Robert P. Walsh

Design of a Superconducting 32 T Magnet With REBCO High Field Coils

The design and fabrication of a 32 T, 32 mm cold bore superconducting magnet with high field REBCO inner coils is underway at the NHMFL. In support of the design, conductor characterization measurements have been made including critical current as a function of field, field orientation, temperature, and strain on conductors and joints. Various conductor and turn insulation systems were examined. The selected coil fabrication method for the 32 T magnet is pancake wind, dry wind coils with sol-gel insulation on a stainless steel co-wind. Quench protection of the REBCO coils by distributed heaters is under development. Small REBCO coils have been made and tested in a 20 T background field to demonstrate performance of the technology. The design of the 32 T magnet is described, including coil configuration and conductor lengths, fraction of critical current, selection of conductor copper content for protection, and stress in the windings.

Critical current of superconducting Rutherford cable in high magnetic fields with transverse pressure

For high energy physics applications, superconducting cables are subjected to large stresses and high magnetic fields during service. It is essential to know how these cables perform under these operating conditions. A loading fixture capable of applying loads of up to 700 kN has been developed by NHMFL for LBNL. This fixture permits uniform loading of straight cables over a 122 mm length in a split-pair solenoid in fields up to 12 T at 4.2 K. The first results from this system for Rutherford cables of internal-tin and modified jelly roll strand of Nb/sub 3/Sn produced by IGC and TWC showed that little permanent degradation occurs up to 210 MPa. However, the cable made from internal-tin strand showed a 40% reduction in I/sub c/ at 11 T and 210 MPa while a cable made from modified jelly roll material showed only a 15 % reduction in I/sub c/ at 11 T and 185 MPa.

Mechanical Properties of Zylon/Epoxy Composite at 295K and 77 K

Zylon fiber/epoxy composites are used at the National High Magnetic Field Laboratory for structural reinforcement of high field pulse magnet coils. Zylon fiber have been chosen to replace glass or carbon fibers primarily because of the fibers extremely high strength and elastic modulus. Although the fiber properties are well documented, there is limited data available with respect to its properties in fiber/epoxy composite form, especially at low temperatures. Here the mechanical properties pertinent to pulse magnet design are characterized and analyzed with respect to manufacturing variables. Specifically the elastic modulus, tensile strength, and fatigue life of the quasiunidirectional composite specimens, are measured at both 295 K and 77 K

High strength kiloampere Bi2Sr2CaCu2Ox cables for high-field magnet applications

Multifilamentary Ag-sheathed Bi2Sr2CaCu2Ox (Bi-2212) wire can carry sufficient critical current density Jc for the development of powerful superconducting magnets. However, the range of its applications is limited by the low mechanical strength of the Ag/Bi-2212 strand. A potential solution is to cable Ag/Bi-2212 wire with high-strength materials that are compatible with the Bi-2212 heat treatment in an oxygen atmosphere. Past attempts have not always been successful, because the high-strength materials reacted with Bi-2212 wires, significantly reducing their Jc. We examined the nature of reactions occurring when Ag/Bi-2212 wires are heat-treated in direct contact with several commonly used high-strength alloys and a new Fe-Cr-Al alloy. INCONEL X750 and INCONEL 600 resulted in significant Jc loss, whereas Ni80-Cr caused little or no Jc loss; however, all of them formed chromium oxide that subsequently reacted with silver, creating cracks in the silver sheath. We found that Fe-Cr-Al did not show significant reactions with Ag/Bi-2212 strands. Scanning electron microscopy (SEM) and energy dispersive x-ray (EDS) examinations revealed that the Fe-Cr-Al alloy benefits from the formation of a uniform, crack-free, continuous alumina layer on its surface that does not react with Ag and that helps minimize the Cu loss found with INCONEL X750 and INCONEL 600. We fabricated prototype 6-around-1 cables with six Bi-2212 strands twisted and transposed around an Fe-Cr-Al alloy core coated with TiO2. After standard 1 bar melt processing, the cable retained 100% of the total current-carrying capability of its strands, and, after a 10 bar overpressure processing, the cable reached a total current of 1025 A at 4.2 K and 10 T. Tensile tests showed that Fe-Cr-Al becomes brittle after being cooled to 4.2 K, whereas INCONEL X750 remains ductile and retains a modulus of 183 GPa. We proposed new cable designs that take advantage of the chemical compatibility of Fe-Cr-Al and high strength of INCONEL X750 for various high-field magnet applications.

Mechanical properties of MP35N as a reinforcement material for pulsed magnets

A cobalt multiphase alloy, MP35N, is studied as one of the reinforcement materials for pulsed magnets. The mechanical properties of this alloy at room temperature and 77 K are examined. The cold-rolled and aged MP35N produces a hardness of 5650 MPa and yield strength of 2125 MPa at room temperature. At 77 K, the yield strength reached 2500 MPa and the work hardening rate was higher than that at room temperature. The Youngs modulus increases about 6% upon cooling from 300 to 5 K. Therefore, the increase of the strength at low temperatures is attributed mainly to the increase of the work hardening rate rather than modulus. The potential for further increasing the strength of this alloy is discussed.

A description of Deinonychus antirrhopus bite marks and estimates of bite force using tooth indentation simulations

ABSTRACT We report the discovery of a specimen of Tenontosaurus tilletti from the Cloverly Formation that bears lesions we interpret as bite marks of Deinonychus antirrhopus. Some of the bite marks are in the form of exceptionally deep punctures through the long bone cortices. These provide a rare opportunity to estimate the bite-force capacities of this taxon through tooth indentation simulations. These experiments showed that approximately 4100 N of bite force were required to generate one of the bite marks, and 8200 N would have been generated simultaneously at a distal-most tooth position. These values are higher than those reported for large carnivoran mammals but similar to values recorded for comparably sized crocodilians. Although current evidence does not indicate how D. antirrhopus actually used its claws and teeth to acquire prey resources, it is clear that large individuals were capable of generating forces great enough to bite through bone.

Properties of high strength Cu-Nb conductor for pulsed magnet applications

Various tests have been undertaken to study the effects of annealing, testing temperatures and volume fraction of the Cu cladding on the properties of Cu-Nb conductors being developed for pulsed magnet applications. The results demonstrate that short time annealing used for insulation had no significant effect on the tensile property of Cu-Nb conductors. The cryogenic temperatures are beneficial to both the conductivity and mechanical properties of the conductors, especially the tensile strength of the Cu cladding. The wire-drawing fabrications showed that wires of 4 mm/spl times/6 mm cross-section-area with a significant volume fraction of Cu cladding could be obtained, leading to final tensile strengths of up to 1100 MPa at room temperature. The strength is increased by about 20% at 77 K. The 77 K-conductivity is about 4.5 times of the room temperature one. The strengthening mechanisms and resistivity variation of the Cu-Nb composite are discussed and it is argued that the distance between the Nb ribbons plays an important role in the variation of these properties.

Development of high strength pure copper wires by cryogenic deformation for magnet applications

A high strength pure copper conductor was fabricated by a cryogenic drawing process at 77 K where the dynamic recovery of copper was reduced. With this method, drawn pure copper wire achieved a strength level of 580 MPa and a conductivity of more than 96% IACS at room temperature. This strength level is about 45% higher than that obtainable by an equivalent room temperature deformation of copper. The material had a strength level of 680 MPa at 77 K and the resistivity ratio was larger than six. The interesting new basic science concerns the understanding of both strain hardening at low temperature, the attainment of high strength due to the stable accumulation of very high densities of dislocations, the change of the work hardening rate induced by cryogenic deformation, and the texture development in cryogenic deformation. In addition, the methodology has the potential to link the development of new approaches to materials selection and production to specific design needs in a variety of magnets. The potential of cryogenic deformation for the development of high strength conductors of pure copper is discussed in this paper.

Materials for 100 T monocoil magnets

The development of improved conductors and reinforcement materials is instrumental for the achievement of ultra-high fields with monolithic magnets. They have the advantage that there is no coupling between the coils of the traditional two-coil systems, and that the magnet can be built much smaller, and it has longer pulse times. The fields that can be achieved are determined by the strain limitation of the conductor, and the strength and stiffness of the reinforcement. We have investigated the material requirements for this type of magnet to define the objectives for material development. It follows that future material development activities for reliable 100 T magnets should focus on conductors with the following specifications (all values are at 77 K): a) Ultimate conductor strength of 1.2GPa, b) Conductivity of 500 percent IACS, c) 10 percent strain to failure, d) Fatigue strength of 1 GPa at about 5000 full load cycles from tensile stress at peak pulse to compressive stress after the pulse of about 0.6 GPa. It is equally important to improve the reinforcement materials. At the NHMFL, we use with great success a combination of Zylon fiber and MP35N superalloy. We have increased the isotropic strength of MP35N at 77 K to 2.6 GPa (modulus 240 GPa), and are working on further improvements.

Characterization of High

The series-connected hybrid magnet under construction at the National High Magnetic Field Laboratory uses cable-in-conduit-conductor with high Jc Nb3Sn strands. One of the candidate Nb3Sn strands is made by the restacked-rod process with a nominal Jc of ~ 2400A/mm2. We characterized the Jc as a function of axial and transverse strain, magnetic field and temperature. The Jc-strain measurements have been carried out by a straight pull device and two different Walters spring probes. In addition, we have investigated the effect of the Nb3Sn reaction heat treatment temperature on the irreversible strain. We found that the irreversible strain increases with the heat treatment temperature. Our results indicate that, in addition to its high J c, this type of strand has moderate strain sensitivity, therefore is suitable for the application of the series-connected hybrid magnet.