Maarten Meinesz

Development of round multifilament Bi-2212/Ag wires for high field magnet applications

Bi-2212/Ag conductor is one of the most promising materials for expanding superconducting magnet applications to higher fields and/or temperatures than present LTS systems. From the view point of practical application, Bi-2212 round wires have significant advantages over more typical HTS tape conductors, such as no anisotropy, ease of handling and simpler coil winding, allowing considerable flexibility in the magnet design. Development efforts at OST have been aimed at enhancing transport properties in long length round wires for high field magnet applications. Recently, significant improvements in the J/sub E/ and J/sub c/ performance have been achieved by optimizing the starting powders, the filament size and fill factor, the deformation processes, and the melt-solidification parameters. The highest J/sub E/ of 266 A/mm/sup 2/ and J/sub c/ of 950 A/mm/sup 2/ at 4.2 K, 45 T with n-value of 17 was obtained in 0.81 mm wire. In this paper progress on the development of Bi-2212 round wires will be reported.

BSCCO-2212 conductor development at Oxford Superconducting Technology

Three types of BSCCO-2212 conductor are in development at Oxford Superconducting Technology (OST). Prototype size batches of multifilament tape continue to be manufactured for use in a 5 T high field insert magnet demonstration in collaboration with the National High Magnetic Field Lab. Progress will be reported in improving uniformity of microstructure and transport properties in these tapes. Development of multifilament wire has been renewed in response to interest from the high energy physics community. Issues of interest in these wires include the effects on J/sub c/ from filament size and distribution, as well as chemical composition and impurity content in the ceramic. Progress in J/sub c/ and J/sub E/ at 4.2 K in these wires will be discussed. Optimization of dip-coated 2212 tape for MRI magnet applications is also underway, as part of a Superconductivity Partnership Initiative. Various cost and performance tradeoffs will be considered, including the silver/ceramic ratio, the thickness of the ceramic, and the choice of batch or continuous heat treatment. The status of these optimization efforts will be reported.

High Field Insert Coils From Bi-2212/Ag Round Wires

Bi-2212/Ag round wire is a promising and practical material for extending high field superconducting magnets beyond the limits of Nb3Sn. Efforts to develop superconducting magnets in the 25 to 30 T range include fabrication and test of practical size insert coils using this wire. Recent studies have focused on improvements in wire performance, wire insulation, and coil fabrication for wind-and-react coils. Continued improvements in the engineering critical current density (JE) and the critical current density (Jc) performance have been achieved by optimizing the starting precursor composition, and the heat treatments. The highest Je of 1580 A/mm2 at 4.2 K, 0 T and 420 A/mm2 at 4.2 K, 31 T were obtained in 0.81 mm wire. In particular, significant progress on braided insulation has been made for enabling a robust procedure for wind-and-react Bi-2212 solenoid coils. Performance of three of these coils has been measured in background fields up to 19 T, showing good prospects for high field magnet application of this conductor.

Progress in

The high field critical current density (Jc) and engineering current density (JE) in multifilament Bi-2212 round wires continue to improve, suggesting this material may well enable the next generation of high field magnets. The critical current in round wires shows no anisotropy with respect to applied field. Recent efforts have focused on characterizing the anisotropy in low aspect ratio rectangular wires; these results show that isotropic 2212 wire should be possible with rectangular cross sections if the aspect ratio is kept below 1.6. A braided insulation is now available for these conductors, enabling a robust procedure for wind-and-react coils. Ic and generated field have been measured in a series of such coils of increasing dimensions. Performance of two of these coils has been measured in background fields up to 19 T, showing good prospects for high field magnet application of this conductor

rm Bi

The critical current density (J_c) of Nb₃Sn strand has been significantly improved over the last several years. For most magnet applications, high J_c internal tin has displaced bronze process strand. The highest J_c values are obtained from distributed barrier strands. We have continued development of strands made with Nb-47 wt%Ti rods to supply the dopant, and have achieved J_c values of 3000 A/mm² (12 T, 4.2 K). Such wires have very good higher field performance as well, reaching 1700 A/mm² at 15 T. To reduce the effective filament diameter in these high J_c strands, the number of subelement rods incorporated into the final restack billet has been increased to 127 in routine production, and results are presented on experimental 217 stacks. A new re-extrusion technique for improving the monofilament shape is also described. For fusion applications such as ITER, we have developed single-barrier internal tin strands having non-Cu J_c values over 1100 A/mm² (12 T, 4.2 K) with hysteresis losses less than 700 mJ/cm³ over non-Cu volume. The J_c-strain behavior of such composites is also presented.

-2212 Wires for High Magnetic Field Applications

The past several years have seen a significant improvement in the maximum critical current density (Jc) in Nb3Sn strand. However for many applications, parameters besides high Jc values are paramount. For fusion applications such as ITER, we have developed single‐barrier internal tin strands having non‐Cu Jc values over 1000 A/mm2 (12 T, 4.2 K) with hysteresis losses less than 1000 mJ/cm3. Our most recent results are presented, with the goal of maintaining the high Jc but further reducing the losses. For high field magnet applications, higher Jc values are obtained using a distributed barrier approach. Results will be presented on a new high Jc Nb3Sn strand that is made with (Nb,Ti)3Sn instead of (Nb,Ta)3Sn. This (Nb,Ti)3Sn strand has a Jc value of 3000 A/mm2 (12 T, 4.2 K), but has improved higher field performance compared with our standard (Nb,Ta)3Sn material, reaching 1700 A/mm2 at 15 T, with further optimizations perhaps still possible. To reduce the effective filament diameter in these high Jc strand...

Internal Tin

The critical current density (J_c) of Nb₃Sn strand has been significantly improved over the last several years. For most magnet applications, high J_c internal tin has displaced bronze process strand. The highest J_c values are obtained from distributed barrier strands. We have continued development of strands made with Nb-47 wt%Ti rods to supply the dopant, and have achieved J_c values of 3000 A/mm² (12 T, 4.2 K). Such wires have very good higher field performance as well, reaching 1700 A/mm² at 15 T. To reduce the effective filament diameter in these high J_c strands, the number of subelement rods incorporated into the final restack billet has been increased to 127 in routine production, and results are presented on experimental 217 stacks. A new re-extrusion technique for improving the monofilament shape is also described. For fusion applications such as ITER, we have developed single-barrier internal tin strands having non-Cu J_c values over 1100 A/mm² (12 T, 4.2 K) with hysteresis losses less than 700 mJ/cm³ over non-Cu volume. The J_c-strain behavior of such composites is also presented.

{\hbox {Nb}}_{3}{\hbox {Sn}}

To evaluate the controlled quench behavior of high temperature superconducting (HTS) coils, particularly when using HTS coils in a hybrid configuration as an insert in a low temperature superconducting magnet, a layer-wound solenoid using Bi2Sr2CaCu2O wire was instrumented with several strip heaters to generate quenches in the axial and azimuthal directions. An array of distributed voltage taps and thermocouples were used to monitor the quench signals. Minimum quench energies (MQE) and quench propagation velocities (NZPVs) were determined. Results show that quench energies were moderate. NZPVs were slow but quench reaction times were of the same order as reaction times obtained at low quench energy densities in Nb3Sn coils.

Conductors Engineered for Fusion and Particle Accelerator Applications

Wind & react Bi-2212 inserts have been manufactured and tested inside a wide-bore NbTi - Nb3Sn magnet providing a background field up to 20 T at 4.2 K. A pair of six-layer concentric coils both achieved critical currents of 350 A (JE = 200 A/mm2) in a 20 T background field. A thicker 14-layer insert made from 119 m of round wire had a critical quench current IQ of 287 A (JE = 162 A/mm2) at the same field and contributed to a combined central field of 22.5 T. This is a record for a fully superconducting magnet at 4.2 K. The 14-layer coil, equipped with an external protective shunt, was used for an extensive series of quench measurements and endured > 150 quenches without damage. Minimum quench energies were found to be in the range of 200-500 mJ in background fields of 15-20 T when the coil carried 70-95% of its critical quench current.

INTERNAL TIN Nb3Sn CONDUCTORS FOR PARTICLE ACCELERATOR AND FUSION APPLICATIONS

The development of a 25 T superconducting magnet is usually envisioned with the use of an HTS insert coil. Previously, we reported the successful development of a 3 T coil in a 19 T background field based on BSCCO 2212 conductor. Here we report on the progress toward a larger 5 T insert with 38 mm free bore. The design is introduced, which calls for 2 stacks of double pancakes and an outer layer wound section, all electrically in series. Reacted conductor is used with insulated steel tapes as reinforcement. Results in terms of field dependence of the critical currents and stress tolerance are presented for both bare conductors and double pancakes. The latter are tested in a 19 T, 0.17 m cold bore, magnet assembly.