bluuebunny/crossref_metadata_2025 huggingface dataset:
Microwave Reflection Measurements of Electron Densities in Electromagnetically Driven Shock-Produced Plasmas
Authors: Funahashi Akimasa , Takeda Susumu
Published: February 2004

The electron densities from 1013 to 1017 cm−3 in plasmas produced by electromagnetically driven, propagating, or reflected shock waves are measured by a microwave reflection method. The Mach number of the shock waves is varied from 9 to 25 in argon, air, and helium at initial pressures from 0.4 to 2.0 Torr. A 35-GHz microwave reflection probe is inserted into a T-type shock tube perpendicular to its axis, or is placed at the tube end parallel with the axis. Despite some perturbation on the gas flow by the inserted probe, surprisingly good agreements are found between the microwave and spectroscopic measurements. The measured electron densities are compared with predicted values calculated from the Rankine-Hugoniot and Saha equations. For Mach numbers higher than 20, the maximum values of the electron densities measured behind the shock fronts agree well with the theoretically predicted values. For low Mach numbers, however, the measured maximum electron densities are lower than the theoretically predicted values. Reasons for the discrepancy between the theoretical and experimental values in low Mach numbers are discussed. In addition, radial and axial density distributions behind the shock fronts are measured for various Mach numbers.
Mössbauer Effect and Magnetic Properties of Iron-Beryllium Compounds
Authors: Ohta Keizo
Published: February 2004

Mössbauer measurements using Fe57 were made on the Fe–Be system in the composition range between 66 and 85 at. % Be. The internal field and the isomer shift in FeBe2 (66.7% Be, MgZn2 structure) are 192±5 kOe and 0.13±0.05 mm/sec, respectively. A metallic iron standard at room temperature was used. The Mössbauer spectrum of a single crystal of FeBe2 shows that the magnetization is parallel to the c axis. When the Be concentration is increased above 66.7%, two or more internal fields are observed, corresponding to Fe (4), Fe (3), and so on. Fe (4), Fe (3), etc., are defined as Fe atoms which have 4, 3 Fe atoms, respectively, in the nearest-neighbor iron sites. When the Be concentration exceeds 75%, the MgCu2 crystal structure develops and the materials are paramagnetic. The (Fe–Mn) Be2 system was also investigated. It was found that the presence of nearest-neighbor Mn atoms decreases the field at the iron sites by approx 14%.
Internal Friction and Related Properties of the TiNi Intermetallic Compound
Authors: Hasiguti R. R. , Iwasaki K.
Published: February 2004

Internal friction and elastic moduli of the intermetallic compound TiNi were measured as a function of temperature from −170° to 800°C. There appear in the internal friction curve two well-defined peaks at −70° and 600°C, respectively, a small peak at 350°C, and a group of several sharp peaks in the temperature range from −50° to 40°C. The elastic modulus has a positive temperature coefficient in the temperature range from 40° to 520°C. These results are discussed in terms of the crystal-structure model of Wang and others.
Relative Ductilities of TiFe, TiCo, and TiNi
Authors: Scholl R. , Larson David J. , Freise E. J.
Published: February 2004

A study of the relative ductilities of binary and ternary compounds of the type TiX, where X is Fe, Ni, Co, or combinations of these, has been performed by means of three-point bending tests at room temperature. The general results of these studies indicate that TiFe or TiCo forms a continuous series of solid solutions with TiNi, provided the concentration of titanium remains at 50 at.%. The ductility of the binary compounds increases in the order TiFe, TiCo, TiNi. Also, nickel additions to TiFe and TiCo increase their relative ductilities. These ductility changes can be correlated with the increasing electron-to-atom ratio as the nickel concentration is increased. In turn, the increasing electron-to-atom ratios are taken to indicate an increasingly metallic character in the bonding of the compounds. This conclusion is consistent with x-ray lattice parameter measurements.
Equiatomic Binary Compounds of Hf with Transition Elements Os, Ir, and Pt; Further Comments on the TiNi Transition
Authors: Wang Frederick E. , Ernst Donald W.
Published: February 2004

By means of single-crystal x-ray-diffraction methods, an underlying structural similarity among HfOs, HfIr, and HfPt has been established. Based on low-temperature (below −100°C) precession photographs, the existence of the previously anticipated ``Mf'' temperature in TiNi has been confirmed. This evidence further supports the existence of unique ``martensitic'' transition ``bands'' in the TiNi, ZrPd, and HfPt series of equiatomic binary compounds.
High-Temperature-Creep Mechanism of TiNi
Authors: Mukiierjee A. K.
Published: February 2004

Polycrystalline specimens of the intermetallic compound TiNi were tested in tension in the temperature range 700°–1000°C. The specimens were observed to deform uniformly over the gage length until the very end of deformation, when it failed by necking. It was noted that the steady-state flow stress was independent of the strain and was a function only of the temperature and strain rate. The shear strain rate γ̇ was given by γ̇∝τn where n=3.0±0.2, r was the shear stress, and n was independent of temperature. The apparent activation energy for creep was found to be 60.0±3.0 kcal/mole and was independent of stress. The result strongly suggests that a viscous creep process, as first formulated by Weertman, is probably the rate-controlling mechanism for this compound in the stress and temperature range investigated.
Nuclear Magnetic Resonance and Susceptibility Measurements in TiCo
Authors: West G. W.
Published: February 2004

Nuclear magnetic resonance and magnetic susceptibility measurements have been made at 295° and 77°K on titanium-cobalt alloys over a range of composition near the equiatomic value and also on ternary alloys near the composition Ti2NiCo. The measurements in TiCo suggest that there may be a small range of solid solution for cobalt-rich alloys near the equiatomic composition. There is a large positive Knight shift for the Co59 resonance in all TiCo alloys. The Knight shift and the susceptibility are substantially larger at the lower temperature presumably due to a large contribution from an orbital interaction. For the ternary compounds both the Knight shift and the susceptibility are smaller, and it is postulated that the orbital contribution has been eliminated.
Spherical-Wave Theory of Dynamical X-Ray Diffraction for Absorbing Perfect Crystals. II. Integrated Reflection Power
Authors: Kato N.
Published: February 2004

Based on the x-ray intensity field due to a spherical wave obtained in Part I, the integrated power ratio of the Bragg-reflected wave is presented for wedge-shaped crystals. No approximation is used except the so-called two-beam approximation. In parallel-sided crystals, the result is practically the same as the previous result obtained by assuming smallness of absorption, based on the plane-wave theory. The result can be used for understanding the traverse-type x-ray topograph.
Nonlinear Excitation of Propagating Surface Waves on a Plasma Column
Authors: Ajmera R. C. , Lonngren K. E.
Published: February 2004

This paper presents experimental results of the nonlinear excitation of a propagating surface wave on a plasma column. The dispersion characteristics of this wave are compared with those obtained from a linear excitation. The possible mechanisms of the nonlinear excitation are also discussed.
Wave Behavior at an Interface of a Semi-Infinite Moving Anisotropic Medium
Authors: Kong Jin-Au , Cheng David K.
Published: February 2004

The wave behavior at an interface between an isotropic medium and a moving anisotropic medium for an arbitrary plane of incidence is studied. Modified forms of Snell's law for refraction are obtained for both the ordinary and the extraordinary waves when the moving medium is uniaxially anisotropic. The critical angle for total reflection is computed for the extraordinary wave as a function of medium velocity and anisotropy. It is found that the reflected and refracted waves behave quite differently, depending on whether the moving medium is a uniaxial crystal or a magneto-ionic plasma in an infinitely strong magnetic field. Conditions for the existence of the Brewster angle, even for a prependicularly polarized wave incident normal to the direction of medium motion, are examined. Curves for reflection coefficient are plotted vs medium velocity.
Twinning and Slip in Zinc by Indentation
Authors: Blish R. C. , Vreeland T.
Published: February 2004

Observations of twinning and slip deformation caused by indentation of zinc reveal that extensive slip on the basal and second-order pyramidal systems takes place at loads up to 5 kg. Prismatic punching through 1-cm crystals is observed at indentation loads in excess of about 2.5 kg. It is concluded that the stress at the tip of the twins cannot be obtained by use of an elastic stress analysis.
Electrode-Limited and Space-Charge-Limited Transient Currents in Insulators
Authors: Weisz S. Z. , Cobas A. , Trester S. , Many A.
Published: February 2004

The equations governing the transient-current flow in insulating crystals are solved for two categories of boundary conditions corresponding to situations in which the current is partly space-charge controlled and partly electrode limited. The carrier reservoir at the injecting electrode is assumed to arise from photo-excitation by a pulse of highly absorbed light. In one category, the light intensity is taken to be sufficiently strong such that initially the current is completely space-charge limited. The time dependence of the current after the collapse of the carrier reservoir is calculated, with the time interval elapsed between the onset of the light pulse and the reservoir collapse taken as a parameter. The other category considered corresponds to sufficiently weak pulse excitation such that the field at the illuminated electrode is never zero. The duration of the pulse for this category is assumed to be short compared to the carrier transit time. The time and voltage dependence of the current is calculated in this case in the presence of surface recombination. Both categories of boundary conditions are often encountered in practice, and a comparison between theory and experiment is expected to yield valuable information on carrier generation and recombination processes at the surface.
Domain Nucleation on the Surface of BaTiO3
Authors: English F. L.
Published: February 2004

A new technique is described which should prove useful in studying the switching mechanism in ferroelectrics. A sequence of electron mirror photomicrographs of a BaTiO3 single crystal taken during switching demonstrates the technique. In this crystal, spike-shaped a domains were observed to nucleate at the positive contact when an external voltage was applied. Domains could also be nucleated at the negative region when the electron beam was allowed to charge the surface.
Calculation of the Piezoelectric Constants of α Quartz on Born's Theory
Authors: Julian Carl L. , Lane F. O.
Published: February 2004

The phenomenon of piezoelectricity in α quartz is related to the phenomena of infrared absorption and Raman scattering, using a model due to Kleinman and Spitzer. For right quartz the values computed for e11 and e14 are −0.052 C/m2 and 0.076 C/m2, respectively. (The values measured by Bechmann are −0.171 C/m2 and 0.0406 C/m2.) The calculation seems to be the first of its type to obtain the correct signs.
Imperfections in Solution-Grown β-Silicon Carbide Crystals
Authors: Bartlett R. W. , Martin G. W.
Published: February 2004

Imperfections in high-purity β-silicon carbide crystal platelets, (111) habit, were studied using Lang x-ray topography, Weissenberg photographs, and etch pitting. Stacking faults on {111} planes with associated partial dislocations of the 〈112〉 type and {111} twinning have been observed. In well-formed platelets, twinning is usually confined to the habit plane. Stacking faults occur parallel with the platelet surface and on {111} planes inclined to the platelet surface, and they tend to cluster in overlapping, closely spaced groups. Etch pits on the surface are caused by emerging 〈112〉 partial dislocations that bound the inclined stacking faults. Partial dislocations in the habit plane have also been identified from diffraction contrast and extinction effects.
Field Dependence of the Rate of Decay of Persistent Currents in an Nb-25% Zr Superconducting Solenoid
Authors: File J.
Published: February 2004

The decay of the magnetic field produced by a persistent current flowing in a superconducting solenoid wound with Nb-25% Zr conductor has been measured at various values of the self-induced magnetic field. The field was varied through the range of 7 to 832 G below the critical field. The results of these experimental runs indicate that over this range of values of the field, the logarithm of the decay rate varies linearly with the field. An estimate of the size of a flux bundle can be made from the result and is 5×10−6 cm.
Field Evaporation of a Screw Dislocation in Tungsten
Authors: Carroll Robert W. , Schubert Clarence C.
Published: February 2004

A complex crystal defect in tungsten has been examined in detail by means of the field ion microscope. Field evaporation has been employed to remove the metal surface, making visible the three-dimensional structure of the defect. The defect has been identified as a grain boundary appearing in(1̄16) of one grain and(11̄6) of the other, so that the grains are misoriented by an angle of 26.5° rotation about the direction [110], which corresponds to the axis of the wire sample. A screw dislocation intersects (110), which is shared by both grains. The intersection of the screw dislocation with the surface produces a spiral on (110) which can be made to rotate under field evaporation. The direction of the Burgers vector of the dislocation has been found to be [110], and the Burgers vector magnitude is an integral multiple of the (110) plane spacing. The Burgers vector magnitude of the dislocation increases as the sample is removed, producing multiple spirals on (110).
Transport Coefficients of Composite Materials
Authors: Jackson Julius L. , Coriell Sam R.
Published: February 2004

The problem of the effective conductivity of a composite material whose local conductivity is a function of position is treated. Using the analogy between this problem and the diffusion of ions in a periodic potential, previously obtained variational results are used to obtain upper and lower bounds for the effective conductivity. These bounds are shown to be the conductivities obtained in certain commonly used equivalent circuit approximations. Although the discussion in the paper is in terms of the electrical conductivity, the theory is equally applicable to other transport coefficients, such as the heat conductivity and the diffusion constant, as well as to a class of response functions such as the magnetic permeability and dielectric constant.
Influence of Previous Positive Streamers on Streamer Propagation and Breakdown in a Positive Point-to-Plane Gap
Authors: Acker Frank E. , Penney Gaylord W.
Published: February 2004

The influence of a positive streamer upon its successor in a sequence of streamers is experimentally studied in a 3.17-cm positive point-to-plane gap using room air at room temperature. Streamers are triggered either by natural processes or by voltage perturbations (pulses) superimposed on the steady gap voltage. Despite the fact that a streamer has an inhibiting effect on the triggering of a later streamer, if another streamer is successfully triggered within a few tenths of a millisecond of the earlier streamer, using gap voltages of 16–25 kV, the later streamer will follow the same path and propagate faster than if the time interval between streamers was longer. Observations are made of the buildup and speed of primary and secondary streamers in sequences culminating in breakdown, and the filamentary spark breakdown of the gap is shown to have different current and light characteristics as a function of time from a streamer. A qualitative model of the streamer buildup mechanism involving metastable atoms or molecules is presented.
Uniaxial Magnetic Anisotropy Induced in Pt–Co Alloy by Tempering under Stress
Authors: Shimizu S. , Hashimoto E.
Published: February 2004

When the 50 at.% Pt–Co alloy is tempered under compressive stress, a uniaxial magnetic anisotropy is induced which has its easy axis along the direction of the stress. When tempered under tensile stress instead of compression, the anisotropy has its hard axis along the direction of the stress. This anisotropy is most developed in the so-called over-aged (perfectly ordered) state, and K=3.1×106 erg/cc is measured in the field of 20 kOe. 4πI=7.4 kG and 4.7 kG are measured in the field of 23 kOe along the easy and the hard axis, respectively. From x-ray-diffraction diagrams a kind of fiber structure is observed in which the 〈001〉 axes of the ordered tetragonal lattices are oriented to the direction of the compressive stress somewhat more than the cross directions.
Brittle Fracture in Iridium Single Crystals
Authors: Brookes C. A. , Greenwood J. H. , Routbort J. L.
Published: February 2004

A study of the brittle fracture of iridium is made using the scanning electron microscope. Except where macroscopic imperfections are present, cracks can only be nucleated after extensive work-hardening. Plastic relaxation of the crack is inhibited because impurities substantially increase the critical resolved shear stress. Brittle fracture in iridium is thus basically an impurity effect.
The Microwave Magneto-Kerr Effect in Silicon and Germanium. I. Measurement of Complex Hall Factor
Authors: Hauge Peter S. , Champlin Keith S.
Published: February 2004

The complex conductivity and Hall factor of lightly doped n- and p-type Si and Ge are measured at 8.6 GHz over the temperature range 77∘ [double-line equal to or less-than] T [double-line equal to or less-than] 300∘K. These phenomenological parameters are determined from observations of microwave reflectivity and low-field magneto-Kerr effect. The experimental analysis applies rigorously to guided waves and treats multiple reflections within the sample exactly. Both Hall and conductivity currents are found to lag the electric field at low temperature. These effects are attributed to the increased influence of the inertia of the charge carriers at low temperature.
Microwave Magnetoresistance of Dendritic n-Type InSb Films
Authors: Wieder H. H. , White D. J.
Published: February 2004

The resistivity ρ0 and transverse magnetoresistance (Δρ/ρ0) of dendritic monophase InSb films were measured in the K-band microwave region. A theoretical analysis of the propagation of a TE10 wave through films covering the cross section of a waveguide was derived subject to the approximation that the resistivity may be treated as a scalar quantity and the attenuation and phase coefficients in the films are equal. It is shown by means of these approximations, good agreement is obtained between the measured dc resistivity and the calculated microwave resistivity. Satisfactory agreement is obtained between the measured dc magnetoresistance, its magnetic field and geometry dependence, and the calculated (Δρ/ρ0) determined from measurements made in the microwave region.
Intense Mercury-Vapor Green-Band Emission
Authors: Carbone R. J. , Litvak M. M.
Published: February 2004

Experiments on a high-pressure and pulsed high-current mercury vapor and argon discharge produced intense green-band emission, but no laser action from the mercury metastable molecular state 3Ou− →1Σg+ (unstable ground-state) transition. Voltage, arc constriction diameter, and band emission intensity are given as functions of current and pressure. The pumping rates are discussed with regard to reaching laser threshold gain and power saturation.
Surface Damage in Fatigued Aluminum by Nucleation of Gold
Authors: Wilkov Melvin A. , Applewhite Barry A.
Published: February 2004

Gold was vapor deposited on fatigue specimens of aluminum previously fatigued in either vacuum or air. Differences in the nucleation of the gold on the aluminum substrate were observed that correlated with the known differences in fatigue surface damage, which have been reported in vacuum and air-fatigue experiments. These differences in nucleation were found to reflect differences in surface morphology, rather than the presence of the oxide layer.
A Unified Theoretical Description of Thermionic Converter Performance Characteristics
Authors: Wilkins D. R.
Published: February 2004

Fundamental principles of molecular chemistry, statistical mechanics, and plasma physics are used to derive theoretical output current and efficiency characteristics of vapor thermionic converters. The results are obtained in terms of basic physical parameters which describe the converter electrodes and interelectrode gas and are in good agreement with corresponding experimental measurements for a broad range of converter operating conditions. The analysis has practical applications for converter physics studies, converter performance evaluations, and thermionic power plant design.
Mössbauer Measurements on Magnetic Anisotropy in Thin Films of Iron
Authors: Rabinovitch K. , Shechter Hanan
Published: February 2004

The Mössbauer technique was used for magnetic anisotropy measurements in thin iron films prepared under various conditions, from which complementary information can be obtained. The average direction of magnetization for two different heater geometries was found to be 58.5° and 44.5°, relative to the normal to the plane of the film. The relation of the Mössbauer results to observations by other methods is discussed.
High-Temperature Superconductors
Authors: Geballe T. H.
Published: February 2004

Superconductors with the highest known transition temperatures to date occur in β-tungsten phases which are rich in Nb3Al. It has been found possible to raise the temperature of Nb3Al (Tc=18.5°K) to above 20°K by replacing some of the Al with Ge.1 The low values found for the electronic-heat-capacity co-efficient, and the almost temperature-independent magnetic susceptibility contrast with the corresponding properties of other high-temperature β-tungsten structures such as V3Si. Thus, the location of the Fermi level at a narrow maximum of the density of states in energy which has been used as a model to explain the high specific heat and temperature-dependent susceptibility of V3Si and the other high-temperature β-tungsten structures2 does not apply to Nb3 (Al, Ge). No distinctive properties of the normal state have as yet been measured. The variation of the transition temperature with valence electron count works well on a coarse-grained scale for β-tungsten structures but does not account in detail for the observed transition temperature as a function of nominal composition. The effect of stoichiometry, order, strain, and the presence of phases other than β tungsten all play some role in determining the observed transition temperature.
Fully Stabilized Superconducting Strip for the Argonne and Brookhaven Bubble Chambers
Authors: Wong J. , Fairbanks D. F. , Randall R. N. , Larson W. L.
Published: February 2004

Composite strip conductors for the two largest superconducting magnets yet to be built are described. The conductors consist of six bands of Nb-48% Ti alloy buried in 2-in.-wide strips of copper. The conductors for the two magnets have, respectively, critical current capacities of 3000 and 5880 A. In addition, the quality of the bond between copper and superconductor is excellent, and the copper conductivity is high. Data are presented for these and other characteristics of the strip as manufactured. The strip conductors are anisotropic in their critical-current behavior with respect to field orientation. The importance of this fact in designing magnets economically is pointed out. Performance results are given for a 24-in. i.d. test coil containing a ton of strip. Many strip configurations are possible and can be easily produced Costs of strip of this type will be competitive with present materials.
Improved Stabilized Superconductors
Authors: El Bindari A. , Bernert R. E.
Published: February 2004

A new theory of the stable transition of a superconductor between normal and superconducting states is reviewed and data supporting this theory are presented. Data on copper-clad Nb-40% Ti are presented, and the slope for the variation of the critical current with wire diameter is found to be 1.5 for wire diameters between 0.005 and 0.05 cm. For copper-clad samples of this material, voltage-current data agree remarkably well with the new theory of stabilization. The importance of the superconducting transitional region is emphasized.
Microstructure Effects in Ti-22 at.% Nb Conductors
Authors: Kramer D.
Published: February 2004

Composite conductors of copper and titanium-niobium, either cable or solid, have been fabricated with low-weight-percentage Nb alloys in the range 22–28 at.% niobium. The fabrication of such alloys and conductors involves heat treatment in the temperature range 300°–500°C. It has been found that omega-phase precipitation occurs in all samples. Details as to the size and distribution of omega precipitates will be given as well as the correlation between omega precipitation and critical current densities in the superconductor.
Radio-Frequency Losses in the Superconducting Penetration Depth
Authors: Victor J. M. , Hartwig W. H.
Published: February 2004

Trapped flux has been shown to be responsible for a large part of the residual ac losses in both types I and II superconductors. The authors have made theoretical and experimental investigations of losses in the 40–400 MHz range to establish a more detailed low-field model. The surface resistance rs, as given by Pippard, is exceedingly small for Sn, Pb, and other pure metals below about 0.95 Tc at these frequencies. As a consequence, trapped-flux effects provide the dominant losses in rolled-foil resonant circuits. The theoretical model is simply Ohmic losses in the normal regions of trapped fluxoids bounded by the penetration depth and the surface. This gives an added resistance rh=rh(0)V(t), where the temperature function is V(t) = (1−t4)−1/2(1−t2)−1. The magnitude of the loss is predicted to be proportional to the density of fluxoids trapped, which in turn is assumed proportional to the background magnetic field for low fields. The experimental technique consisted of pulse determinations of circuit Q in the superconducting and normal state as a function of temperature and weak background magnetic field on cooling below Tc. The most detailed effort was made on pure tin foil from 60–350 MHz with background H fields up to 6 G. Data on these and on Sn–In, Pb–Sn, Ta, and Nb samples showed the function V(t) gave the best fit. The flux-trapping loss was linearly proportional to the cooling field. The Pippard surface-resistance term was separated out along with a very small residual loss, r0. The latter was independent of field or temperature and could be annealed out. The Pippard term had a frequency dependence between ν4/3 and ν3/2 as expected. These results differed from Haden et al. in that no break in the decay rate was observed in the pure Sn circuits. This could be explained by the absence of an observable fluxoid core transition. The absence of a strong frequency dependence is consistent with Ohmic loss and ruled out a dominant hysteresis behavior. The nature of the flux-trapping loss is seen to make it the principle dissipating mechanism for most circuit applications since it resists annealing, does not depend upon impurities, and even though greatly reduced by nulling out external fields, may be partially self-induced due to thermoelectric effects on cooling.
Macrovortex Structure in Hard Superconductors
Authors: Iwasa Y. , Williams J. E. C.
Published: February 2004

The magnetization of hard superconducting wires, in the presence of a transverse magnetic field, can be induced to collapse by the application of a small, local field pulse. Under certain conditions the collapse may propagate along the wire in steps, leaving behind a spatially periodic variation in the remanent magnetization of the wire. The remanent magnetization is associated with discrete vortices of wire. A phenomenological explanation for the formation of the macrovortex structure is proposed. According to this explanation, the energy of magnetization, which is initially stored uniformly over a finite volume, is preferentially dissipated locally during the process of flux jumping, thereby raising the temperature of the region to the critical temperature. Although the formation of the macrovortex structure has thus far been observed only in unclad wire of high pinning strength, it represents a potentially important mechanism for the concentration of energy during a collapse of magnetization.
Radiant-Energy Detection by Superconducting Films
Authors: Bertin C. L. , Rose K.
Published: February 2004

Superconducting films offer promise as fast, sensitive detectors at intermediate frequencies from millimeter waves to near infrared. The superconductor's marked temperature dependence of resistance about its transition temperature suggests that it would have a high responsivity as a bolometer, and operation at cryogenic temperatures suggests that a low NEP can be achieved. In addition, placing the sample in the liquid-helium bath should result in very short response times (τ) on the order of 10 nsec. Films have been studied with sheet resistances from 200 Ω/sq to > 1000 Ω/sq. These films achieve NEP ≈ 10−9 W·Hz−1/2 as bolometers with τ≈20 nsec. There is an enhanced microwave response, NEP ≈ 10−11 W·Hz−1/2 with τ = 20 nsec, which is especially pronounced for high-resistance films. This mode shows promise for use in the far-infrared and submillimeter region. The bolometer mode compares favorably with other bolometers.
Cryoelectric Content-Addressable Memories using Flux-Shuttling Nondestructive Readout
Authors: Keneman S. A.
Published: February 2004

The feasibility of large capacity content addressable memories (CAMs) using the flux-shuttling non-destructive readout (NDRO) technique and designed to handle the multiple response problem is investigated in this paper. A 4×4 CAM incorporating flux-shuttling cells and crossed-film cryotron logic circuitry was designed, fabricated, and tested. On the basis of the results of the tests performed, a highly efficient CAM was designed. The ultimate performance limits of the type of CAM designed are considered.
Frequency and Time-Domain Analysis of a Superconductive Coaxial Line Using the Two-Fluid Model
Authors: McCaa W. D. , Nahman N. S.
Published: February 2004

A miniature 80-ft superconductive coaxial line (Nb inner conductor 0.01 in. o.d., Pb outer conductor 0.034 in. i.d., polytetrafluoroethylene dielectric) has been analyzed in terms of the Gorter-Casimir two-fluid superconductivity model, considering the anomalous and classical cases (without relaxation effects) for the normal components. The dielectric and the metal flux trapping losses are assumed to be negligible. The 106−1012 Hz line attenuation and the 1–100 psec step responses are presented for the temperature range of 2.3°–10°K. Predictions are compared with the experimental results of several independent investigators. It is concluded that if the dielectric and the metal flux-trapping losses are negligible, then (1) The model provides bounds between which the responses of physically realized lines will fall, and (2) The normal conductivity of the conductors must be very good if the maximum bandwidth and minimum risetime are to be obtained.
Some Rate-Dependent Aspects of Flux Jumping in Nb-25% Zr Tubes
Authors: Rothwarf F. , Ford D. , Articola G. , Segal Gail P. , Kim Y. B.
Published: February 2004

A series of experiments have been performed on rate dependence of flux jumping in tubular samples of Nb-25% Zr using pulsed magnetic fields. The field H1 for which the first flux jump occurred was studied as a function of the rate of change of field Ḣ1 at the time of the flux jump. The range of Ḣ1 was 101<Ḣ1<105 kOe/sec. The data were taken using a specially designed inductance-resistance-capacitance circuit for producing pulsed magnetic fields.1 Silicon-controlled rectifiers were used as switching elements in the circuit. The dependence of H1 on Ḣ1 was studied for a series of tubular samples of varying wall thickness (10, 20, 40, and 100 mil). These samples had a 0.36 cm i.d. and were 3.2 cm in length. The results are shown in Fig. 1. It was found that H1 is constant for 101<Ḣ1<103 kOe/sec. The level of H1 in this region increases as the tube outer diameter increases. At some value of rate Ḣ1A, H1 begins to increase monotonically from its constant value and continues to do so up to the highest rates attainable with the present apparatus. Ḣ1A increases for thinner-walled samples and represents the point where the power density S at the surface of the tube begins to exceed S=0.8 W/cm2, the value which characterizes the onset of film boiling2 in the helium bath. The magnitude of the departure of H1 from its constant value is greater for the thicker-walled samples. At Ḣ1≃105 kOe/sec, H1 has increased by a factor of two to three over its value in the plateau region. These results can be interpreted in terms of a recently proposed criterion3–7 for the onset of magnetic instabilities in hard superconductors at a field Hfj=[−π3CJc/(∂Jc/∂T)]1/2,where Jc is the critical current density, ∂Jc/∂T the derivative of the critical current density with temperature, and C the volume specific heat of the superconductor. This function increases from zero at T=0 and rises to a maximum at T≃0.7 Tc for Nb-26% Zr. Thus, increased stability is predicted with rising temperature in the range 0<T<0.7 Tc. It is felt that the present results reflect a heating effect due to the poor heat transfer to the helium bath once the surface power density exceeds 0.8 W/cm2. This result points to the possible stability advantage to be had by operating superconducting devices made with type II materials at a somewhat elevated temperature.
Uses of High-Field Superconducting Magnets in Mössbauer Spectroscopy
Authors: Collins R. L.
Published: February 2004

Two areas of Mössbauer spectroscopy are benefited by the availability of high-field superconducting magnets: chemical-structures studies and research into magnetically ordered systems. The electric field gradient (e.f.g.) tensor at the Mössbauer nucleus is a measure of the distortion of the surrounding charge cloud from cubic symmetry, and this reflects the disposition of strong ligands. The most common Mössbauer nuclides are 57Fe and 119Sn. The e.f.g. tensor produces a splitting into two lines, but this does not yield the two parameters of the tensor. Upon addition of a magnetic field (30 Kg+), the remaining degeneracies are removed and a characteristic distortion ensues.1 This is the only method for extraction of the e.f.g. parameters from powdered samples. Theoretical and experimental examples of this technique will be displayed.
Measurement of the Surface Resistance of Superconducting Lead at 2.868 GHz
Authors: Hahn H. , Halama H. J. , Foster E. H.
Published: February 2004

The surface resistance of superconducting lead at 2.868 GHz was determined from a measurement of the unloaded Q of a TE011 cavity. The results were compared to the analytical expression for the surface impedance of pure superconductors in the Pippard limit, which was worked out by Abrikosov et al. Adequate agreement was found by writing the reduced surface resistance in the form of r=A rAGK+r0, with r0 being a residual resistance and A a constant. The best fit was obtained for A = 3.0, r0=2.2×10−5 and the gap parameter Δ(0)=1.97kBTc.
The Pinning Potential and High-Frequency Studies of Type-II Superconductors
Authors: Gittleman Jonathan I. , Rosenblum Bruce
Published: February 2004

The electrical impedance of type-II superconductors as a function of frequency is discussed in terms of a ``pinning potential.'' Many flux-pinning properties of the mixed state at any frequency can be treated in terms of such a potential and a simple phenomenological extension of the Bardeen-Stephen flux-flow theory. The form of the potential can be determined by various dc and high-frequency experiments. There is a critical frequency above which the impedance for subcritical currents becomes the impedance of the ``ideal'' mixed state. Much below this frequency the impedance is zero. This critical frequency is given by ω0=2πCρnα/φ01/2Hc2H1/2. At microwave frequencies, almost all effects of flux pinning are absent, and the properties of the ``ideal'' mixed state are observed even in the most strongly pinned material. Such measurements have been used to determine the effective mass of the flux tube.
High-Homogeneity Tape-Wound Superconducting Coil
Authors: Graham C. D. , Hart H. R. , Mains E. F.
Published: February 2004

A 100 kG superconducting magnet with field uniformity better than 1 part in 104 over a 1 cm diam sphere has been constructed from composite Nb3Sn superconducting tape. Data showing the performance of the coil are presented. The field profiles depend both on the magnitude of the field and on the field history; field uniformity is considerably reduced at low fields after an excursion to higher fields, because of trapped flux. Neither the field nor the field profile at a given current agrees with the values calculated for a copper coil of identical geometry. The discrepancy results from the anisotropic diamagnetic properties of the superconducting tape winding, and is semiquantitatively understood from an analysis based on the critical-state model. The construction of the coil is described, particularly the electrical connections between modules and the methods used to obtain field uniformity.
A Partially Stable 90 kG Composite Superconducting Solenoid
Authors: Lindenbaum Samuel D. , Becker Joseph F.
Published: February 2004

We have designed and constructed a superconducting solenoid, composed of an outer coil of A-25 (Nb-25 at.% Zr) wound on an inner coil of T-48B (Nb-48 at.% Ti). The solenoid has a ½ in. bore, a length of 4.5 in., and o.d. of 4.3 in., and produces an axial field of 90 kG with a maximum current of 25 A (H/I=3.6 kG/A). A discussion of coil-training effects is included.
High-Field Superconducting Magnet
Authors: Wurz W.
Published: February 2004

A large superconducting magnet has been built which makes it possible to maintain magnetic fields of up to 20 kG for several hours within a horizontal room-temperature work space of 10 cm diameter. The i.d. of the superconducting coil is 16.5 cm. High stability of the magnetic field has been achieved by winding laminated sheets, half-copper, half-stainless steel, between the layers of the coil. The current density is about 20 000 A/cm2. Access to the superconducting coil is provided by a new cryogenic seal, which makes the exchange and rearrangement of the coils possible, and, consequently, permits different field shapes.
Progress in Superconducting Beam Handling and Accelerator Structures Since November 1966
Authors: Kruger P. Gerald , Snyder J. N. , Sampson W. B.
Published: February 2004

At an earlier meeting1 a full-scale superconducting quadrupole which was under construction at Brookhaven National Laboratory was described. The results of tests2 on that quadrupole will be described here. Also, at the meeting a year ago calculations were reported concerning the possibility of constructing a FFAG field with Nb3–Sn, for a 200 BeV accelerator. Those calculations assumed a circular cross section for the geometry of the coil. Since then the calculations3 have been extended to include an elliptical4 cross section for dipole, quadrupole, and diquadrupole superconducting structures. One advantage for the elliptical geometry is that it reduces the cost of the necessary Nb3–Sn by about a ratio of (a+b)/2r where a and b are the semimajor and minor axes of the ellipse and r=a, the radius of the circle for the circular case. Results of the calculations for dipoles, quadrupoles, and diquadrupole fields in elliptical cross-section geometries will be presented. These can all be used in accelerator designs with separated function elements or with combined function elements provided the elements are not pulsed. Pulsing produces losses which are not well understood at present. Theoretically4 the turn distribution for a dipole-ellipse is given by N ∝ (a+b) {Δ[sin(θp)]}, where θp is the parametric angle for an ellipse and is related to the central angle of an ellipse by tanθ=(b/a) tanθp. N is the number of turns per Δθ angle interval and θ is the central angle of the ellipse. The field in the elliptical case for an actual turn distribution of Nb3Sn ribbon is calculated in a way similar to that for the circular case.5 Now, however, ρ is the distance from the axis of the ellipse to the surface of the ellipse at some central θ and the point P, at which the field is calculated is specified by a distance r from the axis of the ellipse to the point but at an angle θi, the initial angle at which calculation is commenced. Then the program calculates the field contribution at the point P from each ribbon, takes the vector sum, and calculates the resultant field. Pictures of a mock-up model will be used to illustrate the type of construction which might be used for a suitable FFAG coil configuration. Various coil configurations of large size including one suitable for a 1000 BeV accelerator will be discussed. Costs for the necessary Nb3–Sn for various coil configurations are estimated assuming present prices, sizes of ribbon, and critical current values. Calculations for bucking coils to produce effective cancellation of the magnetic fields outside of the structures also have been considered.
High-Efficiency Superconducting Homopolar dc Generators
Authors: Atherton David
Published: February 2004

The immediate requirement for flux pumps appears to be a design capable of producing 104 A at 100 W and high efficiency to energize large stabilized magnets. We have, however, successfully operated homopolar superconducting dc generators and also stabilized solenoids in helium vapor rather than liquid, and it appears reasonable to suggest that large magnets could be energized during the latter stages of cooldown, so that the power requirements for reasonable energization times may be less than this. Two designs for homopolar superconducting dc generators will be discussed and their performances reported on; both use permanent magnet and iron rotors. The first design has produced 1600 A and 20 W. The most significant feature of the design is the magnetic circuit used to generate in excess of 6 kG in the normal spot crossing the sheet. This enables reasonable power to be generated in a low-speed (300 rpm) design. There are, however, large losses attributed to sloshing of liquid helium. The second design is evolved from the first by enclosing the rotor in a vacuum container; special cooldown procedures are required. High efficiency is obtained (measured in excess of 100% at the time of writing, the excess being due to measurement error rather than a triumph over the second law of thermodynamics!). The design promises heavy currents and reasonable power outputs. The design allows topological manipulations of the sheet to allow adjustments of the current-carrying capacity of the sheet or output voltage (and hence power) or output impedance (and hence efficiency).
Automatic Superconducting Switches
Authors: Laquer H. L.
Published: February 2004

The design and testing of a simple automatic superconducting switch or current limiter is described. Reliable switches having switching currents up to 670 A at 4°K have been built for use with a flux pump.
Problems in Designing Nonaxisymmetrical Superconducting Magnet Systems
Authors: Gauster W. F. , Coffey D. L.
Published: February 2004

At the Oak Ridge National Laboratory a large superconducting Bmin facility ``IMP'' (Injection into Microwave Plasma) is under construction. In order to build this system in an economical way, optimization methods for designing nonaxisymmetrical magnet systems have been developed. A discussion of these new design methods and of a series of unusual technological tests (for instance, experiments with superconducting test coils in cusp arrangement, exposed to external magnetic fields) is of general importance for the design of nonaxisymmetrical superconducting magnet systems.
Superconductive Microwave Meander Lines
Authors: Gandolfo D. A. , Boornard A. , Morris L. C.
Published: February 2004

This paper describes the fabrication, testing, and application of superconductive meander lines. The meander line is a slow-wave circuit which consists of a number of parallel tapes uniformly spaced and connected in series. The tape length is equal to one-quarter wavelength for the center frequency of the line. Microwave signals propagate in the form of TEM waves along the tapes. Interaction between adjacent tapes leads to a dispersion curve which is in general nonlinear, but which possesses some nearly linear regions. Thus, either linear or dispersive performance may be obtained. Meander lines find application as slow-wave elements in low-noise amplifiers such as traveling wave masers, as linear delay lines, or as dispersive delay lines in pulse-compression filters. The present investigation deals with lead meander lines fabricated by the photoetch process. Reflection from and transmission through the meander line were measured by conventional techniques at room temperature, 77°K, and 4.2°K. A striking decrease in insertion loss was noted when the line became superconductive. Insertion loss and VSWR of the superconductive line were rapidly fluctuating functions of frequency, generally increasing and decreasing together. At numerous frequencies between 4.0 and 4.5 GHz, a 30 nsec delay line had insertion loss of 0.5 dB and VSWR less than 2. (Most of this insertion loss was probably due to nonsuperconductive input lines inside the cryostat.) For a comparison, a copper meander line, identical in geometry to the superconductive line was operated at 4.2°K, and was found to have insertion loss that was as much as 20 dB greater than that of the superconductive line. Meander delay lines should readily lend themselves to operation a closed cycle refrigerator.
A High-Sensitivity Superconducting Detector
Authors: Zimmerman J. E. , Silver A. H.
Published: February 2004

Weakly connected superconductors can function as very low power sensors by utilizing the coherence properties of superconductors. One configuration which has produced significant success incorporates a superconducting point contact and a small resistance element in a low-inductance superconducting circuit. When the critical supercurrent (ic) of the point contact is of the order of φ0/L, (φ0=h/2e, L=inductance of the ring), that circuit exhibits coherent quantum behavior. Used as a parametric amplifier, it has been used to measure ultralow voltages, limited by the thermal noise fluctuations in the resistance element R. The voltage sensitivity is approximated by 8kTR/φ0, where k is Boltzmann's constant and T the absolute temperature of the resistance R. We have measured 4×10−16 V across 1.7×10−10 Ω at 4.2°K with a signal-to-noise ratio approaching 10. The operation of this sensor follows from the Josephson oscillation of voltage-biased superconducting point contacts. A dc voltage V0 across the resistance of the circuit produces an oscillating current at the angular frequency ωJ=2πV0/φ0 and an oscillating voltage across the point contact at ωJ. Upon supplying an rf or microwave current at frequency ωD, the point contact mixes ωJ and ωD. Using a homodyne detector at ωD of bandwidth larger than ωJ, we can retrieve the oscillation at ωJ. Direct measurement of this frequency ωJ yields V0. Such superconducting circuits may be useful as cryogenic thermometers, as thermal radiation sensors, and in low-level spectroscopy.