Scott Bole

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.

Examinations of Cu-Ag Composite Conductors in Sheet Forms

Cold rolled Cu-24 wt% Ag composite was characterized and the effects of crystallographic structure and defect anisotropy and microstructure refinement on properties of the composite were studied. Characterization was carried out with high resolution scanning electron microscopy (SEM), x-ray diffractometer and mechanical testing. All the deformed samples exhibited {110}-texture that had impact on the anisotropic properties of the materials. The yield and tensile strengths were higher in the long transverse (LT) than in the rolling direction (RD). The resistivity was higher in the RD than in the LT. Increasing the rolling strain increased both the mechanical strength and electric resistivity. The property changes with strain were related to the Cu and Ag lamellae thickness. The thickness of the lamella was inversely proportional to the deformation strain. It was observed that the smaller the thickness of the lamellae, the higher the strength and the electric resistivity. A closer examination of the Cu and Ag components revealed that while the lamellae were well aligned in the LT direction, they were curved in the RD. The curved lamellae observed in the RD were attributed to the development of shear bands during rolling. Both the texture and shear bands were related to the anisotropy of the properties.

Test results and potential for upgrade of the 45 T hybrid insert

Construction of the NHMFL hybrid magnet is complete. The resistive insert was tested to full current without the background field from the superconducting magnet on May 17, 1999. Tests of the combined system have been scheduled for October 1999. The resistive insert fits into the 616 mm bore of the 14 T outsert superconducting magnet. The insert consists of a five coil, axially cooled Florida-Bitter design. The two innermost coils are electrically in parallel and this pair is in series with the other four coils. The magnet design uses Florida-Bitter disks made of Cu-Ag, Cu-Be, Cu-Zr and Cu sheet and is heavily based upon the high field resistive magnets previously built at the NHMFL. Details of the coil design, construction and testing are presented.

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.

End effects in the NHMFL 45 T hybrid resistive insert

Design evolutions of the resistive insert of the Hybrid magnet at the National High Magnetic Field Laboratory (NHMFL) have been ongoing based not only on the mid-plane stress state but the effects of the individual coil end-turns. The end-turns inherently have an unconstrained free edge that places additional bending on the Florida-Bitter discs. Finite element analysis of the Hybrid resistive insert has been performed and the results focusing on the end-turns are presented along with a description of the analytical procedure. Numerical results are supported by operational experience. Past, present, and future resistive insert designs are analyzed showing successful operation of the insert to 110% of the original design current.

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

Conceptual design of high transverse field magnets at the NHMFL

The National High Magnetic Field Laboratory in Tallahassee, FL, USA, is designing a high field magnet for condensed matter physics with the field perpendicular to the access tube. The traditional approach to such magnets is to build a split pair of solenoids. Various novel alternate approaches have been examined and compared with split pairs. A particularly attractive option consists of concentric nested Bitter coils tilted 45 degrees to the axis. By energizing the coils in opposition, the axial components of field from the various coils can be made to cancel resulting in a purely transverse field. Preliminary designs including field, power and stress estimates are presented.

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.

Resistive Solenoid Development at the NHMFL

The National High Magnetic Field Laboratory (NHMFL) has several resistive solenoid magnet projects underway presently, including upgrade of four existing magnets, design of the conical bore insert for the Helmholtz Center Berlin (HZB), design of insert coils of the Series-Connected-Hybrid (SCH) for NHMFL and design of resistive split magnet. The upgrades and the design of the conical bore insert are discussed in detail while other programs are presented briefly.