Francis Bitter National Magnet Laboratory Research Articles

Hybrid III system

A magnet system known as Hybrid III under construction at the Francis Bitter National Magnet Laboratory, is described. The superconducting magnet will consist of a Nb/sub 3/Sn magnet surrounded by a NbTi magnet. In a departure from earlier design philosophy, the NbTi magnet is not designed to be cryostable; it is provided with only enough cooling to cope with nominal dissipations generated by joints and field sweeps. The magnet is therefore referred to as a quasi-adiabatic magnet. The aim of the quasi-adiabatic concept is to make a magnet sufficiently adiabatic that there will not be hot spots in the event of a quench, and yet cooled well enough to ride out steady dissipations. The cryostat can operate at 1.8 K and at 4.2 K. The superconducting section is designed to generate 13 T at 1.8 K. The system is to reach 35 T, or perhaps more as water-cooled inserts are developed.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Monohelix research at the MIT Magnet Lab

A discussion is presented of the radially-cooled monohelix to support the high stress and power densities within the highest-field magnets at Francis Bitter National Magnet Laboratory. The design employs a continuous machined helical conductor (to eliminate the radial slits which weaken the Bitter design) whose helical winding is built up from annular plates. The first monohelix is the inner of two coils which constitute the water-cooled insert of a hybrid system of 53-mm bore. The system generates 27.1 T. The coil experiences average hoop stresses up to 370 MPa, and heat fluxes up to 7.2 W/mm/sup 2/ in passages only 0.25-mm deep.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

A 60 cm bore 2.0 tesla high homogeneity magnet for magnetic resonance imaging

A 60 cm warm bore imaging and spectroscopy magnet has been constructed and placed in operation at the Francis Bitter National Magnet Laboratory (FBNML). The magnet achieved its design central field of 2.0 T but is currently being operated at 1.5 T. It operates in the persistent mode with a measured decay rate of less than 0.03 ppm/hr. Employment of both 10 superconducting shims and small ferromagnetic shims located close to the warm bore has resulted in a homogencity of better than 3 ppm throughout the 25 cm diameter spherical volume (DSV). Room temperature shim coils have not been incorporated into the system. A novel form of compact shielded pulsed gradient coil system has been designed, constructed and tested. In such a system, appropriate configuration of an external shield coil results in cancellation of external flux without the introduction of impurity harmonics that degrade the linearity of the gradients. Six sets (X, Y, Z coils, and X, Y, Z shields) have been incorporated into a unit of 6 cm build. The all aluminum cryostat employs a 77 K nitrogen recondenser and a shield cooler operating at less than 20 K. Steady state helium consumption is about 50 ml/hour. The system is currently being used for both high resolution, in-vivo <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">31</sup> P-NMR spectroscopy and a variety of MRI experiments including <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">23</sup> Na imaging of eyes.

68.4-T-long pulse magnet: Test of high strength microcomposite Cu/Nb conductor

A multilayer wire-wound pulsed field magnet was fabricated using a new metal-matrix microcomposite Cu/Nb conductor. The 0.5-in.-i.d. magnet used square cross-section Cu/Nb wire, precompressed in a hardened steel container and cooled to 77 K, to generate 68.4±1 T for a 5.6-ms half-period. The maximum field was generated using a 100-kJ, 4-kV capacitor bank of the Francis Bitter National Magnet Laboratory pulsed field facility. The Cu/Nb composite had an ultimate tensile strength of 180 ksi at 77 K and 140 ksi at 293 K. These new microcomposites have exceptional strength, electrical, and thermomechanical properties for very high stress applications.

Enantioselective deprotonation by chiral lithium amide bases: asymmetric synthesis of trimethylsilyl enol ethers from 4-alkylcyclohexanones

Acknowledgment. This work was supported in part by the National Science Foundation through Grant CHE 83 08 129 and by the John Simon Guggenheim Memorial Foundation. High-field magnetization measurements were made a t the Francis Bitter National Magnet Laboratory, which is supported at M.1.T by the National Science Foundation. We thank J. C. Bonner and L. J. de Jongh for helpful discussions, and L. G. Rubin and B. Brandt for their invaluable help.

30 Tesla hybrid magnet facility at the Francis bitter national magnet laboratory, Massachusetts Institute of Technology

The design, construction and testing of a hybrid magnet system which provides fields up to 30 T is described. The system consists of a superconducting magnet which generates 7.5 T in a 40 cm bore and a family of watercooled insert magnets operating at 10 MW. The superconducting magnet has an overall current density of nearly 6500 A/cm2and a surface heat flux of 0.5 W/cm2. In addition, the paper describes the helium management system whereby both helium and refrigeration are recovered from the magnet cryostat.

The United States superconducting MHD magnet technology development program

The United States Superconducting MHD Magnet Technology Development program is a three-faceted program supported by the U.S. Department of Energy and managed by the Francis Bitter National Magnet Laboratory, Massachusetts Institute of Technology. These facets include basic technology development, technology transfer and construction by industry of magnets for the national MHD program. The technology development program includes the maintenance of a large component test facility; investigation of superconductor stability and structural behavior; measurements of materials' properties at low temperatures; structural design optimization; analytical code development; cryogenic systems and power supply design. The technology transfer program is designed to bring results of technology development and design and construction efforts to the entire superconducting magnet community. The magnet procurement program is responsible for developing conceptual designs of magnets needed for the national MHD program, for issuing requests for quotation, selecting vendors and supervising design, construction, installation and test of these systems. To date two magnets have been delivered, three are under construction and several designs for the MHD Engineering Test Facility are being developed and evaluated.

Worldwide cryogenics-USA The Francis Bitter National Magnet Laboratory, Massachusetts Institute of Technology: Cryogenic research facilities

Activities in cryogenics research and development at the Francis Bitter National Magnet laboratory are described. The water cooled resistive magnets, NMR, low and high field facilities are detailed.

Magnet system of the 500 MHz NMR spectrometer at the Francis Bitter National Magnet Laboratory: I. Design and development of the magnet

A high field nuclear magnetic resonance spectrometer has been put into operation at the MIT, Francis Bitter National Magnet Laboratory. The spectrometer is built around a high homogeneity magnet, the design field of which was 11.74 T, and which in fact operates in persistent mode at a field of 11.71 T. The magnet is wound using niobium–titanium superconductor cooled to 2.3 K and is housed in a special dewar designed to permit continuous subatmospheric cryogenic operation and to minimize helium consumption. This and a companion paper describe the analysis, design, and construction of the magnet.

Magnet system of the 500 MHz NMR spectrometer at the Francis Bitter National Magnet Laboratory: II. Disturbances, quenches, and training

Mechanical disturbances in the NMR magnet winding, some of which resulted in premature quenching, have been monitored. The history of disturbances and quenches is presented. Their nature as well as the training of the winding are interpreted in terms of a comprehensive static and dynamic analysis of the mechanical behavior of the magnet. The analysis accurately predicts the measured deflections of the magnet, and more significantly, critical events in the sequence of training quenches. In particular a repeated quench was shown to be caused by slippage of the winding on the core tube. It was successfully overcome by a change in the field pattern of the magnet. Training quenches are interpreted as the result of axial tensile stress developed locally within the winding.

The MHD component test facility at the Francis Bitter National Magnet Laboratory

The MHD component test facility at the Francis Bitter National Magnet Laboratory

Critical currents at high fields in NB&amp;lt;inf&amp;gt;3&amp;lt;/inf&amp;gt;AL multifilamentary wires

Short sample critical currents have been measured on wires of Nb 3 Al in fields up to 21 tesla. The wires consisted of 7 or 19 filaments of niobium-aluminium foil jelly rolls in a copper matrix. They were reacted at temperatures between 750°C and 950°C to form Nb 3 Al. Critical temperatures were measured inductively and showed transitions with three stages whose onsets were at ∼15K, 10-13K and ∼9.2K. Critical currents measured at low fields reproduced the high values previously reported for this material. Measurements were made at the Francis Bitter National Magnet Laboratory in high fields up to 21 tesla. The magnetic field variation of the critical Lorentz force, J c B, could be fitted to the Kramer relation, b½(1-b)2, for most samples. The bulk Bc 2 , believed to correspond to the intermediate temperature transition was in the range 10-17 tesla. In addition there was a high field tail with Bc 2 from 17-21 tesla, related to high (∼15K) transition onset temperature. These results are discussed in terms of the Nb-Al phase diagram.

High-field facilities at the Francis Bitter National Magnet Laboratory

Publisher Summary This chapter highlights the features of high-field facilities at the Francis Bitter National Magnet Laboratory (FBNML). The laboratory features over two dozen water-cooled, superconducting, and combination magnets with fields ranging up to 30 teslas in a 3.3 cm bore and a homogeneous 25 τ in a 5.4 cm bore. The facility is available free of charge; this attracts scientists worldwide. The power supply consists of two identical shafts of rotating machinery. Each includes a 4.6 MW synchronous motor fed from a 4.16 kV substation, a pair of 240 V direct-current generators, and an 80-ton flywheel to help smooth field ripple and to store energy for multisecond pulses. Together, the four generators can deliver 40 kA continuously. The control system provides a current stability of ∼ 0.1% peak-to-peak; a 6 Hz ripple is the major component of this noise. All of the laboratory's high-field magnets are of the Bitter design. The laboratory's new hybrid system, brought on-line in early 1981, offers two combinations of field and bore: 30 τ in a 3.3 cm bore and a homogeneous 25 τ in a 5.4 cm bore. One of the facilities at FBNML is a group of four spectrometers operating at proton-observed frequencies of 272, 304, 306, and 498 MHz. The last of these incorporates a superconducting magnet generating 11.7 τ by means of niobium–titanium operating at 2.3 K.

Two magnets set records for field intensity

A record for the most intense continuous magnetic field was set in February by a 254‐kG solenoid and then surpassed in July by a 301‐kG solenoid. Both magnets were hybrids built at MIT's Francis Bitter National Magnet Laboratory (NML). The designs were described at the 6th International Conference on Magnet Technology at Bratislava in August by the magnets' developers—Mathias J. Leupold, Robert J. Weggel and Yukikazu Iwasa, all of NML. A hybrid magnet consists of a large superconducting coil surrounding a compact water‐cooled coil. The concept was proposed in 1965 by D. Bruce Montgomery of NML and Martin Wood of Oxford, as a method for efficiently generating a large magnetic field without unduly reducing the volume available for experiments in the central cavity. (The inner bore of both record-setting magnets is 3.2 cm.)

High gradient magnetic separation: Effect of temperature on performance

A simple model is formulated to determine the effect of temperature on the filtering ability of HGMS devices. An optimum temperature range is observed to exist for which the capture of ash-forming minerals and inorganic sulfur in liquid coal is at a maximum. Model will serve as a framework for analyzing experimental data in future. In the past, some pertinent, independent variables such as particle size, slurry velocity and applied magnetic field which affect the performance of HGMS de-Vices, have somewhat been investigated both theoretically and experimentally. The effects of these important variables have been well established. A program is currently underway at the M.I.T. Francis Bitter National Magnet Laboratory to study the applicability of HGMS to products from coal liquefaction processes. As part of this program, the effect of temperature on the performance of filtering ash-forming mineral impurities and inorganic sulfur from liquefied coal is being explored. This introduces an additional variable- temperature -which is to be investigated among other relevant variables of the system. In particular, by varying the operating temperature the trade-off, between the lowering of the viscosity of the fluid as the temperature is increased and the attendant decrease in the magnetization of magnetic particles, is to be studied. In view of these considerations, a simple model has been formulated to predict the effect of temperature on the filtering ability of HGMS devices.

Temperature dependence of the Raman linewidth and frequency shift in Ge and Si : S. Safran and B. Lax, Francis Bitter National Magnet Laboratory and Physics Department, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, U.S.A.

Measuring the thermal conductivity kg of supported graphene is inherently complicated due to uncertainties associated with the heat dissipation into the substrate. We innovate the use of an ultra-thin 8-nm SiO2 substrate to alleviate these uncertainties and thus improve the accuracy of optothermal Raman technique to measure kg of supported graphene. As a result, we present an extensive kg database for a wide temperature range from 325 K to 575 K. Furthermore, we have found that the thermal conductivity of supported graphene before annealing is close to that of suspended graphene at 3000 W m−1 K−1, which is attributable to graphene “suspension” lightly on the substrate roughness, and then progressively decreases over repeated thermal annealing. We elaborate on this annealing-induced kg to occur mainly because of the thermally enhanced graphene-substrate conformity and interfacial scattering by probing the Raman spectroscopic characterization of charge carrier density in graphene and the thermal expansion mismatching strain between graphene and substrate. Repeated thermal annealing also expedites the depletion of intercalated impurities to reduce the graphene-substrate separation distance, which acts to further reduces kg, ultimately to its lower bound under vacuum-annealing. Therefore, manipulating the thermo-mechanical affiliation can offer an alternative route to control the in-plane thermal conductivity of supported graphene.

Superconductor operates in magnetic fields above 500 kG

A compound of lead, molybdenum and sulfur, Pb1.0Mo5.1S6, has remained superconducting in a 510‐kilogauss magnetic field at liquid‐helium temperature, 4.2 K. This is the highest upper critical field Hc2 (4.2 K) yet reported for a superconductor, and the predicted value for Hc2 (0) is 600 kG. Materials that remain superconducting at high magnetic fields could be formed into powerful, lightweight magnets if they can support high current density and are sufficiently strong to withstand the resulting forces. Simon Foner, Edward J. McNiff Jr and Edwin Alexander of the Francis Bitter National Magnet Laboratory, Massachusetts Institute of Technology, described the high‐field measurements at the recent Applied Superconductivity Conference in Oak Brook, Illinois (30 September–2 October) and in the 23 September issue of Physics Letters.

Production of quasistatic high magnetic fields by switching low voltage DC generators

A system is described which permits controlled switching of large energies supplied by the 230 V dc generators available at the Francis Bitter National Magnet Laboratory. Such large energy sources are useful for generating magnetic fields approaching 400 kG for times ∼1 sec if suitable switching and magnet systems can be developed. Results of tests to date are summarized. Currents of ∼10 kA have been switched by an SCR bank into resistive and magnet loads in order to demonstrate feasibility. Quasistatic fields of 260 kG have been generated in a 2 cm i.d. copper solenoid precooled to liquid nitrogen temperatures.

Magnetite used to monitor functioning of human lung

A new technique for monitoring lung contamination and function has been reported by David Cohen of the Francis Bitter National Magnet Laboratory (MIT). The diagnostic handle is magnetite dust, which normally occurs in such low levels in the lungs as to be measurable only in a magnetically shielded room with sophisticated apparatus. Higher amounts of magnetite dust in the lungs can be seen, though, even with a relatively simple and inexpensive flux‐gate magnetometer.

Low-Field Room Built at High-Field Magnet Lab

A magnetically shielded walk-in room whose residual field is about 100 nanogauss is now operating at MIT's Francis Bitter National Magnet Laboratory. The room's designer, David Cohen, has recently reported using a point-contact “SQUID” (superconducting quantum interference device) inside the room to record the magnetic field of the human heart without noise averaging. Cohen also plans to use the room for ac and dc measurements of the magnetic field from the human brain; roughly the brain's field is 1 nanogauss and the heart's peak field is 1 microgauss.