bluuebunny/crossref_metadata_2025 huggingface dataset:
Nonlinear Oscillations in a Weakly Ionized Plasma
Authors: Willett Joseph E. , Bogdanor John L.
Published: February 2004

A perturbation theory is developed for a nonlinear longitudinal monochromatic wave in a weakly ionized plasma. The effect of collisions between electrons and neutrals is treated by means of a relaxation term in the Boltzmann-Vlasov equation. With the distribution function developed to all orders in the amplitude E0 of the oscillating electric field (assumed exponentially damped sinusoidal in form), a constant damping decrement λ is indicated for two cases: Either (i) λ must be negligible or (ii) the collision frequency vR of the resonance electrons must be negligible in the calculation of the distribution function of the resonance electrons in terms of the electric field. With the nonlinear correction developed to lowest order, the Landau damping decrement is reduced from its linear value γk0 by the factor 1−⅛(Ω4/vR4) for case (i), and by the factor 1−116[Ω4/(|γk0|+v̄)4] for case (ii). Here v̄ is the collision frequency appropriately averaged for direct collisional damping, Ω=(eE0k0/m)1/2 is the frequency of oscillation of the resonance electrons, k0 is the wavenumber, and e and m are the electronic charge and mass, respectively.
Nonlinear Interaction of a Microwave Field with a Plasma Slab in a Waveguide
Authors: Sforza P. F.
Published: February 2004

The nonlinear interaction of the dominant TE(10) waveguide mode of a fundamental frequency with a plasma slab in an infinitely long rectangular waveguide is analyzed utilizing an approximate technique. Explicit expressions for the first three harmonic components of the electron density and current density in the plasma are given. General expressions for the nonlinearly generated second-harmonic electric fields are derived. Numerical results show that the second-harmonic power generated in the TM(11) wave-guide mode is more than ten times that generated in the TE(11) waveguide mode.
Polarization of Thin Phosphosilicate Glass Films in MGOS Structures
Authors: Eldridge J. M. , Laibowitz R. B. , Balk P.
Published: February 2004

An investigation has been made of the two polarization processes that occur in phosphosilicate glass films formed on thermally oxidized silicon. The MOS capacitance-voltage technique and ac measurements of capacitance and loss angle were used to investigate the component of the polarization that is operative at lower temperatures (i.e., ≲120° C). The magnitude of this component was found to increase quadratically with the P2O5 concentration of the glass film. The concentration dependence and related data were used to formulate a model for the low-temperature polarization. This model suggests that rearrangement of nonbridging oxygen ions is responsible for the glass polarization. The rate of glass polarization that occurs at higher temperatures (≳200° C) was found to increase very rapidly with increasing P2O5 concentration. It is shown that the polarization can be markedly reduced for practical applications by lowering the P2O5 concentration.
Laser Production of Blast Waves in Low Pressure Gases
Authors: Hall Richard B.
Published: February 2004

It is shown by use of image converter photographs and probe data, that a Q-switched laser can produce blast waves at relatively low gas pressures (p≳0.6 mm Hg). The gas density and time dependence of the laser produced blast waves are found to be in close agreement with predictions based on ideal blast-wave theory. The data were taken using argon gas and laser delivered energies of approximately 0.5 J.
Peak Overpressure at the Foot of a Vertical Wall Facing an Air Blast
Authors: Dresner Lawrence
Published: February 2004

Practical results for the pressure amplification in the right angle between two planes can be obtained by combining the known solutions of the problems of oblique and normal shock reflection. The results so obtained have been checked for various shock strengths and two angles of incidence by experiments carried out in an explosively driven shock tube.
Source Scanning Type X-Ray Grating Spectrometer
Authors: Sawada Masao , Tsutsumi Kenjiro
Published: February 2004

A method is proposed for designing an x-ray spectrometer which can scan a limited wavelength region without movement of the concave grating or the electronic counter. Spectrometers widely used in the past have consisted of a fixed x-ray source and a movable counter. This method is to use a fixed counter and a scanning x-ray source to avoid undesirable movement of the sensitive electronic counter with its delicate and complex cooling system. The scanning x-ray source will be obtained by scanning the cathode ray beam of an x-ray tube. Two cases are analyzed theoretically; the Rowland circle (or Paschen-Runge) mounting, and a new arrangement which will be designated as the Sawada mounting. In the latter, the concave grating and the middle point of the scanning region of the x-ray source lie on the circumference of a circle with the same radius as that of the grating. It will be useful for constructing an x-ray grating spectrometer in the future. The length of a spectral line is shorter in the case of the Sawada mounting than in the case of the Rowland circle mounting, and the width of the line is broader in the case of the Sawada mounting than in the case of the Rowland circle mounting, owing to various abberations. The other advantages and disadvantages of the Sawada mounting are discussed in comparison with the Rowland circle mounting.
Tungsten Filament Life under Constant-Current Heating
Authors: Wilson Alan D.
Published: February 2004

The time required for the evaporation in a vacuum of a given amount of tungsten from a round tungsten filament heated by constant current is computed and experimentally verified. Tables of time vs normalized wire diameter for initial temperatures between 2000° and 2950°K, in steps of 50°K are presented. The results are valid for any initial wire diameter. The increasing electric field is also computed and tabulated. From the results, the ultimate life of the heated filament can be obtained. For example, at an initial temperature of 2400°K, it is found that when the mass of the wire has diminished 10%, the filament has attained about 98% of its life. The life of the wire under constant-current conditions is shown to be substantially less than that under constant-temperature conditions.
Anomalous Low-Temperature Specific Heat of Austenitic Stainless Steels
Authors: Zoller Paul , Decker P. R. , Dillinger J. R.
Published: February 2004

The specific heats of some stainless steels (AISI types 304, 304L, 310, and 321; and US Steel ``Tenelon'') have been measured between 1.7°K and 4.2°K. The results could not be fitted to the usual relation c=γT+BT3, but rather to C=A+γT+BT3. Moreover, the value of γ was extremely high. These results are discussed in terms of magnetic contributions to the specific heat.
Application of Quantum Superconductivity to a New Kind of Instrumentation
Authors: Mercereau J. E.
Published: February 2004

Under the proper circumstances voltage can be developed across a superconductor-this voltage V is the electromotive force induced by flux flow through the superconducting material. Superconducting thin-film techniques have been developed to assure that this flow is composed of individual flux quanta. These techniques usually involve inhomogeneous superconductivity; the inhomogenity either being artificially generated by flow through a constriction (as a Dayem bridge) or intrinsic to the material itself (as a deliberate spacial modification of electron density, Notarys). This resulting superconducting structure may be thought of as a current controlled flux valve-passing quanta φ0 at a rate v=Vφ0−1. These flux valves have been incorporated into various circuits leading to results similar to the behavior of a Josephson Junction. In particular, inserting a valve into a superconducting ring produces a device similar to the Josephson Junction interferometer-highly sensitive to magnetic field. These devices have been adopted into several operating instruments: a differential magnetometer with a one-second time constant and field sensitivity 10−10 G, a digital magnetometer counting increments of 10−7 G at 104/sec, and a voltmeter with a 10−3-sec time constant and sensitivity of 10−14 V.
Heat Transfer from Cylindrical Surfaces to Liquid Helium I
Authors: Efferson Kenneth R.
Published: February 2004

Heat transfer from a cyclindrical copper surface to liquid helium at 4°K has been measured. The sample consisted of a horizontally mounted 3 mm o.d., evacuated glass tube, 6.5 cm long, onto which was deposited a 2000 Å-thick copper film. It was heated by passing an electrical current through the copper film, and the temperature was monitored by a carbon resistor attached to the inside of the glass tube with varnish. In initial experiments, large temperature fluctuations were observed in the nucleate boiling region. These temperature fluctuations disappeared when the sample was either close to the liquid helium surface or when vertical walls were placed around the sample. The large temperature fluctuations were probably due to interactions of the convective flow of liquid helium with the boundaries of the system (Dewar walls, liquid surface, etc.). Similar temperature fluctuations were observed with a stainless steel sample of approximately the same dimensions. The peak heat flux obtained in the nucleate boiling regime was 0.98 W/cm2 for Cu and 0.66 W/cm2 for stainless steel.
Magnetoresistance of Copper at 4.2°K in Transverse Fields up to 100 kG
Authors: Benz M. G.
Published: February 2004

Many high-field superconducting systems make use of copper as part of the conductor configuration. Copper is present primarily because it combines low resistance with moderately high strength. As copper is exposed to high transverse magnetic fields its resistance increases. This resistance increase, or magnetoresistance, has been experimentally measured for several different grades and tempers of commercially available copper at 4.2°K and in the field range from 0 to 100 000 G. Zero field-resistance ratios varied from 60 to 1200 due to differing degrees of purity and amounts of cold work. The results of these measurements fit a normalized equation of the type which gives the fractional change in resistance as a function of transverse field and zero field-resistance ratio. The mechanical properties for these samples have been measured at room temperature and 4.2°K and the yield stress correlated with the resistance properties.
Critical Fields of Nb3Al1−xGex Alloys in Liquid H2
Authors: Foner S. , McNiff E. J. , Matthias B. T. , Corenzwit E.
Published: February 2004

Critical field HC2 vs temperature measurements have been completed in pseudo-binary,1 β-W structure, Nb3Al1−xGex (0.25≤x≤0.3) compounds with both contactless rf and transverse dc resistance measurements. Extrapolation of HC2(T) vs T above 20 kG yields TC=20.7°K. This TC is confirmed by zero-field specific heat measurements.2 HC2(T) vs T is almost linear from TC to 14°K, indicating that paramagnetic limiting is almost completely suppressed. HC2(14°K) ≃200 kG is observed for the TC=20.7°K material. Extrapolated HC2(0) values are deduced from the high-T data and compared with other physical parameters of the earlier TC=20.05°K superconducting materials.3 These new superconducting materials show potential compared with V3Ga and Nb3Sn, particularly in the liquid-H2 temperature range, if practical wires can be developed. A brief summary of the properties of these alloys was presented at the Eleventh International Conference on Low Temperature Physics4 and more detailed results will be published elsewhere.
Fabrication of Niobium Rf Cavities
Authors: Meyerhoff R. W.
Published: February 2004

A new process for fabricating niobium rf cavities for use in beam separators and linear accelerators is described. The best performance to date for a cavity made by this process was obtained with a TE011 mode cavity, operating at 11.2 GHz at a peak magnetic field of ∼500 G, for which Q≈2×1010. The principle advantages of the process described over other methods of fabricating niobium rf cavities are the following. First, a multiplicity of integrally connected cavities can be fabricated in a single joint free section. Second, the finished niobium cavities have a very high-quality surface finish which is produced without the need of mechanical or chemical polishing the cavities after fabrication. Third, the niobium in the completed cavity is ultrapure and thus has an extremely low normal state resistance at liquid helium temperatures. Fourth, a high thermal conductivity between the niobium on the inside of the cavity and the surrounding liquid helium bath can be obtained when the niobium is supported with porous tungsten.
Automatic Tuning of a Superconducting Cavity Using Optical Feedback
Authors: Stone J. L. , Hartwig W. H. , Baker G. L.
Published: February 2004

The resonant frequency of a superconducting cavity has been photodielectrically controlled by a feedback loop which contains an optical path. The cavity is an 870-MHz lead-plated quarter-wave reentrant structure. A 19 000 Ω·cm silicon wafer terminates the quarter-wave stub. The loaded Q is approximately 105 indicating the losses in the semiconductor dominate the superconducting surface resistance. The high Q provides a large phase error between the input and output voltage when the driving oscillator drifts or is modulated away from the very stable resonant frequency of the cavity. The error is processed by a wide-bandwidth phase detector that drives a gallium arsenide diode to follow the frequency excursion of the oscillator by photodielectric tuning. The 9000 Å light is the optical feedback signal which corrects the phase error. The system response typically provides a frequency deviation of ±50 kHz at a 0.2 MHz rate. Higher cavity frequency and purer semiconductor samples extend the deviation limits and shorter free-carrier lifetimes increase the speed of response.
Use of Superconducting Cavities to Resolve Carrier Trapping Effects in CdS
Authors: Hartwig William H. , Hinds James J.
Published: February 2004

The excellent frequency stability and cryogenic environment of a superconducting resonant cavity provides a sensitive method for observing trap-filling in CdS and similar materials. When used with thermally stimulated conductivity and dc photoconductivity, it is possible to solve for trap energy, capture cross section, density of trap states, and free-carrier lifetime. The technique is that used by Arndt, Hartwig, and Stone to observe optically induced changes in the complex dielectric constant by inertia forces on free carriers in Si and other indirect-gap semiconductors. Using TSC, pure CdS crystals showed very weak trapping effects and CdS: Al displayed electron traps at 0.52, 0.35, and a group at 0.265, 0.20, and 0.15 eV. A quenching, or hole trapping, level was seen at 1.6 eV below the conduction band. Deep electron trap densities were about 1015 cm−3 and the shallow set was about 1017 cm−3 each. Hole trap density was slightly in excess of 1018 cm−3. In CdS: Al, the photodielectric frequency shift of an 840 MHz cavity was proportional to the integral of photon flux, indicating the effect was caused by polarization of electrons in traps. Photodielectric data yielded an effective trap energy for the three shallow levels at 0.176 eV and a total density of 3×1017 cm−3. Density of trapped electrons is calculated from frequency change. Capture cross section for the shallow levels was about 10−14 cm2. Additional data from dc photoconductivity provide free-carrier lifetime and location of the electron Fermi level as a function of filled trap density. CdS:Al displays a ``tap'' effect which can erase the accumulated frequency change without warming.
Superconducting Antennas
Authors: Walker G. B. , Haden C. R.
Published: February 2004

A high-efficiency electrically and physically small superconducting antenna is reported. Antenna factors and matching circuit equivalents are developed. Theoretical predictions of efficiency and Q increase are derived on the basis of the superconducting surface resistance of lead. The antenna is a loop supsended through a ground plane and matched through a symmetrical coaxial cavity having a resonant frequency of 400 MHz. The loop is lead-plated and has an open area of approximately 1 cm2. Coupling to the system is achieved through a movable electric-field probe inserted into a slot in the wall of the matching cavity. A normal-state electrically small antenna of this size has a theoretical efficiency of less than 1% due to the fact that the loop resistance is much larger than the equivalent radiation resistance. Upon cooling to 4.2°K, the Q of the system increases to 20 000 from a room-temperature value of 156, indicating limitation by the radiation resistance only. The power radiation efficiency increases by a factor of 500, as measured by a fixed external receiving system. The VSWR of the system is maintained near unity in order to avoid the complications of mismatching.
Steady-State Performance of Multistrand Superconducting Compound Conductors
Authors: Gauster W. F.
Published: February 2004

A careful numerical analysis of a measured steady-state I-V (current-voltage) characteristic of a superconducting compound conductor with very fine strands is reported. The actual, nonlinear heat transfer from copper to the helium bath is considered. It is shown that in the current-sharing state the flux flow resistance of the superconductor, Rf, increases from zero to values much greater than the copper resistance, RCu. For Rf∼RCu, the superconductor current Is is a function of the field H, the voltage V, and the temperature T. For Rf»RCu, Is is a function of H, independent of V, and a linear function of T. The value of this function extrapolated to the bath temperature Tb is appreciably greater than the critical current. In the absence of flux jumps, the current-sharing state should be terminated by the steady-state condition dI/dV = 0. The actual ``take off'' (sudden transition into the normal state) observed, however, took place below the mentioned steady-state limit and before the transition from nucleate to film boiling. The new findings on the steady-state performance of compound conductors with very thin strands make essential modifications of the usual theoretical approaches necessary. It is shown how the new approach can be applied to compound conductors with thick filaments.
Theory of Electrical and Thermal Stability of a Superconductor in the Resistive State
Authors: Bindari A. El
Published: February 2004

The problem of electrical and thermal stability of a superconductor when carrying a steady-state transport current beyond the critical current has been analyzed for a bare superconductor. The following relation is derived ν = I(I−1)/1−θI2, where ν is the nondimensional voltage, I is the current in units of the critical current, Ic, of the superconductor, and θ is a parameter involving only the constants of the superconductor, and the heat transfer coefficient of the cooling medium. To have a reversible voltage current characteristic beyond the critical current, the parameter θ should be «1. Factors affecting this parameter are analyzed. The influence of this parameter on the propagation of a resistive front is discussed. Furthermore, the influence of a normal metal in good electrical and thermal contact with the superconductor is also included.
Coupling of (Ti–Nb) Filaments in a Superconducting Multifilament Composite Conductor
Authors: McInturff A. D.
Published: February 2004

Magnetization measurements were performed on variously prepared samples of multifilament (Ti–Nb) and Cu composites. All measurements were performed at 4.2°K, utilizing peak magnetic fields of 50 kG. The magnetization data were obtained in a manner similar to that described in earlier papers. The impetus for these particular measurements was the hypothesis of P. F. Smith in regard to the coupling of the small filaments of superconductors when they are embedded in a low-resistivity matrix such as Cu. This coupling, of course, is a function of ∂Ha/∂t, but for very long lengths (e.g., magnets), or even short lengths in the case of well annealed Cu, the time constant for the decoupling is extremely long. The coupling, it was suggested by Smith, could be reduced by increasing the resistance of the matrix material, and then twisting the composite, the equivalent of ``transposition'' of the individual filaments in a two-dimensional case. Various resistance layers are formed around each strand by the proper heat treatment of these multifilament composites. The filaments of the sample composites were found to be decoupled to varying degrees depending on applied magnetic field, sample length, and matrix resistivity.
Loss Measurements on Twisted Multifilamentary Superconducting Wires
Authors: Dahl P. F. , Morgan G. H. , Sampson W. B.
Published: February 2004

Measurements have been made on the energy losses produced during pulsed operation of several superconducting magnets. The coils were made from NbTi wires with solid and multifilamentary cores and pulsed to approximately 25 kG. A comparison was made between the losses produced in magnets made from the two types of conductor. The multifilamentary wire was tested first with the filaments straight, and then after the wire had been twisted with a pitch of 2 turns/cm. The losses for both the solid core and the untwisted multicore conductors were essentially identical and increased as the cube of the maximum field. The losses in the magnet made from the twisted material showed a drastic reduction at high fields with the loss varying as the first power of the maximum field.
Dynamic Stabilization against Flux Jumps for Structures Wound from Composite Superconductor-Normal-Metal Tapes
Authors: Hart Howard R.
Published: February 2004

Magnetic instabilities or flux jumps are responsible for many of the difficulties which have accompanied attempts to realize the full potential of high-field superconductors in magnet construction. Superconducting windings will be dynamically stable1 against flux jumps if the flux motion following a disturbance is sufficiently damped by a pure normal metal and if the superconductor is cooled well enough. The critical-state model has been used in developing the criteria for dynamic stability for a geometry approximating a magnet winding formed of a composite superconductor-normal-metal tape. It is found that there are two classes of dynamic stability requirements; both must be met: (i) The thickness of the superconducting layer in the composite tape must be smaller than a critical value which is derived.2 (ii) The winding formed from layers of a wide composite tape tends to exclude (and trap) the component of magnetic field which is perpendicular to the plane of the tape. The winding thus acts much like an anisotropic bulk high field superconductor and is likewise subject to flux jumps. The dynamic stability requirements for this composite structure are derived and discussed.2 Experimental results obtained with simple geometries are used to illustrate the main conclusions.
Use of a Superconducting Modulator in a Voltmeter with 10−12 V Resolution
Authors: Erdman Robert J.
Published: February 2004

Various methods of measuring low dc voltages at cryogenic temperatures have been used for purposes of investigating thermoelectric properties, resistive dependence on various parameters, superconducting phenomena, and other effects. These generally involve resolution on the order of 10−10 V or less and source resistances on the order of 10−6 Ω or less. A voltmeter suitable for this type of work is described. The instrument employs a superconducting parametric amplifier modulator described by Ries and Satterthwaite as a null sensing element in a feedback system. The modulator consists of a tuning fork which creates a time-varying mutual inductance between 2 sets of superconducting coils. The system equations and experimental data indicate that for low source resistance the accuracy and time constant of the voltmeter are essentially independent of source resistance. The noise is less than 10−12 V and the time constant is about 1.5 sec for source resistances of zero to 3×10−7 Ω. The modulator is well suited for production, and is quite insensitive to operating temperature and vibration. The voltmeter has reliability and repeatability typical of commercial room-temperature instrumentation.
Frequency Dependence of the Skin Depth in Superconducting Tin
Authors: Matisoo J.
Published: February 2004

The first measurements of the frequency dependence of the real part of the superconducting skin depth δr(v, t), in polycrystalline tin films, are reported using a new technique of measurement. The technique is similar to the usual cavity technique, except that the cavity in question is a rectangular Sn–SnOx–Sn Josephson tunnel junction. The cavity is self-excited via the ac Josephson effect; the resonant frequencies of the cavity are measured from which δr(v, t) is deduced. The measurements extend from 0≤v≤220 GHz and 0.5≤t≤0.952. The results are compared with Miller's calculations of δr for Sn, which are based on the Mattis-Bardeen theory and are found to be in agreement for t≤0.800. Above this temperature only qualitative agreement exists because the assumptions under which the calculations were made are here only partly satisfied.
Results of Tests on an 8.8 Tesla, 51-cm-Bore Magnet System
Authors: Lucas E. J. , Stekly Z. J. J. , DeWinter T. , Laurence J. , Coles W.
Published: February 2004

As an interim step in a program to build an 8.8 tesla, 51-cm-bore coil system, tests were run on four of the six individual modules that comprise the system. These tests were run on two modules wound with a stabilized reinforced Nb3Sn strip and two modules wound with a stabilized NbTi conductor. All of the modules were instrumented with heaters to determine their stability characteristics. Data were taken on the Nb3Sn modules both at 4.2°K and sub λ temperatures while the NbTi modules were run at 4.2°K only The performance of each module is discussed and compared with the performance predicted from data previously taken on small test coils.
Princeton Floating Multipole-Superconducting Ring Progress
Authors: File J. , Martin G. D. , Mills R. G. , Upham J. L.
Published: February 2004

Two independent concentric Nb3Sn rings carrying persistent supercurrents have been successfully levitated and stabilized against the slide instability. Isochoric Dewar operation has been demonstrated with the ring stabilized against both the slide and flip modes. The optical position sensors have been improved.
Eight-Foot-Diameter 30-kG Superconducting Magnet for the Brookhaven National Laboratory 7-Foot Bubble Chamber
Authors: Prodell A. G.
Published: February 2004

An 8-ft-diam air-core superconducting magnet wound with a superconductor-copper composite strip and capable of generating a central magnetic field intensity of 30 kG has been constructed at Brookhaven National Laboratory for a 7-ft bubble chamber. The magnet, which has an over-all height of 7.5 ft, is installed in an annular Dewar contained with the bubble chamber in a common vacuum envelope. Refrigeration for the magnet system is supplied by a 240-W helium refrigerator which can also be operated as a 60-liter/h liquefier. Details of the magnet, of the cryogenic system, and of magnet test results are given.
Pumped Helium Tests of a 51-cm-Bore Nb3Sn Magnet
Authors: Coles W. D. , Brown G. V. , Meyn E. H. , Schrader E. R.
Published: February 2004

Performance data are presented for tests of 51-cm-bore single coil and coil pair magnets operated at temperatures below 2°K. The magnet coils are wound of silver-plated, vapor-deposited Nb3Sn ribbon. Coil protection was provided by cross-layer shorting strips which also provide very limited stabilization. Each coil was designed to produce a 4.0-T center field. Results obtained at the low temperature are compared with those obtained at 4.2°K. System description and charging characteristics are included in the discussion.
Comparison of NbTi and Nb3Sn Quadrupole Magnet Systems for IMP
Authors: Coffey D. L. , Dunlap J. L. , Gauster W. F. , Horton J. L.
Published: February 2004

IMP (Injection into Microwave Plasma) is a fusion research device which will utilize a superconducting mirror-quadrupole magnet system. The NbTi mirror coils have been built and tested to short sample performance at 60 kG with 104 A/cm2 over-all. Designs of the quadrupole system using NbTi at 8500 A/cm2, or alternatively using Nb3Sn at 13 500 A/cm2, have been examined. Anticipated conductor costs favor NbTi, even though the volume of Nb3Sn is significantly less. However, some details of construction suggest that Nb3Sn would result in easier winding and possibly more reliable operation. Details of these comparisons were given, along with discussions of electrical stability and other technical differences in the two approaches.
Critical Temperature of Some Persistent Nb3Sn Coils
Authors: Duffy Thomas J. , Taylor Clyde E.
Published: February 2004

The critical conditions for several 14-cm-i.d., 2.5-cm-long Nb3Sn tape-wound coils was determined by increasing the temperature at constant current. The coils are inductively energized and have no external electrical leads. At lower temperatures, some of the critical temperatures are well below the ``short-sample.'' Some data for coils energized in a gaseous helium environment instead of in a liquid-helium environment are included. The implications of these results for design of levitated superconducting coils for plasma physics experiments is discussed.
Thin-Film Josephson Junctions Using Getter-Sputtered Niobium
Authors: Nordman J. E.
Published: February 2004

Thin-film niobium-niobium oxide-lead Josephson junctions were fabricated by use of getter sputtering for the Nb films, thermal oxidation of the Nb and evaporation of the Pb top films. These devices have characteristics comparable to good Pb–PbO–Pb devices. Temperature cycling is not harmful to the devices although some drift with time is seen in the characteristics. This drift is in the form of a conductance increase which has been measured as something between a factor of 1.1 to a factor of 20 over a period of months with the devices stored at room temperature.
Superconducting Magnet System for Intravascular Navigation
Authors: Montgomery D. B. , Weggel R. J. , Leupold M. J. , Yodh S. B. , Wright R. L.
Published: February 2004

A magnet system capable of guiding a ferromagnetic-tipped catheter through vessels in the body would greatly facilitate many medical treatments by providing access to parts of the body otherwise inaccessible except by major surgery. A rotatable iron magnet which has been employed in preliminary experiments with laboratory animals has indicated the feasibility of this technique, but a more advanced system is required for sophisticated medical applications. This paper describes a proposed design for a superconducting magnet system, with a 10 in. room-temperature access, which can exert an arbitrarily directed force of 200 times that of gravity on a tip anywhere within the head. A larger but otherwise similar system could be used for cathetarization of any part of the body.
Magnetic Suspension and Guidance for High Speed Rockets by Superconducting Magnets
Authors: Guderjahn C. A. , Wipf S. L. , Fink H. J. , Boom R. W. , MacKenzie K. E. , Williams D. , Downey T.
Published: February 2004

Guidance of high speed, 5000 m/sec rockets in evacuated tubes may be accomplished by eddy-current repulsion between moving superconducting coils (on the rocket) and stationary sheets of copper or aluminum. A method for determining lift and drag forces of moving rectangular coils over an infinite plane sheet is presented. The lift force saturates at high speeds v, the drag forces decrease as v−1/2 at high speeds, while the power dissipated increases as v1/2 at high speeds. Methods of determining lift and drag forces from scale models of finite systems are discussed. The forces as obtained from impedance measurements on scale models in circular conducting tubes are given.
Superconducting Critical Fields and Currents of Nb–Ti–N Thin Films in Continuous Magnetic Fields to 175 kG
Authors: Zbasnik Jon , Toth Louis E. , Shy Y. M. , Maxwell E.
Published: February 2004

Superconducting critical current-field measurements were made on a series of Nb–Ti–N thin film alloys (1000–8000 Å) prepared by reactive sputtering. Samples were measured in dc transverse and parallel fields. A pronounced maximum in the critical field occurs at approximately 20 mole % TiN. The extrapolated zero current value of Hc2 for this composition at 4.2°K was greater than 180 kG. Critical current densities in parallel fields are comparable to those of other high-field bulk superconductors (104 A/cm2 at 150 kG). Films 2000 Å thick and 0.1 in. wide carried a few tenths of an ampere in fields as high as 120 kG in the transverse fields and as high as 140 kG in parallel fields. In the transverse orientation the critical current density appears to be independent of thickness. Possible application of these materials to miniature thin-film solenoids is discussed.
High Field Properties of Pure Niobium Nitride Thin Films
Authors: Deis D. W. , Gavaler J. R. , Hulm J. K. , Jones C. K.
Published: February 2004

Thin films of the rocksalt structure (B1) compounds of the transition metals can be prepared by the method of reactive sputtering. By a refinement of this technique we have produced, for the first time, thin films of niobium nitride (NbN) with transition temperatures similar to those of the bulk materials (≳ 15°K). Films have been prepared on both hastelloy and fused silica substrates. Measurements of the normal state resistivity ρn yield values in excess of 200 μΩ·cm. Preliminary studies of the resistive transitions in these materials near Tc indicate that Hr(J), the field at which ρ/ρn=½, is independent of current density below 200 A/cm2 for films 1000–4000 Å thick. Identifying Hr(J) (J<50 A/cm2) with the upper critical field Hc2(t), the measurements indicate that Hc2(0) is significantly in excess of 200 kOe, and thus is substantially higher than the values obtained for the bulk material. These results are discussed in terms of the current theories of high field superconductivity with particular reference to the importance of Pauli spin paramagnetism and spin-orbit scattering in these materials.
Microwave Conductivity of Granular Superconducting Aluminum Films
Authors: D'Aiello R. V. , Freedman S. J.
Published: February 2004

Measurements of the transmission amplitude and phase of 20 GHz microwaves through superconducting granular aluminum films were made as a function of temperature from 4.2° to 1.75°K. The films were fabricated by vacuum deposition in the presence of controlled amounts of oxygen, and exhibited small grain size (∼40 Å) and enhanced transition temperatures of up to 2.86°K. The temperature coefficients of resistance (TCR) for the films ranged from slightly positive to almost zero, and for several films the TCR was negative. For all films measured, the microwave transmission data agreed very well with that computed by using the Mattis and Bardeen conductivity functions.
Suspension and Guidance of Vehicles by Superconducting Magnets
Authors: Coffey Howard T. , Chilton Frank , Barbee Troy W.
Published: February 2004

A magnetic suspension and guidance system for a very high-velocity (MACH 10) ground-based rocket sled facility has been studied.1 These results have been extended and applied to the study of a high speed (300 mph or greater) passenger train using this system. Suspension and guidance forces are obtained through the interaction of the magnetic field of an on-board superconducting magnet with eddy currents induced in a stationary metallic guideway. It was found that at high velocities the levitation force can be calculated as if the guideway had infinite conductivity. Thus, magnetic image methods can be used resulting in an image force FiFl=[μ0(ni)2πZ][2Z−(L2+Z2)1/2−(W2+Z2)1/2−(W2+Z2)1/2−(L2+W2+Z2)1/2W2+Z2(Z2)−(L2+Z2)1/2−(L2+W2+Z2)1/2L2+Z2(Z2)],where n is the number of turns, and i the current in a rectangular magnet of length 2l and width 2w at a suspension height z. At finite velocities, the resistivity of the guideway reduces the actual lift force Fl below Fi and produces a drag force Fd. Semiphenomenological expressions for these forces as a function of velocity and suspension height have been found for a magnet traversing a guideway of semi-infinite thickness and infinite lateral extent Fl/Fi=exp{−ζ[1+2(z/2l)3/2]−1}Fd/Fi=12(1−exp{−ζ[1+3(z/2l)5/2]−1])exp{−ζ[1+2(z/2l)3/2]−1},where ζ = (4π/μσvz)1/2, v is the velocity, and σ the conductivity of the guideway. Fl is approximately Fi when ζ < 1. These results are in semiquantitative agreement with experiments on normal magnets suspended over rotating metallic plates.2 Six superconducting magnets 0.5 m square, conducting 3×105 A - t are sufficient to levitate a 100-passenger vehicle weighing 60 000 pounds at velocities of 50–300 mph. These magnets would provide silent and smooth suspension and guidance, would require little maintenance, and would not require a high-precision guideway. The important results of this work are: (1) the lift force approaches Fi asymptotically; at speeds of ∼50 mph for magnets 0.5 m square, Fl≈0.8Fi. (2) The drag force at high speeds (>50 mph) decreases as v−1/2. (3) The suspension height is adjustable, 0.15 m seems reasonable. (4) Superconducting magnets of modest dimensions are practical and necessary for this application.
High Tc's in Ultra-Thin Films
Authors: Strongin Myron , Kammerer O. F. , Crow J. E.
Published: February 2004

Measurements are reported which show that the maximum Tc that has been reached in quenched ``soft-metal'' films is correlated with the maximum Tc computed from the analysis of McMillan,1 assuming an optimum value for the electron-phonon coupling. The enhanced electron-phonon coupling may be attributed to the lowering of the phonon frequencies, which would be caused by the small metallic grains and greatly increased surface area expected in these porous quenched films. Molecular dynamics calculations by Dickey and Paskin2 indicate that the changes in the phonon frequencies in small particles are large enough to account for the changes in Tc. Data are presented which show that the region with the greatest increase in Tc occurs within a few monolayers of the metal-vacuum, metal-insulator, or metal-metal surfaces.3 If similar, near monolayer structures of, high Tc, transition metal alloys can be made, temperatures above 25°K should be possible.
Upper Critical Field of Thin-Film Superconductors
Authors: Crow J. E. , Strongin Myron
Published: February 2004

Measurements have been made of the critical field of cryogenically deposited Sn films about 30 Å thick. In these films, where the mean free path is small and the spin-orbit scattering terms are significant, we find that the typical parallel critical field is ∼50 000 Oe at T/TC∼0.9. The excellent quality of the films is illustrated by the strong dependence of the critical field with angle. Near TC, H∥/H⊥ is about 100. If we assume that we can reasonably extrapolate to 0°K from our high-temperature data, we estimate fields well over 200 000 Oe. By using ever thinner films, and higher Z elements to increase spin-orbit scattering, we estimate that parallel critical fields near 1 000 000 Oe are obtainable.
Parametric Excitation of Phonons in Magnetic Materials by Means of Parallel Pumping
Authors: Schlömann E. , Joseph R. I.
Published: February 2004

The instability threshold of magnetoelastic waves has been calculated for arbitrary angle θ between the dc magnetic field and the direction of propagation for the case in which the frequency of the potentially unstable vibrations is either considerably smaller or considerably larger than the crossover frequency of the magnon-phonon dispersion diagram. Under these conditions, the vibrations are substantially pure phonons. When magnetic losses are neglected, the lowest critical field is of the order of hcrit≃ω/γστe, where σ=γb22/C44M0, ω is the pump frequency, γ the gyromagnetic ratio, τe the elastic relaxation time, b2 the magnetoelastic constant, C44 the shear modulus, and M0 the saturation magnetization. If half the pump frequency is less than the crossover frequency, the lowest threshold occurs for transverse waves that propagate at an intermediate (and typically rather small) angle to the dc field direction; if it is larger than the crossover frequency, the lowest threshold occurs for transverse waves having θ=π/2. The influence of magnetic loss upon the threshold is small for low-loss materials such as yttrium-iron garnet as long as the frequency of the unstable vibrations is not too close to the crossover frequency.
Dislocation Displacement Fields in Anisotropic Media
Authors: Hirth J. P. , Gehlen P. C.
Published: February 2004

The methods of Eshelby, Read, and Shockley [Acta Met. 1, 251 (1953)] and of Stroh [Phil. Mag. 3, 625 (1958)] for determining the anisotropic elastic field of a dislocation are reviewed briefly and several problems in their application are discussed. The Stroh method is extended to the dislocation displacement field. The resulting equations are applied to the 〈111〉 screw dislocation in cubic crystals.
Diffraction of X-Rays by Perfect Noncentrosymmetric Crystals
Authors: Holloway H.
Published: February 2004

Application of the Darwin-Prins theory to diffraction by a perfect noncentrosymmetric crystal shows that the ratio of integrated intensities for opposite faces is given by a relationship that is identical to that for a mosaic crystal. I(h,k,l)/I(h̄,k̄,l̄)=|F(h,k,l)|2/|F(h̄,k̄,l̄)|2. This result had been derived previously from the Ewald-von Laue theory. The relationship has been verified experimentally with measurements on nearly perfect crystals of GaSb and InAs.
Effect of Gradient Energy on Diffusion in Gold-Silver Alloys
Authors: Cook H. E. , Hilliard J. E.
Published: February 2004

Interdiffusion coefficients have been measured in Au–Ag alloys containing 18–50 at.% Au at temperatures from 200° to 260°C and over very short penetration distances (7–35 Å). The diffusivities were determined from the rate of decay of composition modulations in vapor-deposited films. The wavelength and amplitude of the modulations were measured by x-ray diffraction. From the dependence of the diffusion coefficient on the wavelength of the composition modulation, it was established that it is necessary (as proposed by Hillert and by Cahn) to allow for the effect of the ``gradient energy'' on the driving force when diffusion occurs in the presence of very sharp composition gradients. The observed gradient-energy coefficient was K=−(2.6±0.5)×10−6 erg·cm−1 in agreement with a value calculated on the basis of nearest-neighbor interactions. The interdiffusion coefficients (corrected to an infinite penetration distance) were in good agreement with an extrapolation of existing high-temperature data obtained with conventional diffusion specimens, thus demonstrating that the vapor-deposited film technique is capable of determining, with good precision, diffusivities of the order of 10−20 cm2 sec−1. This is some eight orders of magnitude smaller than those that can be measured conventionally.
Reflection and Refraction of Electromagnetic Waves by a Moving Uniaxially Anisotropic Slab
Authors: Kong Jin-Au , Cheng David K.
Published: February 2004

The reflection, refraction, and transmission characteristics of a plane electromagnetic wave incident from an isotropic medium, through a moving uniaxial slab, and into another isotropic medium are studied. The conceptual equivalence of a moving medium and a stationary anisotropic one with suitably defined permittivity and permeability tensors is used for the analysis. Complete field expressions are obtained in all three regions for an arbitrary orientation of the plane of incidence and an arbitrary angle of incidence. Special cases where the plane of incidence is parallel, or perpendicular, to the direction of slab motion are considered for both an incident E wave and an incident H wave. Some interesting numerical results are presented for the transmission coefficient at normal incidence when the moving slab is a magnetoionic plasma subject to a strong magnetic field and when the moving slab is a uniaxial crystal.
Analysis of the Electrode Products Emitted by dc Arcs in a Vacuum Ambient
Authors: Davis William D. , Miller H. Craig
Published: February 2004

We have examined the particles emitted radially by dc arcs drawn in a vacuum ambient on cathodes of several elements, as well as the axial (through-anode) ion flux from a copper cathode. The axial results for copper agree grossly with the radial copper results. Significant quantities of multiply charged ions were seen for all elements examined. All arc volt-ampere characteristics were positive in the range of currents observed: 30 to 250 A. We drew the following conclusions (normalizing energy in units of ion energy/ion charge): (1) The energy distributions for the various ions are similar, peaking at potentials well above the arc voltage. (2) The fraction of ions that are singly charged tends to increase with increasing arc current. (3) For a given element, as the degree of ionization increases the location of the ion-energy distribution peak shifts to lower energies. (4) For a given degree of ionization, the location of the peak tends toward higher energies for elements with greater arc voltages. (5) The location of the peak shifts toward lower energies as the arc current increases. A possible mechanism for the production of ions with energies corresponding to potentials greater than the anode potential lies in the theory of a potential peak near the cathode.
Steady-State and Transient Currents in Organic Liquids by Injection from a Tunnel Cathode
Authors: Silver M. , Onn D. G. , Smejtek P.
Published: February 2004

Experimental data are presented on the currents induced in organic liquids by injection from a tunnel cathode. The injection level was varied over a wide range resulting in almost no space-charge limitation to almost complete space-charge limitation. Results were different from that usually observed in solids, in that at low fields, the steady-state current was proportional to V2, while at high fields the current was proportional to V. By proper choice of electrode spacing and applied voltage, space-charge-limited current transients as low as 10−11 A/cm2 and 5 sec transit times were observed. A smooth transition between the electrode-limited and the space-charge limited regimes was achieved by varying the junction voltage that varied the injection level.
Optimum Shapes for Superconducting Solenoids
Authors: Atherton David
Published: February 2004

A technique is described for determining optimum, minimum volume shapes for uniform current density superconducting solenoids. The calculations include the effects of the dependence of current density upon the peak field at the coil winding and lead to coils which are significantly longer than those predicted by previous, field-independent optimization techniques.