Oxford Superconducting Technology (OST) has been continuously improving Bi-2212 round wire performance because of its potential for application in high-field magnets (> 25 T). We focused on Bi-2212 wire configuration design, filament densification and reducing carbon and hydrogen contamination to improve the engineering critical current density (JE). Several wire configurations have been developed to meet different wire diameter and operating current requirements. The swaging, cold isostatic pressing (CIP) and over-pressure heat treatment processes have been demonstrated to effectively increase Bi-2212 filament mass density in the final wire and result in high performance over long length. The JE values exceeding 550 A/mm2 at 4.2 K, 15 T have been achieved on the CIPed 1 m long sample using a 10 bar over-pressure (OP) heat treatment. The twisted Bi-2212 wire significantly reduced ac loss without the critical current degradation.

Bi-2212 round wire is made by the powder-in-tube technique. An unavoidable property of powder-in-tube conductors is that there is about 30% void space in the as-drawn wire. We have recently shown that the gas present in the as-drawn Bi-2212 wire agglomerates into large bubbles and that they are presently the most deleterious current-limiting mechanism. By densifying short 2212 wires before reaction through cold isostatic pressing, the void space was almost removed and the gas bubble density was reduced significantly, resulting in a doubled engineering critical current density (J(E)) of 810 A/mm(2) at 5 T, 4.2 K. Here we report on densifying Bi-2212 wire by swaging, which increased J(E) (4.2 K, 5 T) from 486 A/mm(2) for as-drawn wire to 808 A/mm(2) for swaged wire. This result further confirms that enhancing the filament packing density is of great importance for making major J(E) improvements in this round-wire magnet conductor.

There has been sustained interest in the development of Bi-2212/Ag round wire because of its unique potential for application in ultra-high-field magnets (>; 25 T). Our development activity with this material has been focused on improving the engineering current density. Filament densification by swaging and isostatic pressing processes have been evaluated, as has further optimization of the melt heat treatment conditions. These improvements lead to an increased mass density of the filament in the final wire, and are essential to reduce filament porosity and obtain high performance over long wire length. Engineering current density values exceeding 480 A/mm 2 at 4.2 K, 15 T have been achieved on 1-m-long barrel samples.

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.

The critical current density (Jc) of Nb3Sn strand has been significantly improved over the last several years. For most magnet applications, high Jc internal tin has displaced bronze process strand. The highest Jc 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 Jc values of 3000 A/mm2 ...

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-47wt% Ti rods to supply the dopant, and have achieved J(c) values of 3000 A/mm(2) (12 T, 4.2 K). Such wires have very good higher field performance as well, reaching 1700 A/mm(2) 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/mm(2) (12 T, 4.2 K) with hysteresis losses less than 700 mJ/cm(3) over non-Cu volume. The J(c)-strain behavior of such composites is also presented.

Performance improvements are needed for large scale applications of Nb3Sn, such as ITER or LHC upgrades. The highest critical current density (J(c)) values are achieved in distributed-barrier strand made by the Restacked Rod Process, which can reach 12 T, 4.2 K J(c) values of 3000 A/mm(2), with high residual resistivity ratio (RRR) values. For purposes of accelerator magnet stability, it is desirable to combine high J(c) with a small effective filament diameter (D-eff). Initial experiments show reducing D-eff from 80 mu m to 40 mu m leads to a 10% reduction in J(c). 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 J(c) > 1000 A/mm(2), with losses < 1000 mJ/cm(3).

The high performance Nb3Sn strand produced by Oxford Superconducting Technology (OST) with the Restack Rod Process (RRP) is presently considered as a baseline conductor for the Fermilab's accelerator magnet R&D program. To improve the strand stability in the current and field range expected in magnet models, the number of subelements in the strand was increased by a factor of two (from 54 to 108), which resulted in a smaller effective filament size. The performance of the 1.0 and 0.7 mm strands of this design was studied using virgin and deformed strand samples. 27-strand Rutherford cables made of 1 mm strand were also tested using a superconducting transformer, small racetrack and 1-m shell-type dipole coils. This paper presents the RRP strand and cable parameters, and reports the results of strand, cable and coil testing.

Bi-2212/Ag round wire is a promising and practical material for extending high field superconducting magnets beyond the limits of Nb-3 Sn. 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 (J(E)) and the critical current density (J(c)) performance have been achieved by optimizing the starting precursor composition, and the heat treatments. The highest J(E) of 1580 A/mm(2) at 4.2 K, 0 T and 420 A/mm(2) 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.

Final reactions of 48 or 50 hours were given in a range of temperature from 635 o C to 695 o C to high-J c Ta- and Ti-alloyed Restack-Rod-Process Nb 3 Sn strands to investigate the changes in critical current density and the superconducting properties. J c is the current density in the non-copper region of the wire. Measurements of J c were made at 4.2 K temperature and from 8 to 11.5 T field at BNL and from 12 to 16 T at OST, and these data were fitted to the Summers expression for J c to extrapolate the scaling field . B c2 * ldr All three(Nb, Ta) 3 Sn strand designs investigated displayed peak J c values of ~3000 A/mm 2 at 12 T and ~at 1500 A/mm 2 at 15 T for 665 and 680 o C reactions. B c2 * increased monotonically with increasing reaction temperature, a (Nb, Ti) 3 , By comparison, at each reaction temperature, a strand under development had a B c2 * higher than any of the Ta-alloyed strands. This suggests that Ti-alloyed strands could improve high-field performance if further development can bring their J c values closer to those of the Ta-alloyed strands. Other implications are also discussed.

Advances in high field magnets are driven primarily by the availability of high current density conductors. The restack rod process (RRP), internal Sn superconductors have achieved engineering current densities nearly five times that of bronze route conductors at high fields. Careful utilization of this low temperature superconductor (LTS) enables the production of magnets beyond the previous benchmark of 21 Tesla without an associated increase in magnet and cryostat volume. Steps to realize extremely compact high field magnets for a variety of applications are described. The next significant challenge is to produce magnetic fields beyond 25 Tesla solely using superconducting solenoids. High temperature superconductors (HTS) will be required and, to this end, Bi-2212/Ag matrix wires are at an advanced stage of development. The tangible objective is a new generation of compact, ultra-high field magnets.

The high field critical current density (J c ) and engineering current density (J E ) 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. I c 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

Practical high field superconducting magnets are exclusively built with Nb3Sn multi-filamentary composites. Over the last few years there have been significant improvements in the current carrying capability of Nb3Sn strand, and these improvements offer the possibility to build more efficient and higher field strength magnets. The Nb3Sn composite requirements are somewhat different depending on the application, such as magnetically confined fusion, high energy physics accelerators, and solenoids for NMR or laboratory magnets, and thus require different designs. We will present the current status of Nb3Sn strand development at Oxford Instruments, Superconducting Technology (OST) for these applications, along with magnet results