Jack Toth

The NHMFL Hybrid Magnet Projects

The National High Magnetic Field Laboratory is developing resistive-superconducting hybrid magnets both for internal use and for installation at other facilities. The Tallahassee magnet will have a vertical bore and provide 36 T in a 40-mm bore with 1-ppm homogeneity over a 10-mm diameter spherical volume. The Berlin version will provide a horizontal field of 25 T in a converging-diverging bore configuration suitable for neutron-scattering experiments. A design study is underway for a third magnet for Oak Ridge that will be similar to the Berlin version but provide >30 T. The three magnets will use very similar ~ 13 T Nb3Sn CICC coils for the superconducting outserts. The resistive insert magnets will be different configurations operating at different power levels. In designing the magnet systems we have developed a new numerical model to predict the critical current of Nb3Sn CICCs, tested several conductors in-house and abroad, designed cryostats and refrigeration systems, and developed new resistive magnet technology. An overview of the innovations and present status is presented.

Design of the next generation of Florida-Bitter magnets at the NHMFL

The National High Magnetic Field Laboratory in Tallahassee, Florida designs, builds and operates the worlds highest field dc resistive magnets, providing fields up to 33 T in purely resistive systems and up to 45 T in resistive-superconducting hybrids. The next generation of magnets is presently being designed and used technology developed for our hybrid to upgrade the field in our various resistive magnets. Coil designs are presented for the following 20 MW dc systems: 1) a new 50 mm bore magnet expected to provide 32 T, 2) a new 32 mm bore magnet expected to provide 35 T, and 3) a new high-homogeneity magnet expected to provide 30 T with inhomogeneity of 50 ppm or less over a 10 mm diameter spherical volume.

Large, High-Field Magnet Projects at the NHMFL

The National High Magnetic Field Laboratory has developed and operated large, high-field dc and pulsed magnets for research in condensed matter physics. We are now developing three resistive/superconducting hybrid magnets with fields ranging from 25 to 45 T, and are developing concepts for hybrid magnets up to 60 T as well human-head MRI magnets up to 20 T and repetitively pulsed magnets to reach 60 T every 30 sec.

The Powered Scattering-Magnet Program at the NHMFL

The National High Magnetic Field Laboratory is developing several powered magnets employing novel configurations for use in photon and neutron scattering experiments. First is a split resistive magnet being built for Far-Infrared Scattering at the NHMFL in Tallahassee. This magnet has spurred the development of the novel Split Florida-Helix (SFH) technology. High-field test coils of the SFH concept have been designed and built. Test results are presented. Second, Series-Connected Hybrid magnets with horizontal, conical bores are being designed for neutron scattering experiments at the Hahn-Meitner Institute in Berlin and the Spallation Neutron Source in Oak Ridge, TN. A new resistive magnet technology, the Conical Florida-Bitter (CFB), is being developed suitable for use as the resistive insert of these magnets. A high-field CFB test coil has been designed and is under construction. The conceptual design of the eventual hybrid system is presented along with the detailed design of the high-field test coil.

Cryostat Design for the HZB and NHMFL Series-Connected Hybrids

The National High Magnetic Field Laboratory (NHMFL) is designing two series-connected hybrid magnets, one for the Helmholtz Center Berlin (HZB) and the other for the NHMFL. The one for HZB has a horizontal, conical warm bore with a 30 degree opening angle for neutron scattering experiments. The one for the NHMFL has a 40 mm diameter vertical warm bore with a cylindrical profile. The design of the HZB cryostat will be completed this year. In this paper the design of the HZB cryostat is presented. The results of a structural analysis performed for normal operation and for fault scenario are discussed. The main features of the NHMFL cryostat are described shortly in the introduction section.

Conceptual Design of Powered Scattering Magnets at the NHMFL

The NHMFL is developing conceptual designs of powered magnets with novel configurations. Conical hybrid magnets are being considered using a novel Conical Florida-Bitter technique. A resistive split magnet is being developed and will be installed at the powered magnet facility in Tallahassee. This magnet will be used for scattering experiments with far-infrared light as well as rotation experiments

NMR spectroscopy up to 35.2 T using a series-connected hybrid magnet

The National High Magnetic Field Laboratory has brought to field a Series-Connected Hybrid magnet for NMR spectroscopy. As a DC powered magnet it can be operated at fields up to 36.1T. The series connection between a superconducting outsert and a resistive insert dramatically minimizes the high frequency fluctuations of the magnetic field typically observed in purely resistive magnets. Current-density-grading among various resistive coils was used for improved field homogeneity. The 48mm magnet bore and 42mm outer diameter of the probes leaves limited space for conventional shims and consequently a combination of resistive and ferromagnetic shims are used. Field maps corrected for field instabilities were obtained and shimming achieved better than 1ppm homogeneity over a cylindrical volume of 1cm diameter and height. The magnetic field is regulated within 0.2ppm using an external 7Li lock sample doped with paramagnetic MnCl2. The improved field homogeneity and field regulation using a modified AVANCE NEO console enables NMR spectroscopy at 1H frequencies of 1.0, 1.2 and 1.5GHz. NMR at 1.5GHz reflects a 50% increase in field strength above the highest superconducting magnets currently available. Three NMR probes have been constructed each equipped with an external lock rf coil for field regulation. Initial NMR results obtained from the SCH magnet using these probes illustrate the very exciting potential of ultra-high magnetic fields.

Final Assembly of the Helmholtz-Zentrum Berlin Series-Connected Hybrid Magnet System

The final assembly of the Series-Connected Hybrid magnet system for the Helmholtz-Zentrum Berlin for Materials and Energy (HZB) has occurred with the integration of the superconducting cold mass, cryostat, resistive Florida-Bitter coils, and the cryogenic, chilled water, power, and control subsystems. The hybrid magnet consists of a 13-T superconducting Nb3Sn/CICC coil and a set of 12-T resistive, water cooled coils at 4.4 MW. Much of the cryostat and cold mass functional requirements were dictated by the electromagnetic interactions between the superconducting and resistive coils. This includes the radial decentering and axial aligning forces from normal operations and a 1.1 MN fault load. The system assembly was an international achievement with the cold mass being completed at the NHMFL in the USA, cryostat to cold mass interfaces made at Criotec Impianti in Italy, and final assembly at the HZB in Germany.

Final design of a compact sweeper magnet for nuclear physics

A superconducting dipole, designed for use as a sweeper magnet for nuclear physics experiments, is being constructed by the NHMFL for operation at the NSCL. The magnet operates at a peak mid-plane field of 3.8 T in a 140 mm gap. A multi-particle beam enters the magnet from the upstream side. The neutrons continue straight through to a neutron detector. The charged particles are swept 43 degrees on a one meter radius into a mass spectrometer. Extensive model-based computer analysis (MBA) have been applied for optimizing the coils and the stainless steel bobbin with regard to its shape while keeping the strain and the fraction of actual to critical current within reasonable limits. The structural FEM analysis had to address a variety of complex physical phenomena (composites with orthotropic material properties, cool down, surface-to-surface contacts, etc.) and the loads due to operation (Lorentz forces) had to be obtained from a parallel/coupled magnetic field analysis. One of the challenges magnet designers face in optimizing magnet systems is that the detail required to obtain reasonable accuracy is not well known. This paper presents results of the structural design optimization for the sweeper project and evaluates the adequacy of alternative modeling approaches. Also the final magnetic design of the system is summarized and progress toward fabrication is presented.

Generation of the highest continuous magnetic fields

The generation of the highest magnetic fields is a true challenge to the magnet designer and a stimulus for the materials scientist. Record fields are achieved through a successful interplay between the precise understanding of the physics of magnets, the detailed knowledge and conscious use of the available materials, and a design that optimizes all the different parameters. We describe three technologies essential for the generation of the highest magnetic fields: coils wound from high-temperature superconductors (HTS) operated at liquid helium temperatures in a background field; high-power resistive magnets; and hybrid magnets, which combine the advantages of resistive and superconducting magnets. HTS show high current densities at elevated fields confirmed so far by measurements up to 33 T. In spite of the strain limitation of the conductor, insert coils up to 3.2 T have been built at different laboratories for a total field of 23.5 T. Recently a record field of 25 T using a 5 T Bi/sub 2/Sr/sub 2/CaCu/sub 2/O/sub x/ superconducting insert coil has been achieved at the NHMFL. For resistive magnets, dramatic improvements in magnet design, such as the Florida-Bitter magnet, and new materials, such as the microcomposite CuAg, have made it possible that magnetic fields can now be generated that exceed the fields that hybrid magnets produced 10 years ago. Today, 33 T are provided on a routine basis with resistive magnets. The highest continuous magnetic field ever of 45 T is obtained with a new generation of hybrid magnets. It is available for scientific research in our user facility.