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The Institute of Plasma Physics Chinese Academy of Sciences (ASIPP) has announced a project to design and manufacture a high field MRI magnet with a 900 mm warm bore and comprised of Nb3Sn Rutherford cables in 2017. The maximum magnetic induction of magnet system is 14.3 T and magnetic field homogeneity in a spherical volume with 22 cm diameter is better than 0.5 ppm. The high performance Nb3Sn strand is the potential options to use for such high field magnet system. ASIPP cooperates with Western Superconducting Technologies Company (WST) to develop high performance Nb3Sn strand whose critical current density Jc is higher than 2000 A/mm2 at 4.2 K, 12 T. To obtain the actual performance of WST high performance Nb3Sn strand, three short samples are prepared with different non-copper ratio or different heat treatment, the critical current Ic or quench current of all sample is measured at different field by the voltage-current (V-I) method, the residual resistivity ratio (RRR) of all sample is also measured. To study the strand whether it will be affected by low field instability, a small coil is built with WST strand, and the experimental result is presented and discussed in this paper.

This article focuses on the effect of grain size on estimating the residual resistivity ratio (RRR) of electron beam welded (EBW) high purity Niobium (Nb) for the structural fabrication of radio frequency cavity as a particle accelerator. This also aims to investigate the effects of using different functional models on resistivity. EBW is done by varying the beam current to change the grain size so that its consequent impact on RRR can be evaluated. Grain size was found to be maximum for 70 mA and minimum for 50 mA beam current among the welded samples at HAZ and FZ. The grain size increases from the base metal toward the heat-affected zone and further to the fusion zone like any other fusion welding processes. Pure Nb being electrically superconductive below around 9.2 K in the absence of any magnetic field, it is invariably used as a resonant cavity for a particle accelerator in high energy physics. Two temperature dependent and two field dependent models are used for extrapolation of the RRR value estimation. Temperature dependent models performed the better-fitted residuals for every sample than the field models. The extrapolated RRR values obtained from these models are used to simulate the impact of beam current on grain size at different zones of welding.

For developing a 14 Tesla whole-body superconducting magnetic resonance imaging (MRI) magnet, a new kind of a Nb3Sn composite conductor with Rutherford cable-in-channel (RIC) structure is adopted. This structure of the conductor has a high current carrying capacity with an increased thermal and mechanical stability. The Rutherford cable is the core component of the RIC conductor, and controlling the deformation and performance degradation of Nb3Sn strand are the key points in Rutherford cable development. The strand would experience plastic deformation during the compaction process, which causes sub-elements damage and degrades the transport performance and residual resistivity ratio (RRR). In this paper, the impact of transverse compression on the sub-element of RRP 84/91 Nb3Sn strand before heat treatment was studied with experiments and finite element model (FEM), the experiment results and FEM results were compared and analyzed. Experiment results show that the transverse deformation of the strand should be kept below 20% to avoid the Nb barrier from breaking. The deformation analysis result of the FEM simulation shows good agreement with experimental results.

A project aiming to fabricate a full-length 2.8-m long Nb3Sn superconducting undulator for the storage ring was started last year at the ANL APS. These Nb3Sn undulators operate at a maximum magnetic field on the conductor of about 5 T. To address instabilities at this field, two Nb3Sn wires with small subelement size were used. Specifically, Restacked Rod Processed wires of 0.6 mm in diameter and with 144 and 150 superconducting subelements respectively, over 169 total. The equivalent subelement diameter, DS , of these wires is ∼35 μm. At these small DS values, the critical current density is known to deteriorate, and the Residual Resistivity Ratio is very sensitive to heat treatment. A delicate balance has therefore to be found to obtain parameters within operation specifications. In this paper we show performance results from different heat treatments.

The quadrupole and dipole magnets for the high-luminosity large hadron collider (HL-LHC) upgrade will be based on Nb 3 Sn Rutherford cables that operate at 1.9 K and experience magnetic fields of up to about 12 T. An important step in the design of these magnets is the development of the high-aspect-ratio Nb 3 Sn cables to achieve the nominal field with sufficient margin. The strong plastic deformation of unreacted Nb 3 Sn strands during the Rutherford cabling process may induce non-negligible I c and Residual Resistivity Ratio degradation. In this paper, the cabling degradation is investigated as a function of the cable geometry for both Powder-in-Tube and Restack Rod Process conductors. Based on this analysis, new baseline geometries for both 11 T and quadrupole magnets of HL-LHC are proposed.

MgB 2 films and MgB 2 /BN bilayer films have been fabricated in the same reactor. The MgB 2 films and the bottom MgB 2 layers of the MgB 2 /BN bilayer films were synthesized by hybrid physical-chemical vapor deposition. Furthermore, the BN layers were synthesized by chemical vapor deposition. The degradation of MgB 2 films and MgB 2 /BN bilayer films that are submerged into deionized water at room temperature has been studied. To clarify the protection of BN for MgB 2 films against degradation in water, we analyzed the physical and magnetic properties, including the superconducting critical temperature T c , the critical current density J c , and the residual resistivity ratio. In addition, scanning electron microscopy and X-ray diffraction measurements were taken. The results indicate that the BN films can protect MgB 2 films from degradation in water, which would be beneficial for the device application of MgB 2 films.

Two types of high-purity aluminum-stabilized superconducting conductor were developed and produced for use in large-aperture high-field superconducting solenoids. One (DS1) has transverse rectangular dimensions of 5.27 mm × 20.1 mm, whereas the other (PS) has 5.52 mm × 30.1 mm. The superconductor for both cables is copper-stabilized NbTi strand woven into a Rutherford cable. The stabilizer for the DS1 conductor is 5N aluminum, whereas the stabilizer for PS is an aluminum alloy of 5N aluminum with a 0.1wt% Ni addition. Continuous unit lengths up to 1700 m are required for production. In total, over 18 000 m are required for this contract. A major goal of the R&D is to produce a cable having high shear strength between Cu and Al boundary while maintaining high residual resistivity ratio (RRR) and strength in the Al parts. We report on the cable mechanical properties, RRR of aluminum, sharing stress between Cu and Al, and critical current $(I_{C})$ of the cable, as well as the progress toward the fabrication of long unit lengths for production.

Some of the bronze-route Nb3Sn wires produced for the ITER toroidal field magnets have values of the Cu residual resistivity ratio (RRR) lower than the specification (RRR > 100) when reacted with the longest ITER heat treatment cycle (cycle A: high-temperature plateau 200 h at 650 ◦C). As the low RRR value was suspected to be due to the Cr plating, CERN has carried out extensive investigations on two different Cr-plated Nb3Sn wires in order to assess how the RRR is influenced by the Cr plating. Each type of wire was reacted with three heat treatments differing for the duration of the high-temperature plateau (from 100 to 200 h). The presence of Cr in the Cu stabilizer was investigated using a transmission electron microscope together with energy-dispersive X-ray spectrometry. In the most critical wire reacted for 200 h, CrS nanoparticles and Cr in solid solution (0.02 at.%) with Cu were found up to 13 μm from the Cr–Cu interface. At larger distance, we could not quantify the Cr concentrations because of the detector sensitivity limit (about 0.016 at.% Cr in Cu). We provide a model that can explain the measured RRR degradation purely in terms of the Cr contamination that we observed.

The optimization of the heat treatment of Nb3Sn conductors for toroidal field coils in ITER was attempted to improve the current sharing temperatures (Tcs). Using the strand, we chose the pattern at 570°C for 250 h and 650°C for 100 h as the best, which increased the critical current and maintained the residual resistivity ratio higher than 100. The behavior of the critical current of the strand vs. the magnetic field, temperature, and strain was also improved. This pattern was used on two conductors, and their performances were tested. Tcs was evaluated over 1000 electromagnetic cycles and one thermal cycle. A sharp Tcs degradation occurred at 50 cycles. Then Tcs decreased linearly. Although this tendency was similar to the conductors that were heat treated with the original pattern, the degradation rates were improved. The ac losses (Q) before cycling were approximately 10% lower than those of the original pattern. Q after cycling became almost equivalent between two patterns. The conductor was inspected after the test, which showed that the conductor under the high-magnetic-field zone had contracted by approximately 600 ppm during the test. Some clearly deformed strands were observed under the high-magnetic-field zone, which could degrade Tcs.

High-Jc Nb3Sn strands often exhibit instabilities in 4.2 K liquid helium at low fields ~ 0.5 to 3 T which are associated with magnetization flux-jumps. However at 1.9 K in superfluid helium, a minimum in premature quench currents at intermediate fields of 5 to 7 T has been observed in voltage-current measurements. These measurements are typically used for critical current determinations, and the premature quenching is driven by current redistribution within the strand as the current is increased and is termed “self-field” instability. In this paper, the magnetization and self-field stability of Nb3Sn strands with Jc ~ 2000 A/mm2 at 12 T are described for a series of wires made using the Sn-tube approach with filament diameters ranging from 13 to 65 μm. The copper stabilizer of these wires after reaction has residual resistivity ratio, RRR, of ~5, which in effect means that any dynamic stabilization from thermal conduction effects is negligible. In this regime of RRR, we find that the magnetization stability with transport current increases with decreasing filament diameter as predicted by simple adiabatic theory. We also observed that at 4.2 K the self-field stability improved with decreasing filament size, but became worse with decreasing temperature as seen in measurements at 2.0 K.

MgB2 samples have been synthesized over a wide temperature range and from the stoichiometric and excess magnesium composition. The role of magnesium overdoping on the superconducting characteristics of MgB2 has been studied, and hence, the correlation between the critical current density, porosity, and density of the MgB2 samples has been established. The excess magnesium in the starting composition expands the synthesis temperature range over which the MgB2 phase can be synthesized and also enhances the superconducting properties in terms of Tc, ΔT, and residual resistivity ratio. The reduced weight loss due to Mg vaporization, and hence the lower porosity in the sample synthesized from the stoichiometric composition and at Ts = 900°C, reflected the higher Jc in the MgB2 sample in this series. Whereas with the excess magnesium in the starting composition, the sample with the highest Jc has been synthesized at a lower synthesis temperature of Ts = 700°C. The correlation between weight loss during heat treatment, sample density, resistivity, and active area fraction has been established for vacuum-synthesized bulk MgB2 samples.

A model coil for the 40-T hybrid magnet is being built at the Chinese High Magnetic Field Laboratory, Chinese Academy of Sciences. /restack-rod-process (RRP) strands adopted for the model coil were heat-treated according to the heat treatment (HT) schedule recommended by the manufacturer, which is Oxford Superconducting Technology. The microstructure of the reacted strand was analyzed. Measurements of the critical current, the residual resistivity ratio and the hysteresis losses were also carried out. The results of critical-characteristic measurements confirmed that the performance of the heat-treated wire was good. All characteristics indicated that the HT of the /RRP strands was successfully completed.

It is found that there exist two different but related parameters that control the value of the residual resistivity ratio (RRR) in MgB2, namely, the stoichiometry of the starting powder and the synthesis parameters. The origin of the high RRR in the polycrystalline MgB2 samples has been attributed to the enhanced crystallinity of the samples due to higher Mg concentration in the starting composition. Scanning-electron-microscope images show the improved crystalline nature of the tape samples synthesized with the excess Mg in the starting powder mixture.

Large absorbers and a high counting rate is required for high energy resolution transition-edge sensor (TES) microcalorimeters for future X-ray astronomical missions. However, these two requirements have a relation of a trade-off in terms of energy resolution. Thus, study of the effect of thermal diffusion in large absorbers on energy resolution is essential. Therefore, we performed 3 dimensional FEM simulations for TES microcalorimeter with Bi/Au absorber, where a role of Au layer is boost the thermalization of slow thermalizing Bi, using SPICE to calculate the effect of thermal diffusion in the absorber on energy resolution. The simulation results for square absorbers of size 500 mum and 750 mum with Au thickness 0.5 mum indicate that energy resolution of 3 eV and 5 eV can be achieved, respectively, with a fast response time of 100 mus, provided the residual resistivity ratio of Au is higher than ~10.

We present a systematic study of the superconducting properties of high quality V3Si thin films, grown by high vacuum pulsed laser deposition from a stoichiometric target, a technique never reported in literature for this compound. By changing both the substrate (crystal structure and orientation) and the deposition conditions (substrate temperature, target-substrate distance, laser frequency and pulse energy) the critical temperature and the resistivity values of the samples have been finely tuned. Best results (TC = 16.1 K and Residual Resistivity Ratio RRR = 8.5) are obtained for deposition temperatures higher than 1200degC on both LaAlO3 and MgO substrates. All the samples are polycrystalline, that however does not affect the good quality of the transport properties. The possibility of multiband superconductivity in this compound is investigated, probing as a function of temperature: (i) the critical field up to 28 T using voltamperometric measurements, and (ii) the superfluid density via magnetic penetration depth measurements with a single coil inductive technique operating in the MHz region.

High critical current density (Jc) Nb3Sn conductor is the best candidate for next generation high field (>10 T) accelerator magnets. Although very promising, state of the art high-Jc Nb3Sn strands suffer magneto-thermal instabilities that can severely limit the strand performance. Recently it has been shown that at 1.9 K the self field instability is the dominating mechanism that limits the performance of strands with a low (

Restack Rod Process (RRP) strands with 54 and 108 sub-elements were rolled from 0.7 mm diameter to 0.45 mm thickness to simulate the deformation of strands at the edges of Rutherford cables. Various diagnoses were then applied to assess performance and stability. Transport measurements were used to assess the effect of rolling on the critical current. Magnetization measurements were used to probe superconducting pathway bridging between deformed sub-elements. The copper residual resistivity ratio was also measured to assess tin contamination due to thinned or ruptured diffusion barriers. While systematic changes were observed in all three measurements with increasing deformation, showed the strongest changes. The implications of these measurements for cable stability, and their relationship to observations of the strand cross section by light microscopy, are discussed.

The magnetization and low field stability were measured on a series of high current-density Nb3Sn strands with 54 to 198 sub-elements taken from accelerator magnet development programs. The effective filament size, deff ,, as determined from the width of the magnetization loop at low field and the extrapolation of the transport critical current from high field, was very close to the sub-element diameter determined by the strand geometry. Self-field corrections were vital for obtaining this agreement; without them, deff was overestimated by ~25%. While all strands, even those with deff as small as 35 mum, exhibited flux-jumps at low field in magnetization measurements, smaller sub-elements produced smaller flux-jump magnitude and a smaller range of field over which flux jumps occurred, suggesting stability improved with decreasing deff. . However, good combinations of the residual resistivity ratio RRR and the critical current density became increasingly difficult to obtain by varying the reaction heat treatment as the sub-element number increased. Further analyses of RRR data indicate that tin contamination of the copper stabilizer is underway even before any Nb3Sn is formed in strands with high numbers of sub-elements.

Performance improvements are needed for large-scale applications of Nb3Sn, such as ITER or LHC upgrades. The highest critical current density (Jc) values are achieved in distributed-barrier strand made by the Restacked Rod Process, which can reach 12 T, 4.2 K Jc values of 3000 A/mm2, with high residual resistivity ratio (RRR) values. For purposes of accelerator magnet stability, it is desirable to combine high Jc with a small effective filament diameter ( Deff ) . Initial experiments show reducing Deff from 80 mum to 40 mum leads to a 10% reduction in Jc. For fusion applications, a single-barrier design with well-spaced filaments is used to achieve the low hysteresis losses that are required. The status of our fusion strand development program is presented, including results for strand made using Nb-47 wt%Ti rods to supply Ti dopant. Such strands can reach 12 T, 4.2 K Jc> 1000 A/mm2, with losses < 1000 mJ/cm3.

Magnet programs at BNL, LBNL and FNAL have observed instabilities in high J/sub c/ Nb/sub 3/Sn strands and magnets made from these strands. This paper correlates the strand stability determined from a short sample-strand test to the observed magnet performance. It has been observed that strands that carry high currents at high fields (greater than 10 T) cannot sustain these same currents at low fields (1-3 T) when the sample current is fixed and the magnetic field is ramped. This suggests that the present generation of strand is susceptible to flux jumps (FJ). To prevent flux jumps from limiting stand performance, one must accommodate the energy released during a flux jump. To better understand FJ this work has focused on wire with a given sub-element diameter and shows that one can significantly improve stability by increasing the copper conductivity (higher residual resistivity ratio, RRR, of the Cu). This increased stability significantly improves the conductor performance and permits it to carry more current.

We report on unexpected field-angle dependence of magnetoresistance measurements of commercial, high-purity Nb discovered during our study of residual resistivity ratio (RRR) measurements. The RRR value indicates the purity and the low-temperature thermal conductivity of the Nb and is used as a specification for superconducting radio-frequency cavities. The RRR is typically defined as the ratio of the electrical resistivities measured at 273 K and 4.2 K. One way to obtain the normal-state resistivity at 4.2 K is to measure the resistivity versus magnetic field at 4.2 K and extrapolate to zero field. The field-angle dependence was measured when a specimen was rotated while the field was transverse to the specimen current. The resistance changed by about 10% as the angle varied at 8 T and the local maxima and minima of the resistance were separated by about 30/spl deg/. This was observed on three polycrystalline Nb bar specimens with nominal dimensions of 64 mm /spl times/ 6.7 mm /spl times/ 3.3 mm and voltage taps separated by 30 mm. A similar field-angle dependence was obtained on a polycrystalline Cu wire specimen. This phenomenon has implications for Nb RRR measurements and interlaboratory comparisons.

Highly textured Nb-films were grown by electron-beam evaporation on sapphire substrates. The crystalline quality was examined by Rutherford-backscattering-channeling and X-ray diffraction methods. The channeling minimum yield is 2%. On c-plane sapphire the evaluation of rocking curve measurements yielded 99.98% [111] Nb // (0001) sapphire and 0.02% [110] Nb // (0001) sapphire. Measurements on microbridges with widths ranging from 2 mu m to 100 mu m (film thickness 500 nm) showed a residual resistivity ratio (R/sub 295K//R/sub 10K/) of up to 90 and critical current densities at 4.2 K and 0.2 T as low as 1.7*10/sup 4/ A/cm/sup 2/. These Nb-films form the basis for devices for local generation and detection of vortices. The writing and erasure of vortices and their detection by means of an integrated superconducting quantum interference device (SQUID) are demonstrated. >

The industrial capability for producing significant amounts of Nb/sub 3/Sn superconductor that meet the NET-ITER specifications has been evaluated by extensive tests on thousands meters of multifilamentary composites ordered from two different companies. In particular, the following points have been investigated: weight and length of the delivered units, heat treatment and critical current, chrome plating, residual resistivity ratio (RRR), and AC losses measurements. With regard to RRR and losses some difficulties have been encountered. The consequences for the different coils of the NET-ITER project are discussed. >

Nearly 1500 tons of 8000 filament superconductor are required for the Superconducting Super Collider Laboratory (SSCL) main ring dipole magnets. IGC Advanced Superconductors' (IGC-ASI's) major accomplishments in achieving the goals included in the SSCL Phase 1 program are presented. Under the Phase 1 contract, IGC-ASI has assembled 12 Phase 1A and 9 Phase 1B multifilament billets. As required by the program, all the Phase 1B billets, containing one alloy 'source X', were identically processed under the IGC-ASI baseline. An analysis of data generated in the program is given. All data published are based on alloy source X. The authors discuss mechanical properties, including final die draw parameters, superconducting properties, and cable performance, including data affecting strand residual resistivity ratio (RRR). >

MgB2 has been recognized as a candidate material for fabricating superconducting nanowire single photon detectors with the expectation of having an operating temperature of the detector at 10-20 K. Preparation of ultrathin MgB2 films with high superconducting transition temperature TC is the first crucial step to achieve this goal. Here we present the hybrid physical-chemical vapor deposition of MgB2 superconducting films with thicknesses around 10 nm. The films are epitaxially grown on (111) MgO substrates. The TC of the films, including for a 6-nm-thick film, is found to be above 34 K, close to the bulk value of MgB2 (39 K). The normal-state resistivity at 42 K of a 10-nm-thick film is shown to be 3.1 μΩ cm, indicating that the film is relatively clean. The superconducting critical current density JC of the films has also been measured on strip lines patterned in the films. The JC of the 10-nm-thick film reaches 7.7 ×107 A/cm2 at 2 K, so far the highest value reported for MgB2 films of the same thickness. The high TC, low residual resistivity, and high JC of the ultrathin MgB2 films on MgO substrates suggest that these films have the potential in developing MgB2-based superconducting nanowire single photon detectors or other related superconducting devices.

Nb3Sn strands with large filaments and high-Jc exhibit instabilities due to magnetization flux-jumps at low fields in changing magnetic fields. In addition, at intermediate fields of 5 to 7 T, these strands quench prematurely at currents well below the critical current. Current-voltage measurements are typically used for critical current determinations, and the premature quenching observed is driven by current redistribution within the strand as the current is increased and is termed “self-field” instability. This instability is exacerbated as the temperature is lowered from 4.2 K to 2 K superfluid helium. A previous study examined wires in the “quasi-adiabatic” limit, where dynamic heat transfer mechanisms are suppressed. In this paper, we report on measurements in the temperature range of 4.2-2 K on high-Jc RRP strands with varying copper stabilizer resistivities and Nb3Sn filament diameters. These measurements show that the residual resistivity ratio, RRR, of the copper stabilizer plays an important role in mitigating this instability. Also for strands with similar RRR, we find that the stability improves with decreasing filament diameters, although the improvement is not very dramatic.

Future detector magnets call for the development of next-generation large-sized Al stabilized Nb-Ti superconducting cables exhibiting high yield strength for coping with the large stress in wide bore magnets with peak magnetic fields up to 6 T, while avoiding significant degradation in residual resistivity ratio. Precipitation type alloys obtained by dilute-alloying of high purity Al with suitable additives like Ni, subjected to partial annealing following cold drawing, can feature a yield strength up to 110 MPa at 4.2 K. For the ATLAS central solenoid, a Nb-Ti cable has been plated with a precipitation-type Al-0.1wt%Ni alloy. However, this conductor with a critical current of 20 kA at 5 T, features a cross-section of only 130 mm2. Here, a first step in process scale-up to a 60 kA at 5 T class conductor is described. For the first time, a continuous co-extruded Al-0.1wt %Ni stabilized conductor has been produced with a cross-section as large as 700 mm2. Sections of the conductor are work hardened in order to increase the mechanical properties of the as-extruded temper. The mechanical and transport characteristics as a function of the amount of work hardening have been assessed by removing samples after every subsequent step.

This article describes Nb thin films deposited on Cu using coaxial energetic deposition (CED). CED is a nonequilibrium cathodic arc process that drives medium energy (50-150 eV) ions into a few atomic layers beneath the surface to promote the growth of dense, ordered films via subplantation physics. Nb films deposited on crystalline substrates by CED have shown bulk-Nb like (>; 300-500) RRR values. Here we describe Nb films of 1-10 μm deposited on coupons and complex structures made of Cu at substrate temperatures of 400 °C or less. The coated samples were sectioned and the substrate material etched away so the superconducting transition temperature (Tc), as well as residual resistivity ratio (RRR) at 10 K of the Nb films could be measured. RRR values of >;100 have been achieved on fine-grain Cu with little surface preparation. The vortex penetration magnetic field was measured by DC SQUID and these results are presented along with comparisons between bulk Nb and thin film Nb on crystalline substrates. The CED approach has applications in superconducting bellows interconnects as well as in SRF cavities.

Magnetothermal instability may affect the performance of high critical current density Nb3Sn strands by quenching the superconductor at current values lower than its critical current. This phenomenon has important implications in the design of next-generation superconducting magnets for accelerators. According to the theory, the strand magnetothermal stability can be improved by changing the current distribution within the strand. In this paper, a technique to improve the conductor stability is presented. It consists in partially cutting the strand's outer superconducting subelements in the region where the wire is soldered to the copper block that feeds the current. The special cut forces the current to enter into the inner subelements without degrading the current capacity. As a consequence, the stability is increased, which avoids changing the strand critical current or its residual resistivity ratio because current redistribution during an external perturbation is strongly reduced. The experimental results obtained by applying this technique on a 0.8-mm restacked-rod process strand are presented and discussed.