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The so called nautilus is a winding form in the shape of a double helical spiral onto which a stiff conductor is wound. The conductor is thus bent to a radius smaller than the desired local curvature. When the conductor is allowed to relax from the nautilus form, the required radius of curvature is achieved everywhere. The stiffness in the axial direction is small and compression of the wound coil to a disc is easy. The coil is tightened to a compact disc by pulling the outer tails.

The new National High Magnetic Field Laboratory (NHMFL) at Tallahassee, Florida is committed to putting into operation in 1995 a 45-T Hybrid Magnet System to support research in steady, high magnetic fields. This facility will be accessible by qualified users world-wide on the basis of proposal and review. The more prominent components of this system will be a 24-MW resistive insert and a 120-MJ superconducting outsert. But successful achievement of the performance goals for the 45-T Hybrid System will depend on a number of unique, state-of-the-art subsystems and components. This paper describes the requirements and specifications on the individual subsystems and components in the context of the overall performance gears and reviews the plan for putting the whole together. >

A 35 T Hybrid magnet is to be installed at the National High Magnetic Field Laboratory at Tallahassee. The superconducting background field magnet of this system consists of three coils, two of niobium-tin conductor and one of niobium-titanium. This paper describes the design and construction of the niobium-titanium section. In the design, three issues of principal importance were considered: stability and dissipation under pulsed and steady state conditions; the stressing of the conduit, and the consequences of quenching. The details of construction have centered around the winding and assembly of the pancake coils and the preparation of the joints. The construction of the coil is part of a collaborative program between MIT and FSU. >

The development of a high resolution high-field 1 GHz NMR spectrometer has been undertaken by Battelle Pacific Northwest Laboratories (BPNL). In preparation for the design of the superconducting magnet for that instrument, BPNL sponsored a program to measure the properties at high field (up to 23.5 T) and low temperature (1.8 K) of a number of commercially available superconductors. Lengths of these conductors were obtained and tested as short samples and in small coils. The critical current densities, index values and strain dependence of these properties were characterized. The potential applications of these conductors in construction of a 1 GHz NMR magnet are discussed. >

The authors describe the overall Hybrid III system, present test results, and indicate future plans. Hybrid III, completed in late 1991, has since undergone a sequence of tests in preparation for becoming a facility magnet. When first tested in December 1991, it generated a total central field of 33.5 T. The superconducting magnet (SCM), operating in a bath of superfluid helium at 1.65 K and with a current of 2200 A, contributed 12.7 T to the total. In subsequent runs it was shown that the SCM would reach a critical current of approximately 2230 A when operated at approximately 1.7 K. After the completion of improvements to the cryogenic components as described, two major goals were set: to reach 35 T in two steps and to operate Hybrid III as a facility magnet. Hybrid III is targeted to reach 35 T in the spring of 1993. >

A part of the mission of the new NHMFL is to have available for users in 1995 a hybrid magnet system capable of producing at least 45-T steady field on axis in a 33-mm working bore. Approximately 31 T of the combined field will be produced by a water-cooled insert. The superconducting outsert, which combines NbTi and Nb/sub 3/Sn conductor technologies, will provide more than 14 T. The authors describe an option for this superconducting outsert based on the cable-in-conduit-conductor (CICC) approach, where cabled strands of conductor are contained in intimate contact with helium coolant inside a strong steel sheath that also acts as distributed structure. A departure from the usual practice for CICC technology is in the application of static Hell cooling, which simultaneously provides higher conductor performance and nearly passive extraction of the rather modest heat loads during normal operation of the magnet system. >

The authors chronicle the evolution of hybrid magnets built at the Francis Bitter National Magnet Laboratory. It is noted that resistive water-cooled magnets can generate field according to how much power is available. The hybrid concept has been developed for generating fields beyond a power limit, up to 45 T. Along the way five successively more adventurous designs have been tried. >

A superconducting magnet, consisting of concentric Nb 3 Sn and NbTi coils, will operate in a bath of subcooled helium at 1.8 K in a quasi-adiabatically stable mode. Water-cooled 10 MW insert magnets which fit within the 36 cm cryostat bore are based upon a design which has proven very successful. The system will provide researches with ultrahigh continuous magnetic fields.

We discuss methods and results of calculations for hybrid magnet systems for extremely intense continuous magnetic fields. We focus on a system consisting of a 20 MW radially-cooled insert magnet of 33 mm bore and 406 mm outer diameter, operating within the bore of a 15 T superconducting magnet. We predict such a system to be capable of 45.5 T, given typical cooling-system parameters and the structural efficiency which we expect of our “monohelix” form of construction.

Hybrid III generated fields of 33.6 T in 1991 and 34.2 T in 1992, both in a room-temperature bore of 33 mm. Starting 1993 it has been operated as an FBNML facility magnet, generating either 33 T in a 33-mm bore or 22 T in a 144-mm bore. This paper describes our experience in operating Hybrid III over the past 8 months since December 1992. By the end of 1993, Hybrid III is expected to reach 23 T in the 144-mm bore and, with inserts employing superior alloys, 35 T in the 33-mm bore. >

A 250 dm 3 subcooled cryostat system for superfluid helium was designed and built for a hybrid magnet. The cryostat, which houses a 430 mm bore, 12 T NbTi superconducting magnet, has a refrigeration capacity of ≈20 W at 1.8 K.

The four experiments reported here address the problem of stabilization in large superconducting magnets. The first two experiments concern frictional heating caused by conductor motion; the last two experiments measure minimum quench propagation currents in simulated cooling conditions.

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. >

A superconducting magnet being built for the Francis Bitter National Magnet Laboratory Hybrid III system will contain both niobium-titanium and Nb/sub 3/Sn conductors in two separate coils later joined in a common assembly. Prereacted Nb/sub 3/Sn will be in layer-wound coil. A description is given of the coil winding equipment and process used for the proposed 42-cm-bore coil to be wound with 8 layers of high-field conductor and 10 layers of low-field conductor. Each layer will contain about 60 turns. The outside diameter will be 62 cm, and the length 57 cm. When operated at 1.8 K with 2300 A, it will add 4 T to the 9.3 T from a NbTi magnet which surrounds it. >

A case study of an electrical short that occurred in a large superconducting coil is presented.

The laboratory provides a wide variety of magnets and equipment to about 250 experimenters involved in ≈ 100 programs each year; these account for over 100 publications covering a broad scientific spectrum. The hybrid magnet has surpassed 1500 hours of operation and recently attained a field of 31.8 T.

A description is given of the design and construction of a magnet generating over 17 T with multiple radial access ports to allow water cooling. There are seven ports, 19-mm high, at 30 degrees intervals around half the circumference of the magnet midplane. The magnet consists of a split pair of coils of 330 mm (outer diameter) and 100-mm length spaced 23 mm apart. Together they consume approximately 8.5 MW. They are Bitter coils with cooling provided by water flowing outward through shallow passages etched in the plates. Heat fluxes are as high as 700 W/cm/sup 2/. In an unusual overall configuration, hydraulics, coil clamping, support, and electrical connections were all made as unobtrusive as possible. >

Critical current data in the temperature and field ranges of 1.8 to 4.2 K and 8.5 to 12.5 T are presented for two size-groups of commercial NbTi conductors. The first group of conductors had critical currents at 4.2 K and 9 T of 6 to 77 A, while the second group's ranged from 1400 to 1570 A.

Original work on heat transfer by circulations of supercritical helium for cooling superconducting cables was done in this laboratory in 1965 by Kolm et al [1]. Two recent papers[2,3] have described the computer program for which experimental verification is being sought. The pros and cons of a hollow-conductor system have also been discussed in these pubications.

The term “hybrid” magnet has been coined to describe coils in which part of the field is generated by superconductors and part by non-superconductors. Such systems have the advantage in that they can produce higher fields than can be produced by either of their parts alone. In addition, they can represent a rather economical approach to very high fields for laboratories that already possess either power supplies or refrigerators of reasonable capacity.

A method of making joints between superconducting multifilamentary wires has been developed in connexion with the construction of a 7 T nmr magnet. Test results indicate that they have acceptably low resistances and carry sufficient currents.