Mark D. Bird

Florida electro-mechanical cable model of Nb3Sn CICCs for high-field magnet design

Performance degradation of Nb3Sn cable-in-conduit-conductors (CICCs) is a critical issue in large-scale magnet design such as in the International Thermonuclear Experimental Reactor (ITER) and the series-connected hybrid (SCH) magnets currently under development at the National High Magnetic Field Laboratory (NHMFL). The critical current Ic of Nb3Sn conductors is strongly affected by thermal pre-strain in strand filaments in a CICC from differential thermal contraction between strands and conduit during cooling down after heat treatment. Mitchell and Nijhuis recently introduced strand bending under locally accumulated Lorentz force for the interpretation of observed transverse load degradation, defined as the Ic reduction due to strand bending and contact stress at strand crossing with respect to the expected Ic from strand data at the thermal compressive strain. In this paper, a new numerical model of CICC performance has been developed based upon earlier work by Mitchell and Nijhuis. The new model, called the Florida electro-mechanical cable model (FEMCAM), combines the thermal bending effects during cooling down and the electromagnetic bending effects during magnet operation, as well as effects due to strand filament fracture. We present the FEMCAM formulation and benchmark the results against about 40 conductor tests of first-cycle performance and 20 tests that include cyclic loading. We also consider the effects of different jacketing materials on CICC performance. We conclude that FEMCAM can be a helpful tool for the design of Nb3Sn-based CICCs and that both thermal bending and transverse bending play important roles in the performance of Nb3Sn CICCs.

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.

High-field NMR using resistive and hybrid magnets.

Resistive and resistive-superconducting hybrid magnets can generate dc magnetic fields much higher than conventional superconducting NMR magnets but the field spatial homogeneity and temporal stability are usually not sufficient for high-resolution NMR experiments. Hardware and technique development addressing these issues are presented for high-resolution NMR at magnetic fields up to 40T. Passive ferromagnetic shimming and magic-angle spinning are used effectively to reduce the broadening from inhomogeneous magnetic field. A phase correction technique based on simultaneous heteronuclear detection is developed to compensate magnetic field fluctuations to achieve high spectral resolution.

A new series-connected hybrid magnet system for the National High Magnetic Field Laboratory

The National High Magnetic Field Laboratory (NHMFL) proposes to build a new hybrid magnet system with three specific goals: 1) address the needs of users requiring high homogeneity, high temporal stability, and long residence times at moderately high fields; 2) enhance user service through use of a single 10 MW power supply (allowing simultaneous operation of multiple systems); and 3) make the system attractive in terms of combined capital and operating costs. The basic configuration are series-connected hybrid, wherein resistive insert and superconducting outsert are connected in series and powered by a single 10-MW unit (20 kA and 500 V) of the NHMFL dc power system, a configuration with advantages for handling faults and off-normal operating conditions and with improved temporal stability. Specific design goals are 35 T, 10-ppm uniformity over a 10-mm DSV, and access for 40-mm diameter probes. By powering the superconducting outsert with 20-kA high-temperature-superconductor (HTS) current leads, cryogenic loads are kept small, substantially smaller even than the present 45-T Hybrid. The net result is a magnet design: 1) with capability for both higher field and higher field quality than present resistive-only systems, 2) that permits simultaneous service of two or more users, 3) that is sufficiently compact to fit within the standard resistive-magnet cell, 4) whose ease of operation is comparable to resistive-only systems, and 5) whose lifetime cost (including construction and operation) can be significantly lower than comparable resistive-only systems. We discuss details of the existing conceptual design, the methodology for its creation, the perceived development needs, and the projected program to realize this system.

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}

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

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

The worlds highest-field dc magnets have, for roughly the past thirty years, consisted of resistive-superconducting hybrid magnets. These magnets use superconducting technology for the outer coils, where the magnetic field is moderate, and resistive-magnet technology for the inner coils, where the field is highest. In such a configuration, higher fields are attained than is possible with purely superconducting magnet technology, and lower lifetime (capital and operating) costs are attained than with a purely resistive magnet. The resistive coils of these magnets represent the pinnacle of high-field resistive-magnet technology and have been the focus of much of the resistive magnet technology development over the past thirty years. The evolution of high-field resistive magnet technology is presented, focusing on the development of hybrid inserts.

Mechanical Design of the Series Connected Hybrid Magnet Superconducting Outsert

The National High Magnetic Field Laboratory (NHMFL) in Tallahassee, Florida, USA, continues research and development of transverse field magnets (with the field perpendicular to the access tube). Presently, the emphasis is on a novel approach with concentric nested coils tilted at an angle to the central axis; current flows in opposite directions within the coils at opposite tilt angle, generating a transverse dipole field. Superconducting tilted coils using wire-wound technology and resistive tilted coils using advanced technology are being examined. Some very preliminary, conceptual designs and magnetic field calculations are presented. Related problems, including behavior under the Lorentz forces are discussed briefly.

Resistive magnet technology for hybrid inserts

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.

New concepts in transverse field magnet design

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.