High-temperature superconductors (HTS) could enable high-field magnets stronger than is possible with Nb-Ti and Nb 3 Sn, but two challenges have so far been the low engineering critical current density J E , especially in high-current cables, and the danger of quenches. Most HTS magnets made so far have been made out of REBCO coated conductor. Here we demonstrate stable, reliable and training-quench-free performance of Bi-2212 racetrack coils wound with a Rutherford cable fabricated from wires made with a new precursor powder. These round multifilamentary wires exhibited a record J E up to 950 A/mm 2 at 30 T at 4.2 K. These coils carried up to 8.6 kA while generating 3.5 T at 4.2 K at a J E of 1020 A/mm 2 . Different from the unpredictable training performance of Nb-Ti and Nb 3 Sn magnets, these Bi-2212 magnets showed no training quenches and entered the flux flow state in a stable manner before thermal runaway and quench occurred. Also different from Nb-Ti, Nb 3 Sn, and REBCO magnets for which localized thermal runaways occur at unpredictable locations, the quenches of Bi-2212 magnets consistently occurred in the high field regions over a long conductor length. These characteristics make quench detection simple, enabling safe protection, and suggest a new paradigm of constructing quench-predictable superconducting magnets from Bi-2212.

The European Union contributes around 20% of the cable-in-conduit conductor lengths needed for the ITER toroidal field (TF) magnet coils. For that purpose, 97 tons of Nb 3 Sn superconducting strand have been fabricated over five years, the production being completed in 2014. This superconducting strand has been manufactured by two companies, namely, Bruker EAS (Germany) and OST (USA), through the bronze route and the internal tin diffusion, respectively. This paper reports the outcomes of this strand mass production and of the strand characterization as performed by the suppliers and cross-checked on a regular basis by Durham University.

Flexibility of design means internal tin Nb3Sn strands can be tailored for particular applications. For particle accelerator applications, the development work is focused on reducing the effective filament diameter whilst maintaining high Jc and RRR. We will present our latest results on new distributed barrier strand designs that modify Nb, Sn, and Cu ratios to enhance RRR and maintain Jc in strands having subelement diameter of less than 45 μm. For laboratory magnets, cryogen-free operation is becoming the norm. To that end, we are continuing to develop single barrier internal tin strands having minimal ac losses but high Ic values of critical current. For high field NMR magnets, where the highest Jc in the highest magnetic field is critical, highest performance strand has average Jc values over 1600 A/mm2 at 4.2 K, 15 T. We will also present summary of strand performance for the ITER TF coil and our latest results on RRP strand for cable-in-conduit applications.

Flexibility of design means internal tin Nb 3 Sn strands can be tailored for particular applications. For particle accelerator applications, the development work is focused on reducing the effective filament diameter whilst maintaining high J c and RRR. We will present our latest results on new distributed barrier strand designs that modify Nb, Sn, and Cu ratios to enhance RRR and maintain J c in strands having subelement diameter of less than 45 μm. For laboratory magnets, cryogen-free operation is becoming the norm. To that end, we are continuing to develop single barrier internal tin strands having minimal ac losses but high I c values of critical current. For high field NMR magnets, where the highest J c in the highest magnetic field is critical, highest performance strand has average J c values over 1600 A/mm2 at 4.2 K, 15 T. We will also present summary of strand performance for the ITER TF coil and our latest results on RRP strand for cable-in-conduit applications.

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.

Bi2Sr2CaCu2Ox (Bi-2212) superconducting wires are of interest for producing the next generation of high field (greater than 20 T) magnet windings. However, improvements in critical current must be achieved to make them fully viable for such applications. Fully optimizing the thermal reaction in the melt-processing is a means of maximizing the capabilities of wires presently available. Using methods of Statistically Designed Experiments, the process variables may be simultaneously optimized to maximize c, and in this work, we show the effects of including and varying the parameters of a pre-anneal in the thermal reaction cycle. We find that the critical current is less sensitive to changes in the pre-anneal temperature, and is more responsive to changes in the dwell time. We examine the microstructure for varied dwell times and show the fitted response surfaces for c as a function of the pre-anneal parameters.

Compositional analysis of the Bi2Sr2CaCu2Oy (Bi-2212) and secondary phases in high-current Bi-2212 conductors with dilute (<;1 wt.%) second phase additions was performed. Aluminum or zirconium based oxide secondary phases were added to the powders in work designed to gauge their usefulness as processing aids in the development of long length, high critical current density Bi-2212 wires and tapes. Dilute amounts of the additions did not appreciably change the composition of the Bi-2212 phase in the conductors. In all cases, there was an excess of bismuth and a deficiency of the alkaline earths with respect to the ideal 2212 stoichiometry. Scanning and transmission electron microscopy were used to determine the key microstructural features of these conductors with the dilute additions and relate them to their superconducting properties.

Sub-scale coils are being manufactured and tested at Lawrence Berkeley National Laboratory in order to develop wind-and-react Bi(2)Sr(2)CaCu(2)O(x) (Bi-2212) magnet technology for future graded accelerator magnet use. Previous Bi-2212 coils showed significant leakage of the conductors' core constituents to the environment, which can occur during the partial melt reaction around 890 degrees C in pure oxygen. The main origin of the observed leakage is intrinsic leakage of the wires, and the issue is therefore being addressed at the wire manufacturing level. We report on further compatibility studies, and the performance of new sub-scale coils that were manufactured using improved conductors. These coils exhibit significantly reduced leakage, and carry currents that are about 70% of the witness wire critical current (I(c)). The coils demonstrate, for the first time, the feasibility of round wire Bi-2212 conductors for accelerator magnet technology use. Successful high temperature superconductor coil technology will enable the manufacture of graded accelerator magnets that can surpass the, already closely approached, intrinsic magnetic field limitations of Nb-based superconducting magnets.

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 (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 Jc 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 Jc values over 1100 A/mm2 (12 T, 4.2 K) with hysteresis losses less than 700 mJ/cm3 over non-Cu volume. The Jc-strain behavior of such composites is also presented.

Bi-2212/Ag conductor is one of the most promising materials for extending the field strength of superconducting magnets over present low temperature superconductor systems. From the view point of practical application, Bi-2212/Ag round wires have significant advantages over more typical HTS tape conductors, such as no anisotropy, and easier handling and coil winding, which allows considerable flexibility in the magnet design. Recent development efforts at Oxford Superconducting Technology have been aimed at manufacturing high quality multifilamentary Bi-2212/Ag round wires with the varied sizes to fabricate HTS insert coils for high field magnet applications. However, further improvement of critical current density (J(c)) and engineering current density (J(E)) in larger diameter wires is desirable for practical applications. Recent results show a strong dependence of the wire J(E) and J(c) performance on its microstructure, in particularly, the interface of Bi-2212/Ag. Significant improvements of microstructure and J(c) have resulted from the optimization of wire size and filament numbers, but not obviously on starting powder fill factors. The highest J(E) of 3 20 A/mm(2) (non-Ag J(c) of 1103 A/mm(2)) at 4.2 K, 25 T was obtained in 1.15 mm wire with 85 x 19 filament configuration.

The production of high magnetic fields using low temperature superconductors (LTS) has become common place. However, large magnet sizes and associated high cooling costs have often precluded the full utilization of these research capabilities. Recent advances in internal Sn superconductors and cryogen free technology have opened up a new era in superconducting magnet development. Ultra-compact, laboratory sized magnets producing fields up to 22 Tesla are available. This new class of high field magnet weighs under 200 kg and is suitable for general laboratory installation. In addition, extremely compact, high field, split pair magnets with open access are now operating at the elevated temperature of T = 4.2 K. Beyond conventional wet magnet technology, there is a growing trend to utilize cryogen free technology. Cryogen free magnets do not require liquid Helium baths and, with the addition of active shielding, both the experimental sample access and siting availability is improved. The influence of enabling technologies required to realize the above practical applications for high field, superconducting magnet systems is described.

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.

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.

Multifilamentary Nb3Sn superconducting strands fabricated with high niobium fractions have exceptionally high critical-current densities but are sometimes marginally stable during testing. We report a technique for determining the pre-strain in such conductors, in which additional stabilizing copper is electroplated onto the conductor and the pre-strain is determined by extrapolation to the as-fabricated niobium fraction. This technique is used to measure the pre-strain in conductors with high niobium fractions of 20% to 30%. Values of the pre-strain epsilon(max) in these conductors are reduced to the range 0.1% to 0.2%, which is significantly less than the epsilon(max) values of 0.2% to 0.4% in traditional bronze-process Nb3Sn conductors (where niobium fractions are typically about 10% to 15%). However, including about 20% dispersion-strengthened copper into the conductor matrix restores epsilon(max) to the range 0.25% to 0.35%, thus providing practical levels of epsilon(max) for magnet design in high-niobium-fraction strands.

Magnetization measurements have been made on several high J/sub c/ Nb/sub 3/Sn strands fabricated by different internal-Sn designs. In general these conductors have high magnetization at low fields, often exhibiting flux-jumps that are characteristic of large superconductor diameter. The effective filament size d/sub eff/ is approximately the size of the sub-element because the filament pack within each sub-element is fully coupled. Dividing the filament pack of the sub-element by adding Ta is effective for reducing d/sub eff/ and magnetization instability. But, some residual coupling across the dividers seems to remain below 6 K, perhaps due to Ta/sub 3/Sn. Implications for accelerator magnets are discussed.

Oxford Instruments, Superconducting Technology (OI-ST) produces Nb 3Sn wires by two major process routes: bronze and internal Sn, including modified jelly roll (MJR). Each process has its own strengths for particular applications. We report on MJR wire designed for prototype high energy physics accelerator magnet studies which achieved short sample non-Cu critical current density at 12 T, 4.2 K exceeding 2000 A/mm2. We also report recent results from wires fabricated by other internal Sn methods, the bronze process, and powder-in-tube processes

This paper describes the development and testing of a 3 T Class Bi-2212 insert magnet. The magnet consists of three sections, each built by stacking double pancakes using Powder-In-Tube conductor and the Wind&React approach. Conductor with a pure Ag matrix was used for the inner section and conductor with a mixed Ag and AgMg matrix was used for the outer two sections. Elements of the design, conductor properties, construction, and testing are presented. The successful generation of 3 T in a 19 T background magnetic field and some of the development issues are discussed.

Based on a successful 1 T Class PIT insert coil, the authors are now pursuing a 3 T Class insert coil. This paper describes the design and the latest conductor and coil test results, as well as supporting experiments. The final product is envisioned to contain 3 concentric sections, requiring over one kilometer of conductor. This will be tested in a 20 T large bore resistive magnet at the NHMFL. Experimental work focuses on the use of conductor with a silver-alloy matrix in the outer sections, that are subject to the largest stresses when operated in a background field. Results from heat treatment optimization for wound coils, mechanical test of conductors and coil design studies are reported.