Jeff Parrell

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}}

Oxford Instruments, Superconducting Technology (OI-ST) produces Nb/sub 3/Sn 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/mm/sup 2/. We also report recent results from wires fabricated by other internal Sn methods, the bronze process, and powder-in-tube processes.

Conductors Engineered for Fusion and Particle Accelerator Applications

The high performance NbSn strand produced by Oxford Superconducting Technology (OST) with the Restack Rod Process (RRP) is presently considered as a baseline conductor for the Fermilabs accelerator magnet R∓mp;mp;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.

Progress with Nb/sub 3/Sn conductors at oxford instruments, superconducting technology

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.

Performance of Nb

Multifilamentary Nb/sub 3/Sn 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 /spl epsiv//sub max/ in these conductors are reduced to the range 0.1% to 0.2%, which is significantly less than the /spl epsiv//sub max/ values of 0.2% to 0.4% in traditional bronze-process Nb/sub 3/Sn conductors (where niobium fractions are typically about 10% to 15%). However, including about 20% dispersion-strengthened copper into the conductor matrix restores /spl epsiv//sub max/ to the range 0.25% to 0.35%, thus providing practical levels of /spl epsiv//sub max/ for magnet design in high-niobium-fraction strands.

_{3}

About 450 km of high JC Nb3Sn wires have been procured for the construction of two series-connected hybrid magnets by the National High Magnetic Field Laboratory (NHMFL). In addition to the quality assurance tests by the wire manufacturer, a receiving quality assurance inspection with a predetermined sampling frequency is carried out at the NHMFL. These tests include wire diameter, twist pitch, Cu/non-Cu ratio, IC, residual resistance ratio, hysteresis loss, and IC strain dependence. In this paper, the test procedures are described and the test results are presented. Based on these test results, all the wires have been accepted.

Sn RRP Strands and Cables Based on a 108/127 Stack Design

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.

Present and Future Applications for Advanced Superconducting Materials in High Field Magnets

Fermi National Accelerator Laboratory has been developing a 15-T Nb3Sn dipole demonstrator for a future very high energy pp collider based on an optimized 60-mm aperture four-layer “cos-theta” coil. To increase the magnet efficiency, the coil was graded by using two cables with same 15xa0mm width and different thicknesses made of two different restacked rod process (RRP) wires. Due to the nonuniform field distribution in dipole coils, the maximum field in the inner coil will reach 15–16 T, whereas the maximum field in the outer coil is 12–13 T. In preparation for the 15-T dipole coil reaction, heat treatment studies were performed on strands extracted from these cables with the goal of achieving the best coil performance in the corresponding magnetic fields. In particular, the effect of maximum temperature and time on the cable critical current was studied to take into account actual variations of these parameters during coil reaction. In parallel and in collaboration with Oxford Instruments-Superconducting Technology, development was performed on optimizing Nb3Sn RRP wire design and layout.

Compressive pre-strain in high-niobium-fraction Nb/sub 3/Sn superconductors

A strategy to obtain higher critical current density (Jc) in Nb3Sn conductors is to increase the Nb fraction. This can be achieved by improving the deformation behavior of the Nb filaments so less Cu is needed between filaments enabling smaller diameter filaments and more filaments in the subelement region. The objective of this project is to better understand the influence of different pre-processing strain path (texture) on the deformation of a Nb filament in a Cu-matrix. Initial severe plastic deformation by equal channel angular extrusion (ECAE) was used to obtain a uniform fine grain size with slightly different textures in starting Nb rods. Cu-Nb composite bars were warm extruded and wire drawn to evaluate the performance of different starting Nb fine grained microstructures on Nb filament deformation behavior. The initial textures in the various Nb samples were different but quite weak; little difference is seen in the final Nb filament circularities. However, this study indicates the Cu-Nb interface roughness is influenced somewhat by non-uniformity in the Nb microstructure. The evidence suggests that to obtain better deformation characteristics in Nb filaments, one should start with fine-grain weak or untextured Nb with a globally uniform microstructure. This should enable finer as-drawn Nb filaments with better concentricity, composite wires with a higher Nb loading fraction, and conductors with higher Jc.

Quality Assurance Tests of

The Restacked Rod Process (RRP) is the Nb3Sn strand technology presently producing the largest critical current densities at 4.2 K and 12 T. However, when subject to transverse plastic deformation, RRP subelements (SE) merge into each other, creating larger filaments with a somewhat continuous barrier. In this case, the strand sees a larger effective filament size and its instability can dramatically increase locally leading to a cable quench. To reduce and possibly eliminate this effect, Oxford Instruments Superconducting Technology (OST) developed for FNAL a modified RRP strand design with larger Cu spacing between SEs arranged in a 60/61 array. Strand samples of this design with sizes from 0.7 to 1 mm were first evaluated for transport current properties. A comparison study was then performed between the regular 54/61 and the modified 60/61 design using 0.7 mm round and deformed strands. Finite element modeling of the deformed strands was also performed with ANSYS.