K.R. Marken

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.

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)).

Design of Multifilamentary Strand for Superconducting Supercollider (SSC) Applications -- Reduction of Magnetizations Due to Proximity Effect and Persistent Current

Nonsuperconducting saddle magnets can in principle be designed to produce an undistorted dipolar magnetic field. But if the coils are wound from superconducting strands, residual magnetization, MR, (i.e. “persistent current”) resident in the filaments is responsible for multipolar distortions of the desired field. Recognizing that the height (or thickness) of the M(H) hysteresis loop -- ΔM(H) ≡ (MR+ - MR-) -- is proportional to the product of critical current density, Jc(H), and the filament diameter, d, there is a strong interest in producing, on a commercial scale, multifilamentary strands with smaller and smaller filaments. In order to preserve filament quality in small filaments, some authors have deemed it necessary to confine the ratio of filament spacing (s) to filament diameter (d) to s/d ≤ 0.15±0.02. The combination of small d with low s/d results in interfilamentary spacings sufficiently close to proximity-effect couple the filaments. But if the interfilamentary matrix is alloyed with ~0.5 wt.% Mn, coupling is barely perceptible even with 1 μm diameter filaments. But having disposed of excess, interfilamentary, magnetization one is still faced with the intrafilamentary magnetization of the NbTi filaments themselves. Since during the operating cycle (field-increasing) of the SSC magnet this magnetization is diamagnetic (throughout most of the winding) it can be neutralized by including in the superconducting strand a material with a large positive magnetization, such as Ni. Possible methods of deploying and administering the Ni are discussed.

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>>

Design, fabrication, and properties of magnetically compensated SSC strands

The addition of Ni to a superconducting strand can compensate its diamagnetic persistent-current magnetization over a portion of the beam-injection and acceleration stroke of the Superconducting Super Collider (SSC) magnet. The ferromagnetic addition may thus relieve the SSC magnets need for extremely fine filaments, the initial purpose of which was to minimize conductor magnetization. It is shown how the Ni could be incorporated into the strand itself, either in the form of replacement filaments or as a coating on the outside surface.

EXPERIMENTS TO IMPROVE MATERIALS FOR SSC MAGNETS

The standard 6 μm diameter filamentary SSC conductor is now produced, with Jc’s as high as 3000 A/mm2 at 5 T, in relatively long piece lengths. The design of the billet and the thermomechanical treatments which are given to the strands were established some years ago1. The chief concern in recent years has been the achievement of long and reproducible piece lengths while maintaining the required Jc level.

Design of Coupled or Uncoupled Multifilamentary SSC-Type Strands with Almost Zero Retained Magnetization at Fields Near 0.3 T

Multifilamentary Cu-matrix strands with interfilamentary spacing as small as 0.2 μm can be almost fully decoupled by the addition of 0.5 wt.% Mn to the interfilamentary Cu. Decoupling in this way seems to be beneficial from a field-stability standpoint. On the other hand, the elimination of coupling does little to reduce residual strand-magnetization at the injection field of about 0.3 T when that field is approached, as usual, along the shielding branch of M(H). This residual diamagnetic magnetization (say MR) of the winding material is responsible for unwanted distortion (multipole formation) of the dipolar field. It is demonstrated that MR can be locally cancelled to zero by associating the strand with a small volume-fraction (less than 2%, depending on filament diameter) of pure Ni or any other low-field-saturable ferromagnetic material. The presence of the Ni has little effect on the shape of the M(H) hysteresis loop of the strand, other than to shift its wings uniformly in the + M (when H is positive) and -M directions, respectively. In practice, the Ni could be administered as: (a) additional filaments, (b) interfilamentary barriers, or (c) an electroplated layer on the outside of the strand.

Ferromagnetic Material in the Superconductor and Its Effect on the Magnetization Sextupole and Decapole in the SSC Dipoles at Injection

It has been shown that the magnetization of a multi-filamentary superconductor can be altered by adding nickel to the composite strand. This report presents the results of calculations of the magnetization sextupole and higher multipoles in a 5 cm Superconducting Super Collider (SSC) dipole with and without nickel as part of the strand composite. The relative distribution of the nickel in the inner and outer coil conductors can be used to effectively eliminate sextupole and decapole at the SSC dipole injection field. Calculations of magnetization sextupole in the dipole are presented for strand with substituted nickel filaments and strand with electroplated nickel. The effect of nickel in the strand on the SSC dipole field quality at fields above the injection field is described. The effect of nickel in the strand on magnetization sextupole flux creep decay and the magnetization sextupole temperature dependence is also discussed.

Position and amplitude of proximity effect peaks in the magnetization curves of NbTi/Cu and NbTi/CuMn multifilamentary strands

The magnitude and position of proximity-effect-related magnetization peaks in the M-H loops of multifilamentary NbTi superconductive composites have been studied. M-H loops were taken at T=4.2 K as a function of field-sweep amplitude, H/sub M/, for several specimens. Three regimes emerge: (i) low field, where proximity effects are mainly shielding in nature; (ii) high field, where there are anomalous trapping effects: and (iii) an intermediate-field region, corresponding to a crossover between these two regimes. Correlations are made between these regimes and the specimen parameters H/sub C1,NbTi/ and H/sub p,NbTi/. A relationship is found between the magnitude of the maximum of the high-field magnetization and the breakdown field of the copper.