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Journal of Applied Physics

Issue 3 • Date Mar 1980

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Displaying Results 1 - 25 of 112
  • Issue Table of Contents

    Page(s): toc1
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    Freely Available from IEEE
  • Unidirectional magnetic gradiometers

    Page(s): 1267 - 1272
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    The response of a magnetic gradiometer to various types of magnetic sources is analyzed. Both semiquantitative and quantitative methods for determining the noise immunity and the spatial sensitivity of the gradiometer are presented. A new class of magnetic gradiometers with unidirectional properties is introduced. View full abstract»

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  • Generalized concepts in large‐scale laser isotope separation, with application to deuterium

    Page(s): 1273 - 1285
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    Optical laser isotope separation (LIS) and chemical process concepts are developed that arise in the large‐scale application of LIS processes with continuous process streams. The discussion applies to one‐photon, two‐photon and multiphoton LIS and the separation of a multiple‐component mixture. It is partly based on a novel reaction chamber which provides throughout its volume a constant energy fluence or power density. Concepts discussed are the isotopic depletion, isotopic and process selectivity, isotopic abundances, abundance quotients α* and β*, the photon utilization factor, and the material balance equations in the LIS plant. An expression is derived that gives the total process cost per unit separated product in terms of the isotopic selectivity, the isotopic depletion, and the natural abundance of the desired species. A total‐cost minimization is carried out which gives the optimum isotopic depletion of the process as a function of isotopic selectivity, photon cost parameters, and parameters characterizing the photochemical reaction and the feed costs. The expressions are applicable to ’’closed cycle’’ as well as ’’once through’’ feed processing. The results are used to estimate the cost per kg of heavy water as a function of laser efficiency and isotopic selectivity, for a formaldehyde‐based deuterium separation method. View full abstract»

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  • Photoionization cross sections, electron‐impact inverse mean free paths, and stopping powers for each subshell of silvera)

    Page(s): 1286 - 1289
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    Using the Herman‐Skillman potentials and bound wave functions for each subshell of silver, we have computed the continuum wave functions, and subshell‐by‐subshell photoionization cross sections with photoelectron energies up to 10 keV. Applying a relationship between photoionization and electron impact ionization, we have obtained inverse mean free paths and stopping powers, again by subshell, for electrons penetrating through silver. The maximum electron energy considered is 100 keV. For the total photoionization cross section, comparison of our work with experiment shows excellent agreement for photon energies down to 100 eV, below which solid‐state effects should be included. Theoretical total inverse mean free paths, being strongly dominated by contributions from 4d electrons, are in good agreement with data around 1 keV, but about a factor of 2 larger at energies below 100eV. Our stopping power is in good agreement with other theoretical work above 400 eV and approaches the relativistic Bethe formula above 10 keV. Range is also computed and is in good agreement with other theoretical work. View full abstract»

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  • Transition radiation as a source of x rays

    Page(s): 1290 - 1293
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    The generation of transition radiation resulting from the passage of relativistic charged particles through a periodic structure can produce intense and highly directional x rays. An optimization analysis accounting for material properties, multiple scattering, and statistical random errors in foil thicknesses is used to design an x‐ray emitter. The results demonstrate that transition radiation sources can be comparable in brightness to synchrotron emitters. View full abstract»

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  • Emission of particles from a charged sphere into a magnetic field

    Page(s): 1294 - 1296
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    Trajectories of particles emitted normally from a charged sphere into a constant magnetic field are examined. Calculations are made which separate such trajectories into two classes, those which return to the sphere and those which do not. View full abstract»

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  • Photorefractive effects and light‐induced charge migration in barium titanate

    Page(s): 1297 - 1305
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    We propose a new theoretical model for the light‐induced migration of charges which mediates the ’’photorefractive effect’’ (light‐induced refractive index change) in barium titanate and other crystals. We also present experimental results of various effects of this light‐induced charge migration in a single‐domain crystal of barium titanate, specifically, (1) energy transfer between two intersecting optical beams, (2) optical four‐wave mixing and optical‐beam phase conjugation, (3) erasure of spatial patterns of photorefractive index variations, and (4) photoconductivity. The theoretical model predicts the observed dependences of these effects on (1) beam intensities, directions, and polarizations, (2) crystal orientation, and (3) on an externally applied dc electric field. Time dependences of transients as well as steady‐state magnitudes are predicted. In this model, identical charges migrate by hopping between adjacent sites, with a hopping rate proportional to the total light intensity at the starting site. The net hopping rate varies with the local electric potential that is calculated self‐consistently from the charge migration pattern. In barium titanate the charges are positive with a density of (1.90.2) ×1016 cm-3 at 514 nm. The origin of the charges and sites is at present unknown. The hopping rate constant determined from optical beam interactions is used to predict the observed photoconductivity of 1.3×10-10 cm Ω-1 W-1 at 514 nm. View full abstract»

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  • Models of high‐power discharges for metal‐Xe excimer lasers

    Page(s): 1306 - 1320
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    High‐power (∼108 W/l) discharges in metal‐doped Xe are modeled for typical metal atom densities of 1015–1017 cm-3 and Xe densities of ∼1020 cm-3, and electron densities of 1014–1017 cm-3 as appropriate for proposed excimer lasers. Na is used as a prototype species, while its properties are varied to indicate some of the changes that could result from the use of different metals. The model includes sixteen excited levels of Na, three ionic species, the excimer levels of NaXe, and Na2. The degree of ionization is determined by collisional multistep excitation and ionization of excited atoms versus dissociative recombination of electrons with Na+2. Steady‐state conditions in the positive column are calculated for typical gas temperatures of ∼0.06 eV and electron temperatures Te of 0.3–0.5 eV. The Na population distribution is largely Boltzmann at the electron temperature and the electron density is close to the Saha equilibrium value except at low electron temperatures and very high extracted laser powers. Useful gain and extracted powers of ∼10 MW/cm3 are predicted for the higher Te and Na densities, with the pulse width limited to ∼10-7 sec by gas heating. The model indicates that a metal which produces a deeply bound product state via dissociative recombination could yield a very efficient high‐power laser or gain cell. View full abstract»

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  • Simultaneous HF and DF laser excited by a ferroelectric ceramic capacitively coupled discharge

    Page(s): 1321 - 1324
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    With ferroelectric ceramic, principally BaTiO3 dielectric capacitive coupling electrodes, simultaneous HF and DF pulsed laser action has been obtained as a function of [H2]/([H2]+[D2]) in H2+D2+SF6 mixtures. The laser had a multiline output of 30 μJ for an efficiency of 5.4×10-4 at 120 pps. After 107 pulses there has been no degradation of electrodes or decrease in laser performance. Alternate H‐source gases of CH4, C2H6, and C3H8 had lower HF laser output than H2. View full abstract»

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  • GaAs homojunction rib waveguide directional coupler switch

    Page(s): 1325 - 1327
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    An electro‐optic directional coupler switch has been fabricated with a GaAs homojunction structure and tested at 1.15 μm. Each single mode guide is made in a n‐LPE layer grown on a n+ substrate, the p+ rib used to confine the light being obtained by Zn diffusion and chemical etching. A calculation of the losses in the planar approximation roughly yields the large values of attenuation measured for TE and TM modes (∼8 cm-1). By reversely biasing the ’’stepped Δβ’’ junctions with less than 17 V, more than 13 dB power isolation has been achieved for both switching states on a 6.3‐mm‐long device. Large improvements of the characteristics are expected from the additional design parameters provided by GaAs‐GaAlAs double heterostructure. View full abstract»

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  • Phonon‐assisted recombination and stimulated emission in quantum‐well AlxGa1-xAs‐GaAs heterostructures

    Page(s): 1328 - 1337
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    Extensive data are presented on various photopumped multiple‐quantum‐well AlxGa1-x As‐GaAs heterostructures, grown by metalorganic chemical vapor deposition, showing the variety of laser operation that can be observed one and two phonons below the lowest (n=1, n′=1′) confined‐particle electron‐to‐heavy‐hole (e→hh, n) and electron‐to‐light‐hole (e→lh, n′) recombination transitions. These experiments are performed on small cleaved rectangular samples that, because of two identifiable sets of orthogonal coupled modes, permit unambiguous identification of laser operation on LO‐phonon sidebands below the confined‐particle transitions. For a small number (two) of closely coupled (∼50 Å) GaAs quantum wells of size Lz ∼50 Å laser operation occurs on multiples of h̸ωLO from one phonon below transition n=1 (E1) to within a phonon or two of the L indirect band edge. AlxGa1-x As‐GaAs heterostructures with more coupled GaAs quantum wells readily permit observation of laser operation two phonons below the lowest confined‐particle transitions. Besides the use of size to reduce carrier scattering to lower energies in a GaAs quantum well, high Zn doping is used to screen and reduce the electron‐phonon interaction and thus reduce the carrier scattering to lower energies. The strong phonon participation in the laser operation of these quantum‐well heterostructures, which are not superlattices, demonstrates that the electron‐phonon interaction observed in this work is a true two‐dimensional effect (and not, e.g., the effect of Brillouin‐zone folding). An analysis and arguments are presented justifying this statement and, in addition, leading to the important conclusion that stimul- ated phonon emission is possible and might play (or likely plays) a role in the quantum‐well heterostructures of this work. View full abstract»

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  • Pulsed laser heating profile width and changes in total coupling with pulse length and pressure

    Page(s): 1338 - 1344
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    Thermal coupling involves the heat absorbed by a surface in air under irradiation by a laser beam. At high incident intensities plasmas are ignited and there is an enhancement of total thermal coupling. Part of this occurs as some energy is transferred to the region outside of the laser beam spot because of plasma growth with increasing energy. The diameter of disk‐shaped targets was used as a parameter in one set of experiments in order to determine the width of the radial heating profile. Second, when the axial temperatures of thin targets were measured as a function of time, there was a relationship for those and the assumed radial heating profiles which allowed the use of the Laplace inversion technique to obtain their widths. When the laser pulse length was varied over the range 1–25 μsec, the thermal coupling peak and threshold showed only small changes. No real optimization was seen when an initial spike was present. Thermal coupling at 0.1 Torr air pressure was found to lack the normal 1‐atm enhanced regime. No abrupt intensity or fluence threshold was seen for either aluminum or titanium and the variation of thermal coupling with fluence was material dependent. Total thermal coupling maxima at 0.1 Torr were lower than those seen at 1 atm. View full abstract»

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  • Characteristics of a TEA CO2 laser preionized by ultraviolet light

    Page(s): 1345 - 1350
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    The discharge characteristics of the main electrode both in low‐voltage and in high‐voltage regimes at 1 atm pressure are described. The number of initial electrons effective in the main gap discharge depend on the polarity of the main electrode. The photoemission of the electrons from the cathode situated near the uv source is very large and accounts for about 70 –80% of the total electrons. Also described is the influence of additive gas on both the electrical discharge and laser output power. View full abstract»

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  • Development of highly repetitive phosphate glass lasers

    Page(s): 1351 - 1356
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    High repetition rate operation of glass lasers up to 30 Hz has been successfully achieved for the first time by developing and using new kinds of phosphate glass. The key point of the success is to make the variation of optical path measured at any distance from the center of the glass rod nearly equal to zero by canceling out the effect of the temperature change of the index of refraction with that of the thermal expansion. This provides a new criterion on the glass design. The glass laser will not suffer any more from the disadvantage of the low thermal conductivity of the material, and its quality may become comparable with that of the YAG laser. The application of these kinds of glass into high‐power lasers has enabled us to construct a high repetition rate high‐power laser system (1 GW, 10 Hz), which will provide new applications to basic science and engineering. View full abstract»

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  • Stimulated Raman and Brillouin backscattering along a magnetic field

    Page(s): 1357 - 1359
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    The decay of a circularly polarized electromagnetic wave propagating along a magnetic field into a backscattered circularly polarized electromagnetic wave and a Langmuir wave or ion acoustic wave in a bounded plasma is analyzed. View full abstract»

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  • Vibrational kinetics in CO electric discharge lasers: Modeling and experiments

    Page(s): 1360 - 1369
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    Numerous models have been developed to predict the performance of CO electric discharge lasers. Although many comparisons of predicted and observed laser power have been made, few comparisons exist at the more fundamental level of predicted and measured CO vibrational population distributions. Such comparisons provide a critical test of the vibrational kinetic mechanisms assumed in the models and may help explain discrepancies between predicted and measured laser output. In the present study, a model of CO laser vibrational kinetics is developed, and predicted vibrational distributions are compared with measurements. The experimental distributions were obtained at various flow locations in a transverse cw discharge in supersonic (M=3) flow. Good qualitative agreement is obtained in the comparisons, including the prediction of a total inversion at low discharge current densities. The major area of discrepancy is an observed loss in vibrational energy downstream of the discharge which is not predicted by the model. This discrepancy may be due to three‐dimensional effects in the experiment which are not included in the model. Possible kinetic effects which may contribute to vibrational energy loss are also examined. View full abstract»

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  • Performance of a cw double electric discharge for supersonic CO lasers

    Page(s): 1370 - 1378
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    The results of an experimental investigation of a cw double discharge in supersonic CO mixtures are reported. Stable discharges in CO/N2 and CO/Ar mixtures, with a maximum energy loading of 0.5 eV/CO molecule, were achieved in a small‐scale continuous‐flow supersonic channel. Detailed measurements of the discharge characteristics were performed, including electrostatic probe measurements of floating potential and electron number density and spectroscopic measurements of the CO vibrational population distributions. The results of these measurements indicate that the vibrational excitation efficiency of the discharge is approximately 60%, for moderate levels of main discharge current. These experiments, on a small scale, demonstrate that the double‐discharge scheme provides adequate vibrational energy loading for efficient CO laser operation under cw supersonic flow conditions. View full abstract»

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  • Phase matching temperature variation of second‐harmonic generation in Li out‐diffused LiNbO3 layers

    Page(s): 1379 - 1384
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    Li out‐diffusion occurs in LiNbO3 during Ti in‐diffusion for waveguide fabrication. Phase matching temperature measurements of 1.06‐μm second‐harmonic generation are used to observe waveguide parameters. Undesired Li out‐diffusion effects can be minimized by using a high (over 49%) melt mole fraction of Li2O or by using a high Ti‐diffusion temperature. View full abstract»

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  • Growth of Gaussian instabilities in Gaussian laser beams

    Page(s): 1385 - 1387
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    We present a theory for the growth of a Gaussian perturbation superimposed on a Gaussian profile laser beam. This theory gives an exponential growth of the perturbation for small distances z traveled inside the nonlinear medium. For larger values of z, the growth is not exponential. The growth parameter α is defined and an analytical expression for this parameter is obtained. Our theory gives a smooth matching between the exponential growth of perturbations in a linearized instability theory and the sharp self‐focusing thresholds expected for smooth Gaussian profile laser beams propagating in nonlinear media. View full abstract»

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  • Magnetostrictive effects in evaporated nickel films

    Page(s): 1388 - 1393
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    We have made Ni films by electron beam evaporation for use as a variable delay surface acoustic wave (SAW) device. The films have been characterized by static magnetization and magnetostriction measurements as well as by measurements of SAW phase shift attenuation in applied magnetic fields. A unified theoretical treatment of the field dependence of all the measured quantities is developed, based on a free‐energy expression containing Zeeman, demagnetization, anisotropy, magnetoelastic, and elastic energy terms. We introduce an analytical technique by which a quantitative measure of a film inhomogeneity such as the variance of the uniaxial magnetic anisotropy field may be deduced from static field experiments. The dynamic field analysis establishes basic analytical expressions for phase shift and attenuation of SAW devices which may serve as a useful guidance to the design of practical surface acoustic wave devices. Reasonable agreement between experiments and theoretical predictions of these calculations is obstained. View full abstract»

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  • Technological aspects of cryogenic laser‐fusion targetsa)

    Page(s): 1394 - 1402
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    Most current laser‐fusion targets consist of hollow spherical glass shells which have been filled with a mixture of gaseous deuterium‐tritium fuel. Theoretical considerations suggest that optimum yields can be obtained from these targets if the fuel is condensed as a uniform liquid or solid layer on the inner surface of the glass shell at the time it is irradiated. In principle, this can be accomplished in a straightforward way by cooling the target below the condensation or freezing point of the fuel. In practice, cryogenic targets can appear in routine laser experiments only when the necessary cryogenic technology is reliably integrated into experimental target chambers. Significant progress has been made recently in this field. The authors will discuss the scientific basis and the various technological features of a system which has allowed the successful irradiation of uniform solid‐fuel‐layer targets. View full abstract»

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  • On the use of the photoacoustic effect for investigating phase‐transitions in solids

    Page(s): 1403 - 1406
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    By exploring the dependence of the photoacoustic signal on the thermal properties of the sample, we demonstrate experimentally the usefulness of the photoacoustic effect for investigating phase transitions in solids. Special application is made for the case of Al‐doped VO2. We also describe the photoacoustic cell utilized and the complete characterization of the cell is done by using a piece of germanium in single‐crystal form which has well known thermal properties. View full abstract»

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  • MHD electrical potentials: Uniqueness of solutions to an extended class of elliptic boundary value problems

    Page(s): 1407 - 1409
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    The electrostatic potential describing the fields interior to a magnetohydrodynamic (MHD) power generating channel obeys a linear uniformly elliptic partial differential equation. The physically natural boundary conditions, however, do not fall within a class for which the solution has previously been shown to be unique. We discuss the boundary conditions appropriate to this situation and then generalize the discussion in Courant and Hilbert to demonstrate the uniqueness of the solution to an extended class of boundary value problems which includes the MHD channel as a special case. View full abstract»

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  • Gas‐flow‐controlled arcs for power collection

    Page(s): 1410 - 1413
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    Arcs in axial, free or constricted, gas flows were found to be promising as a means of contactless power collection for high‐speed ground transportation vehicles. Satisfactory current collection at speeds up to 250 km/h has been achieved with a 6‐mm gap. Higher speeds or larger gaps, however, may require partial aerodynamic shielding or magnetic trapping of the arc column. View full abstract»

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  • Mechanism of pulsed surface flashover involving electron‐stimulated desorption

    Page(s): 1414 - 1421
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    A simple model is proposed to explain how a breakdown avalanche of secondary emission electrons can lead to surface flashover when an insulator in vacuum breaks down a few nanoseconds after high voltage is applied. The case of a plane insulator–vacuum interface perpendicular to parallel electrodes is considered. Positive surface charging is assumed to occur almost immediately upon application of the voltage, and the attendent secondary emission avalanche is assumed to be maintained at saturation throughout the prebreakdown time delay by field emission from the cathode electrode. Bombardment of the insulator by avalanche electrons desorbs a cloud of gas, which is partially ionized as it drifts through the swarm of electrons in the avalanche. The electric field at the cathode end of the insulator becomes enhanced as positive ions accumulate, which in turn increases the field emission and the rates of gas desorption and ionization. This and other regenerative processes rapidly lead to breakdown. Field enhancement at the cathode end of the insulator and increased field emission are individually considered in determining the prebreakdown time delay, with very similar results. The model predicts a time delay of the order of 10 ns at E=10 MV/m, which is in reasonable agreement with experimental observations. The proportionality we have observed between the time delay and the inverse square of the applied voltage is also predicted, as well as a dependence of the time delay on the insulator length. The model may also account for the improved performance of insulators coated with certain metal oxides. View full abstract»

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Aims & Scope

Journal of Applied Physics is the American Institute of Physics' (AIP) archival journal for significant new results in applied physics

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Editor
P. James Viccaro
Argonne National Laboratory