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Friction experiments are reported for relative motion between NbTi/Cu windings coated with formvar, i.e. for a superconductor with low transition temperature ( T c ). A heat flow meter technique is used based on ZNMF heat piping in He II ( ZNMF = zero net mass flow ). The friction factors obtained are consistent with results deduced from other techniques. From the data obtained, adiabatic, limiting displacements d ad (at quench) are calculated for high transition temperature (high T c ) composite conductors. The comparison with low T c results shows that d ad will not be a critical parameter for the present winding friction model at high T c .

Pushing accelerator magnets beyond 10 T holds a promise of future upgrades to machines like the Tevatron at Fermilab and the LHC at CERN. Exceeding the current density limits of NbTi superconductor, Nb 3 Sn is at present the only practical superconductor capable of generating fields beyond 10 T. Several Nb 3 Sn pilot magnets, with fields as high as 16 T, have been built and tested, paving the way for future attempts at fields approaching 20 T. High current density conductor is required to generate high fields with reduced conductor volume. However this significantly increases the Lorentz force and stress. Future designs of coils and structures will require managing stresses of several 100’ s of MPa and forces of 10’ s of MN/m. The combined engineering requirements on size and cost of accelerator magnets will involve magnet technology that diverges from the one currently used with NbTi conductor. In this paper we shall address how far the engineering of high field magnets can be pushed, and what are the issues and limitations before such magnets can be used in particle accelerators.

The early design and test results have been previously reported. During the subsequent operation of "D20" the accelerator prototype dipole has provided both additional and more detailed data as to its characteristics and performance. D20's use as a test facility for high field critical current measurements has provided operational experience and history pertaining to accelerator required characteristics. There has been recently obtained data related to field quality, operational reproducibility and reliability, which will be presented. This prototype "D20" has attained the highest magnetic field of any accelerator prototype dipole constructed and tested to date. The magnet has continued to operate routinely.

A 5cm SSC superconducting dipole that develops 6.6 tesla at 5790 A is proposed. The two layer magnet has 12% more transfer function than the present design as a result of using thin collars and close in'' iron. The thin collars provide precise positioning of the coils; they also provide minimum prestress (perhaps 2000 psi) as aid for magnet assembly. A welded skin around the iron provides the final prestress and shapes and the coil geometry. A prestressed aluminum bar placed between the vertically split iron yokes provides precise control of the gap between yokes halves and is designed to allow gap to close tightly during cooldown so that there is no decrease of prestress. In order to reduce the effect of iron saturation on the field multipoles the iron ID has been optimized to an elliptical shape. The coil inner layer is a 30 strand cable with 1.3:1 cu/sc. The outer layer is a 36 strand cable wit 1.8:1 cu/sc. At the operating field of 6.6 tesla the current density in the copper is 666 A/mm{sup 2} and 760 A/mm{sup 2} in the inner and outer layers respectively. The magnet short sample performance is limited by the inner layer.more » Operating at 4.35 K the maximum current and central field are 6896 A and 7.95 tesla. The calculated operating short sample temperature at 6.6 tesla and 5798 A is 5.17 K (0.82 K temperature margin). The magnet stored energy is 100.0 (KJ/m) at the 5790 A operating current. A mechanically similar 5cm bore two layer dipole for the cable test facility (D-16B-1) has been recently built and tested. The magnet had no collars and the iron was placed directly on the coil OD. The magnet's first quench was at 7 tesla with 6000 A and it reached 7.6 tesla at 6600 A. This paper contains tables and figures associated with the design.« less

Transitions from the superconducting to normal state at 4.4 and 1.8 K in several model accelerator dipoles were recorded by a fast data acquisition system. The resistive voltage rise in the conductor during the transitions is used to determine accurately the location of the quench source in the magnets and to estimate the axial and turn-to-turn quench velocities. The quench velocity, temperature evolution and energy deposition in the coil were calculated using the program QUENCH and are in reasonable agreement with the data. In the two dipole magnets studied, the transitions almost always occurred in the regions of highest field. In one coil the high field region is in the straight section because the field in this region is enhanced by iron support rings. In the other magnet the high field region is at the end, in the innermost turn of the first layer. Some quenches were preceded by large voltage spikes that can be ascribed to conductor motion. Other quenches do not appear to be associated with any large energy release. Acoustic emission (AE) was monitored during the tests and AE bursts were observed simultaneous with the initial voltage spike. An increased AE signal continued as the quench progressed.

In continuation of single-conductor studies, quench onset data of quasi-steady operation are reported for a formvar-coated composite (Nb48Ti/Cu) in bundle geometry (conductor thickness ≈ 0.2 cm, approximately square cross section). Overall thermal conductances of the heater simulation technique applied are of the order 0.1 W/(cm2K), comparable to single-specimen results, around 4 K and below the lambda transition in agreement with model predictions. Considerable deterioration however occurs below 3 K and down to the lambda temperature during operation in near-saturated He I. Consequences for magnets of intermediate energy density are discussed with emphasis on the modifications necessitated by coatings.

Experiments probing He II-He I coexistence show that in the presence of heat flow there exists a thin interfacial domain. A thermohydrodynamic shock model is proposed to predict the location of the shock domain in the4He phase diagram. Experimental domain thicknesses are reported that are consistent with the model prediction. Consequences of the model for other cases are discussed.

Magnetization of superconducting material can be introduced into POISSON through a field dependent permeability table (in the same way that iron characteristics are introduced). This can be done by representing measured magnetization data of the increasing and decreasing field by two independent B-γ curves (γ = 1/μ). Magnetization curves of this type were incorporated into the current regions of the program POISSON and their effect on the field coefficients observed. We have used this technique to calculate the effect of magnetization on the multipole coefficients of a SSC superconducting dipole magnet and to compare these coefficients with measured values.

The electromagnetic stresses in projected designs of high-field magnet coils sometimes exceed the yield points or creep strengths of the presently used coil winding materials. This is especially true of react-after-winding Nb 3 Sn coils, plastic insulating materials, and the crossed strands of twisted cable. This report presents compression stress-strain curves of stranded and twisted flat cable coil winding specimens for reacted and epoxy-impregnated Nb 3 Sn coils, at 300 K and 80 K. The lateral and longitudinal strains of one specimen were measured along with the direct compressive stress-strain, at 300 K. A similar specimen was compressed to 40 MPa (6 kpsi) at room temperature and then to 145 MPa (20 kpsi) at 80 K. Plastic flow occurred up the maximum stress. Stress cycles subsequent to maximum stress were elastic. Some specimens were mounted in a rigid enclosure to simulate hydrostatic containment of the magnet coil. Pure annealed Cu was tested at 300 K, and pure epoxy was tested at 300 and 80 K. In addition the thermal contractions from 300 to 80 K were measured.

Superconducting magnets may exhibit a significantly increased critical surface when the operating temperature is lowered. It is also known that the stability associated with superconductor-coolant interaction is enhanced in superfluid Helium-4 (He II), provided the coolant ducts have been optimized. The use of He II has been promoted in several large systems. The authors have investigated magnet cryosystem options which utilize the thermomechanical effect (fountain effect) of He II and its inverse, the mechano-caloric effect, for refrigeration (called vortex refrigeration). Examples are presented for simplified thermodynamic state changes of cycle operation. Topics covered include heat leak utilization for the He II range and the quantification of vortex refrigeration. >

Three model superconducting dipole magnets, lm length and having a bore diameter of 76mm, fabricated without epoxy resins or other adhesives, have been built and the first two have been tested in He I and He II. The conductor is the 23-strand Rutherford-type cable used in the Fermilab Doubler/Saver magnets, and is insulated with Mylar and Kapton. The two-layer winding is highly compressed by a system of structural support rings and tapered collets. Little "training" was required. Quench currents greater than 95 percent of "short sample" were obtained in He I with rise-times of 15 to 20 seconds to a central field of 4.6T; 6.0 T in Helium II.

An eight-pancake Nb-Ti dipole magnet, with bent up ends, called D-108B has been built and tested. This magnet is a Nb-Ti version of a Nb/sub 3/Sn magnet designed to produce a 10-tesla dipole field in a 40 mm diameter aperture. The pancack design is used for the heavy 12,000 ampere Nb/sub 3/Sn cable because of the mechanical difficulty in winding such a heavy cable into the conventional nested cylindrical shell configuration with a 2'' inner winding diameter. The Nb-Ti version operates at 1.8K, in He II, has superconducting cable half as thick as the Nb/sub 3/Sn cable, and operates at half the operating current: 6000 A rather than 12,000 A at 10 tesla. Both magnets are approximately one meter long. D-10B was tested from January 26 to February 2, 1983 and reached short-sample performance in both He I and He II after moderate training. The central field at 4.3K is 7.0 (+- 0.1) tesla, and at 1.8K is 9.1 (+- 0.2) tesla. Ramp rate sensitivity and cyclic heating data were also measured.

A superconducting dipole magnet is discussed in detail and test results are presented. The performance of the dipole magnet demonstrates that it can operate successfully at least at the lower end of fields in He 1 at about 4.3K and in He 11 between 1.8 and 2.16K. A photograph of the magnet is shown and a cross section is given. One of the goals of fabrication was to maintain the first turn of each layer in positive compression against its respective pole island during operation at the highest current. Target values for the precompression were determined for each layer.

Designers of cryostable superconducting magnets, which operate in pool boiling He I, are aware of critical heat flux densities. The regions of most general interest are the peak nucleate boiling and recovery heat fluxes at a conductor temperature no more than a few hundred millikelvin above the pool temperature. Systems which depend on nucleate boiling can face a convective region prior to the nucleate boiling transition. Thermal fluctuations can easily effect the pressure to an extent where the liquid becomes subcooled. Depending on the degree of subcooling the temperature difference at the transition to nucleate boiling, can exceed the temperature difference in all of the nucleate boiling region.

Among the various approaches to superconducting magnet design, several categories of magnets may be distinguished, which include liquid-helium-cooled magnets. There are generally accepted stabilization criteria [1] for these large superconducting magnets [2], such as steady-state stability. An extension of related stability data to transient conditions has been reported for a number of bare conductors, but not for coated composites of large magnets.