wavelength Dispersive Spectroscopy, (WDS) were both used to obtain compositional data.

Various types of internal-Sn multifilamentary strand were studied in an attempt to correlate fracture, subelement hardness, and interfacial conditions. Wire fracture (one of the main factors limiting the piece-length of multifilamentary precursor wires) is empirically known to increase with strand complexity. In an attempt to quantify this, a number of internal-tin multifilamentary precursor wires exhibiting various levels of drawability were investigated by microhardness as well as optical and electron microscopy. The progression of cross sectional stability was observed via optical microscopy as a function of wire drawing. Microanalysis both in the Scanning Electron Microscope (SEM) and the Scanning Transmission Electron Microscope (STEM) were used to study the interdiffusion of various elements through the interface between the restacked subelement and between the outer Cu sleeve, as well as inside the subelement. Comparing the hardness distribution along different regions of the interfaces between sleeve and subelement as well as between the subelements suggests possible causes for the occurrence of fracture.

We report a systematic study on the effect of sintering temperature on the phase formation, critical current density, upper critical field and irreversibility field of nanoscale SiC doped MgB2. Bulk and Fe sheathed wires doped with different nano-SiC particle sizes have been made and heat treated at temperatures ranging from 580 to 1000 °C. A systematic correlation between the sintering temperature, normal state resistivity, RRR, Jc, Hc2, and Hirr has been found in all samples of each batch. Samples sintered at a lower temperature have a very fine and well consolidated grain structure while samples sintered at a high temperature contain large grains with easily distinguishable grain boundaries. Low temperature sintering resulted in a higher concentration of impurity precipitates, larger resistivity, higher Jc up to 15 T and lower Tc values. These samples show higher Hc2 and Hirr at T near Tc but lower Hc2 near T = 0 than samples sintered at high temperature. It is proposed that huge local strains produced by nano-precipitates and grain boundary structure are the dominant mechanism responsible for higher Hc2 at T near Tc. However, higher impurity scattering due to C substitution is responsible for higher Hc2 in the low temperature regime for samples sintered at a higher temperature. In addition to high Hc2, it is also proposed that the large number of nano-impurities serve as pinning centres and improve the flux pinning, resulting in higher Jc values at high magnetic fields up to 15 T.

Nb/sub 3/Al strands with four different designs have been processed and tested. Strand specifications and some generic aspects of strand fabrication, processing, and testing are described. One strand type was long-length RHQT processed and transport tested at various lengths. Transport J/sub c/ results for short samples and small solenoid coils are presented and discussed for various RHQT protocols. Critical current measurements on various strands have been performed in fields from 18 to 26 T. Various strand stabilization techniques have been investigated including electroplating, plasma spray coating, and direct Nb-Cu high temperature diffusion bonding; these results are described. Fifty meters of strand was RHQ processed and then cabled into a short length of experimental 12-strand Nb/sub 3/Al Rutherford cable in preparation for transformation (T) and testing. Cabling properties are described.

Multifilamentary PIT Nb-Al and Nb-Sn strands have been fabricated and tested. Three main conductor variants were investigated; Nb/sub 3/Al, Tube-Wall-Reaction (TWR) Nb/sub 3/Sn, and Internal Reaction (IR) Nb/sub 3/Sn. Multifilament restacks of 7, 19, 30, and 37 filaments were fabricated and tested. The powders were formed by mechanical alloying in a planetary mill with subsequent jet milling for particle refinement. In the case of Nb/sub 3/Al, ductile Al was added to the intermetallic compound before filling; while for Nb/sub 3/Sn based conductors, Cu was added. All Nb/sub 3/Sn strands were alloyed with Ta. Phase formation and transport properties are described. In the case of TWR Nb/sub 3/Sn strands, the nature and thickness of the reaction layer was investigated. For monofilaments, the reaction layer was roughly 50 /spl mu/m thick, for 37 filament strands it was only 5 /spl mu/m. SC layer J/sub c/ values of 2680 A/mm/sup 2/ at 12 T were obtained, but quench limited because of the short samples used in the present work.

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.

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.

The intrawire resistance and alternating current (AC) loss of two MgB2 wires with filaments surrounded by Nb barriers have been measured and analyzed. Relatively high values of filament-to-matrix contact resistivity are found in the MgB2 wires; the values are two or three orders higher than those commonly found in NbTi or Nb3Sn wires. Considering the high porosity of the MgB2 filaments, cold high-pressure densification has been applied on the two MgB2 wires to investigate its impact on intrawire resistance and AC loss. The intrawire resistance is measured with a direct four-probe voltage-current method at various temperatures. The AC loss is measured by vibrating sample magnetometer measurements at 4.2 K. In addition to the intrawire resistance measurements, the critical current of MgB2 wires before and after densification is measured with a U-shaped bending spring at 4.2 K as function of axial strain. The critical current in densified MgB2 wires is found to be higher than that in the same wire without densification; it is also less sensitive to the applied axial strain.

Rapid heating to different temperatures in the vicinity of 2000/spl deg/C followed by quenching to room temperature was used to prepare short samples of jelly-roll-type Nb/sub 3/Al superconductor. The as quenched material has either a bcc or A15 structure depending on the presence or absence of a specific endothermic reaction during the rapid heating, but after a secondary heat treatment at 800/spl deg/C an A15 structure is obtained for both cases. Significantly higher critical current densities at magnetic fields up to 18 T (4.2 K) are obtained for samples quenched as bcc, while the A15-quenched material is superior at higher fields due to a larger upper critical field. For the bcc-quenched samples the critical current density is optimized (16 T, 4.2 K) when they are quenched from temperatures just above or 400/spl deg/C higher than the temperature of the endothermic reaction, with a noticeable drop in the middle range. A 10 hour secondary heat treatment gives rise to better superconducting properties than a 3 hour heat treatment (both at 800/spl deg/C). Some of the observed dependencies are explained using SEM microstructure analysis.

Niobium-aluminum precursor strands were fabricated using a modified form of the rod-in-tube method (MRT) and also the conventional jelly-roll (JR) method. Monoelements with Nb/Al volume ratios of 2.3, 2.7, 3, and 5 were drawn down and restacked into groups of 7/spl times/37 and 37/spl times/37 (MRT), also 7, 19, and 36 (JR) elements. Suitably clad, these were reduced to final strand sizes of 1.6, 1.2, and 0.8 mm (MRT) also 0.8 and 0.4 mm (JR). In this way, the thickness of the Al component of the Nb/Al composite was reduced to sizes in the range of 2.5 to 0.1 /spl mu/m. Although most of the material produced to date has been fully cold drawn, warm hydrostatic extrusion has been explored for use in the next phase of strand production. The MRT and JR strands, in short-sample form, have been subjected to pulsed resistive heating to temperatures of typically 1800 to 2100/spl deg/C followed by either radiative cooling or liquid-Ga quenching. Studies of the relationship between Al-element thickness and final properties have been made.

The critical current density (J/sub c/) of the monofilament Monel/Fe/MgB/sub 2/ wire was measured between 4.2 K and 30 K with a Variable Temperature Insert (VTI) in a 20 T superconducting magnet using the analog DC pulse four-wire method with 1 /spl mu/v/cm criterion. The critical current density, J/sub c/ was measured to be 0.16/spl times/10/sup 5/ A/cm/sup 2/ at 4 T at 21 K for a MgB/sub 2/ sample which was annealed at 700/spl deg/C for 5 min and at 900/spl deg/C for 5 min under Ar gas flow. Scanning Electron Microscopy (SEM) and X-ray diffraction (XRD) analysis were used to characterize the microstructure of the samples.

Intra-wire resistance and AC loss of various multi-filamentary MgB2 wires with filaments surrounded by Nb barriers have been measured and analyzed. The intra-wire resistance is measured with a direct four-probe voltage–current method at various temperatures. The AC loss is acquired by both vibrating sample magnetometer (VSM) and magnetization measurements at 4.2 K. With the aid of finite element method simulations, the filament-to-matrix contact resistance and effective transverse resistivity are derived from direct intra-wire resistance measurements. The effective transverse resistivity values are in good agreement with those analytically derived from the AC coupling loss measurements. Surprisingly, very high values of filament-to-matrix contact resistivity are found, being 2 or 3 orders higher than commonly found for NbTi or Nb3Sn wires.

High field pinning centers in MgB2 doped with 2 at. % carbon under a low and a high hot isostatic pressures have been investigated by transport measurements. The field dependence of the transport critical current density was analyzed within the different pinning mechanisms: surface pinning, point pinning, and pinning due to spatial variation in the Ginzburg-Landau parameter (Δκ pinning). Research indicates that a pressure of 1 GPa allows similar pinning centers to Δκ pinning centers to be obtained. This pinning is very important, because it makes it possible to increase the critical current density in high magnetic fields at 20 K and 25 K. Our results indicate that the δTc and δl pinning mechanisms, which are due to a spatial variation in the critical temperature (Tc) and the mean free path, l, respectively, create dislocations. The high density of dislocations with inhomogeneous distribution in the structure of the superconducting material creates the δl pinning mechanism. The low density of dislocations...

The change in the lattice parameters or the lattice disorder is claimed as a cause of the slight reduction in the transition temperature by carbon doping in MgB2. In this work, an extensive investigation on the effects of carbohydrate doping has been carried out. It is found that not only the a-axis but also the c-axis lattice parameter increases with the sintering temperature. A linear relation between the unit cell volume and the critical temperature is observed. Compared with the well known correlation between the lattice strain and the critical temperature, the X-ray peak broadening itself shows a closer correlation with the transition temperature. The residual resistivity and the critical temperature are linearly correlated with each other as well and its implication is further discussed.

Cold high pressure densification, a method recently introduced at GAP in Geneva, was applied for improving the transport critical current density, Jc, and the irreversibility field, Birr, of monofilamentary in situ MgB2 wires and tapes alloyed with 10 wt% C4H6O5 (malic acid). Tapes densified at 1.48 GPa exhibited after reaction an enhancement of Jc from 2 to 4 × 104 A cm−2 at 4.2 K/10 T and from 0.5 to 4 × 104 A cm−2 at 20 K/5 T, while the Birr was enhanced from 19.3 to 22 T at 4.2 K and from 7.5 to 10.0 T at 20 K. Cold densification also caused a strong enhancement of B(104), the field at which Jc takes the value 1 ×104 A cm−2. For tapes subjected to 1.48 GPa, B(104) _ and B(104) ⊥ at 4.2 K were found to increase from 11.8 and 10.5 T to 13.2 and 12.2 T, respectively. Almost isotropic conditions were obtained for rectangular wires with aspect ratios a/b < 2 subjected to 2.0 GPa, where B(104) _ = 12.7 and B(104) ⊥ = 12.5 T were obtained. At 20 K, the wires exhibited an almost isotropic behavior, with B(104) _ = 5.9 T and B(104) ⊥ = 5.75 T, Birr(20 K) being ∼10 T. These values are equal to or higher than the highest values reported so far for isotropic in situ wires with SiC or other carbon based additives. Further improvements are expected on optimizing the cold high pressure densification process, which has the potential for fabrication of MgB2 wires of industrial lengths.