E. W. Collings

Design considerations for high Tc ceramic superconductors

Abstract The flux-jump stability and cryostability of model conductors based on the ceramic superconductor YBa 2 Cu 3 O 7 (YBC) are considered. The higher specific heat of materials at 80 K than at 4 K guarantees a much greater adiabatic flux-jump stability. In fact, YBC should be immune to adiabatic flux jumping in applied fields of less than about 5 T, which accounts for most fields of practical interest. As for the dynamic flux-jump stability of a model metal-matrix composite, the high thermal conductivity of YBC at 80 K is partly responsible for relieving the customary requirement for 100 μm filaments. Depending on the assumed J c dynamically flux-jump stable filaments can be many millimetres thick. The general conditions for cryostability in composite conductors are investigated as the operating temperature is increased from 4 to 80 K and as, for pool boiling coolant, liquid helium is replaced by liquid nitrogen. Finally, it is pointed out that although thick conductors will satisfy the electrical requirements for stable d.c. operation, fine filamentary subdivision will always be needed to reduce a.c. hysteretic losses under both external-field and self-field conditions.

Influence of twist pitch and sample length on proximity effect coupling in multifilamentary composites described in terms of a field independent, two current region model☆

Abstract Magnetization and loss have been measured via vibrating sample magnetometry for a set of multifilamentary NbTi superconductive strands exhibiting a wide range of proximity effect (PE) coupling. Measurements extending over a large range of twist pitches L p and sample lengths L demonstrated that the magnetization and loss behaved similarly with respect to both parameters. For small L p ( L ) values, magnetization and loss were linear with L p ( L ), while for larger values a saturation was found (with either L p or L ). Additionally, critical fields and critical currents for the proximity effected matrix, as well as penetration fields for the composite, were measured. Critical state models for anisotropic media were used to calculate the magnetization and penetration fields of these strands over a large range of L p values. A calculation of the transfer length and magnetization of short samples is also presented.

Eddy-current effects in twisted and wound SSC strands

An analysis has been made of the effective pitch (with respect to an applied field transverse to the winding axis) of a pair of filaments that have been first twisted and then helically wound. In case of very different values of twist pitch and winding pitch, the effective pitch is simply the smaller of these. However, as the value of the winding pitch approaches that of the twist pitch, the effective pitch may become quite large. Results of this analysis are compared with magnetization measurements of a series of helically wound strands with varied twist pitch. These measurements show large eddy-current losses in a twisted strand with a twist pitch comparable to the winding pitch; more generally, it is shown that eddy-current loss depends upon the superposition of twist and winding. The general shape of the magnetization as a function of applied field M(H) loop due to the combined effects of filamentary hysteresis and eddy currents has been mathematically modeled.

Low temperature magnetic susceptibility and magnetization studies of some commercial austenitic stainless steels

Abstract The magnetic susceptibilities and magnetizations of seven austenitic stainless steel alloys, AISI 310S, 304L, 304N, 316 and 316L, and AWS 330 have been measured at temperatures down to 4.2 K. The experimental results, presented here, reveal in these four types of alloys four distinct classes of magnetic behaviour, and support a general magnetic description of austenitic stainless steels in terms of a two-magnetic-phase (spin glass plus superparamagnetic cluster) model.

Transverse and longitudinal resistivities in NbTi multifilamentary strands with Cu and CuMn matrices

Transverse and longitudinal resistivities have been measured for a pair of multifilamentary strands with filament diameters of 2 mu m, interfilamentary spacing to filament diameter ratio of 0.19, and about 5000 filaments. One of these strands has a Cu matrix and the other has a CuMn matrix. 4.2-K transverse resistivities were extracted from the per cycle eddy current loss and also the magnetization of samples of various twist pitches. Longitudinal resistivities were measured with a four-terminal method at T=12 K. These results are interpreted in terms of size, proximity, and current-path effects. It is found that size effects significantly enhance both the transverse and longitudinal resistivities of the Cu matrix material, and that current-path effects significantly decrease the transverse resistivity of the CuMn material. Additionally, the proximity effect causes the resistivity of the Cu matrix to vary with magnetic field. Resistivity for the Cu matrix at high fields is about 0.6-0.8 that of the CuMn.<<ETX>>

A conductor, with uncoupled 2.5 mu m diameter filaments, designed for the outer cable of SSC dipole magnets

A conductor with a stabilizer-to-superconductor ratio of 1.8/1 and containing 22900 2.5- mu m diameter filaments at a wire diameter of 0.65 mm has been produced for the outer cable of the Superconducting Super Collider (SSC). The fabrication procedures used to make this material from a full production-size billet 305 mm in diameter are described. Metallographic, electrical, and magnetization data are presented, and these illustrate the filaments are completely uncoupled at the size used in the outer SSC cable. This work indicates the feasibility of the commercial manufacture of such a 2.5- mu m-diameter filamentary material. Preliminary tests show relatively low J/sub c/ and n values. It is likely that materials with smaller spacings than those used here will be required before the J/sub c/ values specified for the SSC can be obtained reproducibly. >

Magnetic Studies of Proximity-Effect Coupling in a Very Closely Spaced Fine-Filament NbTi/CuMn Composite Superconductor

Magnetization studies have been conducted on a 23,000-filament composite (with a filament-spacing/filament-diameter ratio, s/d, of about 0.19) drawn down to d = 11.5 to 0.5 μm. Various techniques have been used to explore the occurrence and properties of proximity-effect coupling between the filaments across the Cu-0.5wt.%Mn matrix. This coupling, which sets in at d < 1.5 µm — much smaller than 2.5 /im intended for superconducting supercollider (SSC) magnet applications — is studied both at low fields (well below the Hc1 of the NbTi) and at high fields (of up to 1.5 tesla (15 kG)).

Enhanced static magnetization and creep in fine-filamentary and SSC-prototype strands via helical cabling geometry enhanced proximity effects

Helical-cabling-geometry enhanced proximity effect (PE) magnetization and creep have been found in multifilamentary NbTi/Cu superconductive composites with filament diameters (interfilamentary spacings) of 2.0 (0.39) mu m and 6.0 (1.14) mu m. Single strands were wound into helical coils to simulate round-cable geometry. Additionally, oval coils were wound to verify the existence of enhanced PEs in 6.0 mu m filament diameter material with a large effective coupling length. For the helical coils, when the strand twist pitch was nearly equal to the cable perimeter, PE related magnetization was enhanced, and this magnetization creeps at a significantly greater rate than that of bare NbTi.<<ETX>>

Mictomagnetism in austenitic stainless steels

Abstract Mictomagnetism provides a useful qualitative description of the magnetic behaviour of austenitic stainless steels, alloys which occupy a transition region between long-range ferromagnetism and long-range antiferromagnetism. Near the ferromagnetic side, large-cluster superparamagnetism dominates the magnetic properties; in the middle region, mictomagnetism exhibiting superparamagnetism in a short-range antiferromagnetic matrix is evident. Eventually the superparamagnetic clustering begins to disappear as long-range antiferromagnetism sets in, but a region does seem to exist in which both kinds of property are present. Within the framework of mictomagnetism in general, this Paper discusses the magnetization versus temperature and magnetization versus field characteristics of three selected austenitic stainless steels and the conditions under which they exhibit magnetic remanence during field cycling at constant low temperature, and thermomagnetic remanence during low-temperature thermal cycling at constant field.

Hysteretic method of lower critical field determination in high-Tc superconductors

The per unit volume per cycle AC hysteretic loss of samples with different radii, R0 (filament radii, in the case of multifilamentary materials), can be meaningfully compared as function of field-sweep amplitude, Hm, by normalizing the loss to some reference radius, Rref.. The R0-normalized Qh can then be used to compare the Jcs of different samples in both the low- and high-field regimes. A new function, Qh=(10/3 pi 2JcR0)((3/2)Hc1(Hm-Hc1)2+(Hm-Hc1)3), has been developed for describing the low-field hysteretic loss in transverse-field cylindrical samples. Two methods are outlined for using this equation to extract Hc1 from thick high-Jc samples-samples which defy the conventional deviation-from-linearity approach. Applications of these procedures to samples of melt-processed YBCO and multifilamentary NbTi are presented.