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Journal of Applied Physics

Issue 7 • Date Apr 1985

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Displaying Results 1 - 25 of 45
  • Analysis of fluid light valve control layers for high‐definition television large picture projection

    Page(s): 2355 - 2360
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    Here, proceeding from a previous study, the deformation problem of a viscose fluid is analyzed under simplified conditions as regards the physical model. This viscose fluid is used as the light control organ in the light valve method of large‐screen television projection. By using simple approximate relationship, the important properties, such as dynamic behavior, frequency response, stability behavior, and resolution limit of the control layer and their dependence on material and system parameter are discussed in detail. The theoretical knowledge obtained from this analysis agrees to a large extent with the experimental results. View full abstract»

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  • Submillimeter wave absorption of n‐type InSb at low temperatures

    Page(s): 2361 - 2365
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    The absorption coefficient of two high‐purity n‐InSb samples is measured in the 10–40 cm-1 range using Fourier transform spectroscopy. The absorption coefficient spectrum is presented for both samples at 4.2 K. It is also shown for the lower resistance sample cooled to 2.2 K and heated by dc bias to elevated electron gas temperatures of 7.5 and 17.9 K. ac Drude theory gives rather poor agreement with experiment at 2.2 and 4.2 K but does much better when the sample electron gas is heated. In contrast, a simple quantum mechanical theory of absorption based on inverse Bremsstrahlung yields promising agreement at the lower temperatures although its applicability is questionable. The non‐Drudian absorption is shown to have a favorable effect on the performance of InSb hot‐electron bolometers. View full abstract»

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  • An application of a statistical model for the calculation of the logarithmic mean excitation energy of molecules: Molecular hydrogen

    Page(s): 2366 - 2369
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    A statistical model, the local plasma approximation, is considered for the calculation of the logarithmic mean excitation energy for stopping power of chemically bound particles by taking into consideration chemical bonding. This statistical model is applied to molecular hydrogen and leads to results that suggest a value for the logarithmic mean excitation energy of molecular hydrogen that is larger than the accepted experimental and theoretical values. View full abstract»

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  • Electromagnetic missiles

    Page(s): 2370 - 2373
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    Under transient excitation the energy transmitted by an antenna of finite size to a faraway receiver can decrease much more slowly than the usual R-2. Instead, by a suitable choice of excitation, this quantity can decrease as slowly as one wishes, under the physical restriction that the total energy radiated by the antenna is finite. Such cases of slow decrease are referred to as electromagnetic missiles. Another way of stating the result is that the product of the following two quantities can approach zero as slowly as one wishes: (1) the energy transmitted per unit area of receiver, and (2) the total area of the receiver. Examples are constructed when either one, but not both, of these quantities remains finite at infinite distance. View full abstract»

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  • Measurement of spatial correlation functions using image processing techniques

    Page(s): 2374 - 2384
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    A procedure for using digital image processing techniques to measure the spatial correlation functions of composite heterogeneous materials is presented. Methods for eliminating undesirable biases and warping in digitized photographs are discussed. Fourier transform methods and array processor techniques for calculating the spatial correlation functions are treated. By introducing a minimal set of lattice‐commensurate triangles, a method of sorting and storing the values of three‐point correlation functions in a compact one‐dimensional array is developed. Examples are presented at each stage of the analysis using synthetic photographs of cross sections of a model random material (the penetrable sphere model) for which the analytical form of the spatial correlations functions is known. Although results depend somewhat on magnification and on relative volume fraction, it is found that photographs digitized with 512×512 pixels generally have sufficiently good statistics for most practical purposes. To illustrate the use of the correlation functions, bounds on conductivity for the penetrable sphere model are calculated with a general numerical scheme developed for treating the singular three‐dimensional integrals which must be evaluated. View full abstract»

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  • Unipotential electrostatic lenses: Paraxial properties and aberrations of focal length and focal point

    Page(s): 2385 - 2401
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    An experimental study of electrostatic electron lenses as a function of geometrical and electrical parameters is described. The lenses are of the symmetrical three‐electrode unipotential type. The parameters are the thickness of the center electrode and the interelectrode spacing, both relative to the center electrode aperture diameter, and the ratio of lens voltage to cathode voltage. The lens properties are characterized in terms of the focal length and focal distance, and the spherical and chromatic aberrations of these quantities. In general, the principal surfaces of a lens are not plane, and the aberrations of focal length and focal distance are not the same. Expressions are derived relating the focal length and focal distance aberrations to the spherical and chromatic imaging aberration coefficients Cs and Cc, and the magnification aberrations. The advantages of formulating the lens properties in terms of focal length and focal distance and their aberrations, and the usefulness of the data presented here, are illustrated with several examples. View full abstract»

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  • Monitoring the growth of nonuniform gratings written holographically by Gaussian laser beams

    Page(s): 2402 - 2405
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    Interfering Gaussian laser beams are sometimes used to photochemically etch gratings into materials; the gratings which result are nonuniform since the groove depth varies over the grating cross section. The grating growth can be monitored in real time by observing the diffraction of the writing beams into their various orders. In order to relate the maximum groove depth to the observed diffraction efficiency, the grating nonuniformity must be accounted for. We present a short calculation which does this. The results are checked with measurements on actual gratings and good agreement is obtained. The results can help to interpret previously published measurements. View full abstract»

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  • Multidimensional modeling of transverse avalanche laser discharges: Applications to the HgBr laser

    Page(s): 2406 - 2423
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    Geometrical considerations are important with respect to the stability and efficiency of avalanche electric discharge lasers. Parameters such as the electrode contours and the distribution of preionization electrons affect excitation rates through the relative values of the local electric field, local depletion of initial species, and through the response of the discharge circuitry to spatially dependent conditions within the plasma. Constriction of the discharge and subsequent impedance mismatch of the discharge to the pulse forming line result from these spatial nonuniformities. In this paper geometrical effects in the mercury bromide electric discharge laser are examined by comparing the results from a multidimensional discharge and kinetics model with experimental observations. The code models electron and heavy particle kinetics and laser intensity in time and one spatial dimension: parallel to the electrodes and perpendicular to the optical axis. Quantities whose spatial dependence is perpendicular to this dimension, such as the contours of the electrodes, are accounted for through their impact on the local electric field. HgBr laser efficiency and spatial characteristics are examined as a function of stored electrical energy, the impedance of the pulse‐forming circuitry, electrode contours, and profile of the preionization electron density. Parasitic discharges within the gas, but exterior to the optical cavity, are examined as a mechanism through which the magnitude of the preionization density can effect laser efficiency. View full abstract»

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  • Thermal analysis of the catastrophic mirror damage in laser diodes

    Page(s): 2424 - 2430
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    A new thermal analysis of a catastrophic mirror damage in laser diodes is presented in the present paper. Three‐dimensional heat spreading and temperature dependence of a thermal conductivity is taken into account in the model. Both an active layer heating and a nonradiative recombination of carriers generated by absorbed radiation in an active area close to a facet mirror are considered as heat sources. The model is strictly correct for times ranging from 6.2 to 500 nsec for a standard stripe laser diode, i.e., for currents not higher than 4.5 A and not lower than 0.8 A. The analytical solution of the thermal conduction equation gives a dependence of a catastrophic‐degradation time, i.e., a permissible length of current pulses from a point of view of the catastrophic mirror damage, on the amplitude of the pulses for the standard stripe laser diode. View full abstract»

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  • Measurement of kinetic parameters relevant to the operation of an electron‐beam initiated atomic iodine laser

    Page(s): 2431 - 2436
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    Parameters relevant to the operation of an electron‐beam initiated atomic iodine laser, which would lase on the transition 5 2P1/2 (I*)→5 2P3/2 (I), have been measured by pulse radiolysis. Kinetic data for the parent compound quenching of I* were obtained by variation of the parent compound pressure at constant buffer gas pressure and observing I* decay rates versus time. Deactivation rates for perfluoroalkyl iodides were found to be much lower than for the corresponding alkyl iodides. Values obtained (in cm3 molec-1 s-1) are as follows: CH3I, (2.0±0.1)×10-13; C2H5I, (5.0±0.3)×10-13; CF3I, (8.8±0.3)×10-16; C2F5I, (9.7±1.0)×10-15; n‐C3F7I, (2.5±0.2)×10-15; i‐C3F7I, (1.7±0.1)×10-15; n‐C4F9I, (1.8±0.1)×10-14. The extent of population inversion was investigated by measuring initial excited state and ground state atomic iodine concentrations, which gives the branching ratio [I*]/[I]. Values obtained are as follows: CH3I, 2.7; CF3I, 3.8; C2F5I, 2.7; i‐C3F7I, 3.2; n‐C4F9I, 1.8. Comparing both branching ratios and lifetimes, it is seen that CF3I is the most promising candidate for an electron beam initiated atomic iodine laser. View full abstract»

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  • The effect of slight surface roughness on the scattering properties of convex particles

    Page(s): 2437 - 2454
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    The average scattering cross section for reflection of electromagnetic waves by an ensemble of randomly oriented convex particles is calculated. The particles are assumed to be large compared to the incident wavelength and to have a slightly rough surface such that the deviations from a smooth surface are smaller than wavelength. A perturbation expansion resulting from the Rayleigh–Rice approach leads to incoherent and coherent corrections to the zero‐order specularly reflected light. The quantity that in this limit completely determines the scattering behavior of the particles is the two‐point correlation function ρ(r) of the roughness structure. The main effect of roughness is a diminution of the reflected intensity for a wide range of the scattering angle θ and a shifting of the maximum of the polarization curve to larger θ. It turns out that measurements of the backscattered intensity in some wavelength limits provide information on the correlation function ρ(r). For conducting material, knowledge of the cross‐polarized backscatter intensity over a wide frequency range even enables one, at least in principle, to calculate the complete shape of the correlation function. View full abstract»

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  • Laser damage studies of polymer oxide coated silica optics at 350 nm

    Page(s): 2455 - 2459
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    A single layer SiO2 film with graded porosity applied from a polymer solution derived from organometallic silicon compounds on fused silica lenses provides antireflectivity over the transmission range covering the first, second, and third harmonics of neodymium glass lasers. The damage thresholds of initial samples at 350 nm (3ω) varied from less than 1 to 9 J/cm2. The main causes of this large variation in the damage threshold were determined to be substrate polishing, substrate surface cleaning, and the coating solution chemistry and processing, which, under certain conditions, leads to carbon formation during the heat treatment of the coating. View full abstract»

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  • Pyroelectric enthalpimetric detection

    Page(s): 2460 - 2463
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    A pyroelectric enthalpimetric calorimeter for the H‐O reaction was constructed using Pd and Au electrodes on a 2.5‐cm‐diam, 0.3‐mm‐thick LiTaO3 wafer. The counter (or common) electrode was of Au. The detector was placed in a chamber and various gases were admitted at pressures slightly above atmospheric. 100 ppm H2 in N2 was used as the hydrogen source and pure O2 as the oxygen source. A thermal signature was obtained on the Pd electrode that was confirmed by a thermocouple attached to the pyroelectric calorimeter. A delayed thermal signature was observed on the Au electrode resulting from thermal diffusion in the LiTaO3 substrate of the heat generated at the Pd electrode. The results on the H‐O reaction are in general agreement with the literature on Pd metal oxide semiconductor (MOS) structures. The results demonstrate the utility of the pyroelectric calorimeter. View full abstract»

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  • Piezoresistance response of longitudinally and laterally oriented ytterbium foils subjected to impact and quasi‐static loading

    Page(s): 2464 - 2473
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    Resistance change measurements in ytterbium foils, encapsulated in a polymethylmethacrylate matrix and subjected to impact loading (to 25 kbars) and quasi‐static triaxial loading (to 4 kbars), are presented. Two gauge orientations were examined. Residual resistance measurements were obtained for impact experiments. Both sets of experiments confirm the overall validity of the theoretical analysis described in Ref. 5. In particular, the crossover in resistance change‐stress curves, predicted at low stresses, was measured. The present results show that gauge resistance measurements cannot always be uniquely related to a particular matrix stress component. Restricting the use of ytterbium foils to 20 kbars is inferred from the experimental results. The residual resistance data depend on both foil and matrix response. Finally, analysis of the resistance measurements, along the two orientations in the impact experiments, shows that polymethylmethacrylate retains significant material strength to 25 kbars. View full abstract»

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  • A stretched betatron

    Page(s): 2474 - 2476
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    We have demonstrated trapping, confinement, acceleration, and dumping of electrons in a betatron which has a geometry similar to the Astron. 76±22 nC were accelerated to 800±100 keV. This corresponds to a circulating electron current of 60 A. The betatron has a 6‐cm orbit radius and is 1 m long. Electrons are injected tangentially from a 70‐kV, 15‐A field emission diode. A toroidal field roughly equal to the betatron field is used. View full abstract»

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  • Thermally stimulated scattering in plasmas

    Page(s): 2477 - 2481
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    A theory for stimulated scattering of a laser beam is formulated where the dominant nonlinearity is the ohmic heating of the plasma. The analysis is carried out with particular reference to experimental investigations of CO2 laser heating of a linear discharge plasma. In the conditions characterizing this experiment local heat conduction is of little importance and the dynamic evolution for the electron temperature is dominated by heating and energy exchange with the ion component. These features are incorporated in the analysis. The resulting set of equations gives a growth rate and characteristic scale size for the filaments, which agrees well with the experimental results. View full abstract»

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  • Plasma properties and thin‐film formation in a pulsed electromagnetic inductive silane discharge

    Page(s): 2482 - 2485
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    A new approach to prepare thin films in a pulsed inductive silane discharge is attemped. The time‐resolved spectral lines qualitatively clarified dynamics of decomposition process in the pulsed silane discharge and the local thermodynamic equilibrium in this discharge was investigated by the population density distribution estimated from excitation temperature measurement. The films had peculiar deposition patterns with good adhesion due to electromagnetic effects and main group of infrared spectra was the SiH3 complex. View full abstract»

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  • Arc expansion in xenon flashlamps

    Page(s): 2486 - 2500
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    Plasma arcs in large diameter (d≫1 cm) xenon flashlamps often do not completely fill the bore of the discharge tube. The arc is usually initiated on one side of the discharge tube, adjacent to the ground plane, and the fraction of the discharge tube filled with plasma varies as a function of axial location. A model is presented that describes, from first principles, arc expansion in xenon flashlamps. The model simultaneously solves a coupled set of one‐dimensional transport equations in different regions of the discharge tube to simulate two‐dimensional effects in hydrodynamics, electron kinetics, and radiation transport. Using this method, expansion of arcs initiated at arbitrary locations within the discharge tube can be studied. Arc filling fractions are found to decrease with increasing filling pressure of xenon, increasing diameter of the discharge tube, and decreasing stored energy in the discharge circuit. The arc filling fraction also decreases as the breakdown filament moves away from the axis of the discharge tube and towards the wall. Arc expansion is slowed and ultimately halted by a lowered E/N (electric field/gas density) in the gas exterior to the arc, rapid conversion of atomic ions to molecular ions and their subsequent recombination, and by the efficient manner in which radiation dissipates energy which might otherwise be available for thermodynamic expansion of the arc. The asymmetric expansion of the plasma arc results in asymmetric heating of the inside wall of the discharge tube, also calculated in the model. The growth of the plasma arc is also found to be in part responsible for changes in the spectrum of radiation emitted from the arc for various angles of observation. This effect results from wavelength‐dependent absorption coefficients in the plasma. View full abstract»

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  • Point explosion simulation by fast spark discharges

    Page(s): 2501 - 2506
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    It is shown here that the late hydrodynamic effects of fast spark discharges (dI/dt∥t=0≥1010 A/sec), i.e., after the discharge period, can be described by a point explosion simulation. The solution is not self‐similar since counterpressure has to be taken into consideration. This non‐self‐similar model is confirmed experimentally by shock wave position and velocity measurements. View full abstract»

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  • A new design concept for field distortion trigger spark gaps

    Page(s): 2507 - 2511
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    A common field distortion triggered spark gap utilizing geometric field enhancement at sharp edges usually operates in a cascade mode via the trigger electrode. A new trigger concept is proposed allowing strong field enhancement and direct breakdown between the two main electrodes. A test setup was designed to prove the feasibility of this concept. Experimental results on delay and jitter depending on percent breakdown voltage are presented. Best results achieved are a delay of 9 ns and a jitter of 2 ns at a self‐breakdown voltage of 15 kV. View full abstract»

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  • Time‐dependent behavior of the contraction regions of high‐pressure mercury arcs

    Page(s): 2512 - 2518
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    The paper describes an investigation of the time‐dependent behavior of the impedance of high‐pressure mercury lamps operated with pulsed current. The lamp impedance was obtained experimentally from electric as well as spectroscopic measurements, the latter also allowing a determination of axial impedance profiles. These measurements show that during current‐off the most significant part of the lamp impedance is located close to the electrodes, whereas during current‐on a more homogeneous impedance profile is observed. The arc channel close to the electrodes is found to be rather narrow and therefore its high impedance during current‐off can be reduced significantly by an additional current which is small compared to the average lamp current. View full abstract»

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  • Experimental study of the current penetration in a dense Z pinch

    Page(s): 2519 - 2523
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    We study the oscillations superimposed on an intense discharge feeding a dense Z pinch. The periods, measured in different conditions, allow the measurement of the plasma inductance. We show that the variation of this inductance is related to the current penetration in the plasma, the anomalous current distribution, and the superconductive behavior of a dense turbulent plasma. View full abstract»

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  • Optical properties of sputtered gold clusters

    Page(s): 2524 - 2528
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    Gold particles, sputtered at different substrate temperatures, have been analyzed by both optical absorption and transmission electron microscopy techniques. Changes in the absorption band depending on the preparation of the films was observed and related to the bulk properties of the material. However, the bandwidth was nearly independent of particle concentration. It is suggested that a high concentration of microscopic and macroscopic defects, not observed in clusters grown with other techniques, is probably responsible for this behavior. View full abstract»

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  • Applicability of the microwave cavity perturbation method for conductivity measurements on carbon fibers

    Page(s): 2529 - 2531
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    A microwave cavity perturbation technique has been used to measure the resistance per unit length of carbon fibers. These materials and other test materials used in these measurements such as copper, evenohm, and tungsten, have much higher conductivities than polymeric materials which have been investigated previously with this technique. The 1500–5000‐Ω resistance of the carbon fibers lay in the region near the limit of applicability of the perturbation technique, thereby affording a means of investigating this limit. The condition Rs/Zc≫2 where Rs is the sample resistance and Zc is the characteristic impedance of the cavity, appears to be a useful condition for estimating the limit of applicability. View full abstract»

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  • Photocarrier transport and trapping processes in doped polyethylene terephthalate films

    Page(s): 2532 - 2537
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    The radiation‐induced conductivity of polyethylene terephthalate film is modified by doping this polymer with electron acceptor molecules. Detailed results of x‐ray induced thermally stimulated current and photoconductivity experiments are reported for one dopant, 2, 4, 7‐trinitro‐9‐fluorenone. At dopant concentrations≲1019 molecules/cm3, this impurity acts as a deep trap for photocarriers, radically decreasing the radiation‐induced conductivity of the doped film. The kinetics of photocarrier trapping in a host material exhibiting dispersive transport is discussed. At dopant concentrations ≳1020 molecules/cm3, the radiation‐induced conductivity increases due to the onset of photocarrier transport among the electron acceptor molecules. Utilizing the physical processes discussed in this work, it is possible to develop dielectric materials displaying selected levels of radiation‐induced conductivity. 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