Iain R. Dixon

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.

Current Sharing and AC Loss Measurements of a Cable-in-Conduit Conductor With

Performance verification of the Nb3Sn cable-in-conduit conductor (CICC) for the series-connected hybrid magnets at the National High Magnetic Field Laboratory (NHMFL) and Helmholtz Centre Berlin is performed through short sample testing. The superconducting outsert coil consists of three CICC configurations, graded for the applied magnetic field. The CICC for the high field section of the coil is tested in the SULTAN facility at EPFL-CRPP. Measurements of the current sharing temperature at field current combinations comparable to what is expected in the magnet are made. Electromagnetic cycling is performed to investigate the Nb3Sn strand sensitivity to transverse loads. In addition, measurements of AC loss and pressure drop along the conductor are made and compared to thermal-hydraulic computations.

{\rm Nb}_{3}{\rm Sn}

A wide bore, high resolution NMR magnet with a proton resonance frequency of 900 MHz, field of 21.1 T, and stored energy of 40 MJ has been designed and fabricated at the NHMFL. The magnet has five Nb/sub 3/Sn and five NbTi coils, each of which contains stainless steel strip heaters. An active protection system with analog quench detection circuitry is employed to determine the onset of a quench and to subsequently open a superconducting protection switch and energize a portion of the strip heater network. A passive protection system with diode sets and shunting resistors exist for redundancy. Details of the protection system are presented and its performance during the magnets bucket test is discussed.

Strands for the High Field Section of the Series-Connected Hybrid Outsert Coil

An innovative hybrid magnet configuration is being developed at the NHMFL, consisting of a Florida-Bitter resistive magnet nested within a cable-in-conduit conductor (CICC) superconducting magnet to provide high fields for less power than traditional hybrid magnets. The resistive and superconducting magnets, connected in series, will be capable of producing 23.1 T and 13.8 T respectively for a total central field of 36.9 T. The CICC uses a cable of multifilamentary Nb3Sn/Cu strands inside a superalloy jacket that confines flowing supercritical helium in direct contact with the cable strands. The design of the magnet system is presented along with the design criteria used to evaluate the superconducting magnet and its integral components. The results of a structural analysis performed using finite elements for normal operational and fault loads are discussed for the most critical component, the conduit

Quench detection and protection of the wide bore 900 MHz NMR magnet at the National High Magnetic Field Laboratory

The National High Magnetic Field Laboratory, in collaboration with Intermagnetics General Corporation, is in the process of fabricating a wide bore 900 MHz NMR magnet. The magnet is in many ways similar in concept to the typical high field NMR spectrometer magnet, employing Nb/sub 3/Sn and NbTi conductors in a set of epoxy impregnated long solenoids plus compensation coils for uniformity. The magnet will operate at a temperature of 1.8 K to produce a field of 21.1 T. The high field and large bore result in large mechanical stress in the coils and large magnetic stored energy. Innovations in the technology are introduced into the design to satisfy the requirements, including the manner of reinforcement of the windings and the active protection system. The design parameters of the magnet are presented. The technological solutions to the requirements of large high field magnets are discussed.

Mechanical Design of the Series Connected Hybrid Magnet Superconducting Outsert

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.

900 MHz wide bore NMR spectrometer magnet at NHMFL

A stress analysis assuming generalized plane strain conditions is presented for a solenoid magnet with orthotropic material properties. Analytic expressions are provided for the longitudinal, radial and axial stress and strain valid in a neighborhood of the mid-plane of a long solenoid. Calculations are made for an example magnet with multiple coil sections and external reinforcement. The results of generalized plane strain and finite element methods are compared with plane stress and plane strain analyses. >

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

A 900 MHz NMR magnet with a room temperature, clear bore of 105 mm was recently brought to full field at the NHMFL. The magnet was ramped to 21.1 T on its first run without incurring a quench, since installation into its final cryostat was completed. Issues on superconductor stability were present during magnet charging and are discussed. In addition, the magnet performance in terms of field uniformity and of field stability, with the implementation of a novel design concept of current injection, is presented.

Generalized plane strain analysis of solenoid magnets

The mechanical properties of epoxy impregnated coils are critical to proper stress analysis of high field superconducting magnets. In order to identify the engineering properties of coil composites, mechanical test specimens of epoxy impregnated coil windings are prepared. The samples consist of multiple layers of superconductor separated with interlayer insulation and are epoxy impregnated. Mechanical tests are performed at liquid nitrogen and liquid helium temperature. Orthotropic elastic properties are measured for the composite and used for mechanical analyses. Properties in a direction longitudinal to the conductor (hoop direction) are measured in tension. The compression modulus is measured for properties transverse to the conductor, representing radial and axial directions.

Performance of the ultra wide bore 900 MHz NMR magnet at the national high magnetic field laboratory

The program at the National High Magnetic Field Laboratory for the design and development of 1 GHz class NMR magnets is described. The parameters are given for a 1.066 GHz magnet incorporating an HTS inner coil. The design of the related wide bore 900 MHz conventional superconductor magnet is described. Aspects of the technology development program supporting these designs are presented.