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15 May 1991

Volume 69, Issue 10, pp. R21-7375

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Plasma points and radiative collapse in vacuum sparks

K. N. Koshelev and N. R. Pereira

J. Appl. Phys. 69, R21 (1991); http://dx.doi.org/10.1063/1.347551 (24 pages) | Cited 62 times

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This review discusses the intense x‐ray emitting regions, called plasma points, that appear in low‐inductance vacuum sparks and other high‐current discharges. Accurate x‐ray spectroscopy indicates the existence of two types of plasma points with different plasma parameters. One type is extremely small (∼microns), dense (∼1023/cm3), and hot (≳1 keV), while the second type is an order of magnitude less extreme. A dynamic model (Vikhrev 1982a) based on radiation cooling with axial outflow of plasma predicts a radiative collapse that is consistent with many features of the plasma points.
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52.80.Vp Discharge in vacuum
52.50.Lp Plasma production and heating by shock waves and compression

Ellipsometric study of variations on the interlayer in chemically prepared CuxS/CdS bilayer thin film

K. P. Vijayakumar

J. Appl. Phys. 69, 6771 (1991); http://dx.doi.org/10.1063/1.347666 (5 pages) | Cited 3 times

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Ellipsometry has been used to study the variation in the irregularities at the interface of CuxS/CdS p‐n junctions with annealing temperature. It is found that annealing of CdS films before chemiplating affects the structure of the layer at the interface drastically. The p‐n junction prepared using CdS films annealed at 200 °C is found to have a rather smooth interface; but, as the annealing temperature is increased beyond 200 °C, the interlayer becomes rough again. This is explained on the basis of the difference in reaction rate in and outside the grains. It is found that for CdS films annealed above 200 °C, the reaction rate inside the grains is greater than that in the boundaries. This is verified by measuring the CuxS thickness for different CdS films annealed at different temperatures using ellipsometry. In situ studies on CuxS/CdS junctions revealed during heating that the interface remains rather smooth for a larger range of temperature (up to 200 °C) if the junction is prepared by chemiplating CdS films annealed at 200 °C.
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73.40.Lq Other semiconductor-to-semiconductor contacts, p-n junctions, and heterojunctions
78.66.Fd III-V semiconductors
78.66.Hf II-VI semiconductors
07.60.Fs Polarimeters and ellipsometers
68.35.Fx Diffusion; interface formation

Energy deposition in direct nuclear pumped optical cavities

L. M. Montierth, W. A. Neuman, D. W. Nigg, and B. J. Merrill

J. Appl. Phys. 69, 6776 (1991); http://dx.doi.org/10.1063/1.347667 (13 pages) | Cited 4 times

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A two‐dimensional, time‐dependent model has been developed to determine the coupled fluid dynamic and charged particle transport behavior of a flowing gas, nuclear pumped laser cavity. Stationary results are presented for a typical cavity that uses a flowing He buffer gas pumped with charged particles produced by the 10B(n,α)7Li reaction. The boron is coated on fuel plates mounted parallel to the flow direction. The effects of changing the buffer gas inlet flow velocity and outlet pressure are investigated. For a fixed inlet velocity, the results presented show that the gradients of the charged particle energy deposition and density increase in magnitude primarily in the direction perpendicular to the gas flow, i.e., normal to the fuel plates, as the outlet pressure is increased. With increasing inlet velocity and a fixed outlet pressure, the density variations decrease, whereas the variations in energy deposition increase in the direction perpendicular to the flow and decrease in the direction parallel to the gas flow. For higher inlet velocity cases, the deposition is nearly one‐dimensional, varying primarily in the direction perpendicular to the flow. Qualitatively similar results can be found with an argon buffer gas and fission fragment pumping for similar charged particle ranges. In general, for a fixed cavity geometry similar charged particle energy deposition behavior can be obtained for different laser gases by adjusting the outlet pressure and charged particle source.
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42.55.Vc X- and γ-ray lasers
52.50.Dg Plasma sources
42.55.Lt Gas lasers including excimer and metal-vapor lasers
25.40.Fq Inelastic neutron scattering

Beam quality of direct nuclear pumped laser amplifiers

W. A. Neuman and J. R. Fincke

J. Appl. Phys. 69, 6789 (1991); http://dx.doi.org/10.1063/1.347668 (10 pages) | Cited 4 times

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The achievable beam quality resulting from heavy charged particle pumping of transverse flow, gas laser amplifiers is presented. The charged particles are generated by nuclear reactions in either B4C or UO2 coated on plates parallel to the lasant flow. The character of the laser medium aberration resulting from the nonuniform energy deposition and fluid dynamic effects is discussed as well as the effects on the far‐field beam quality of the amplifier. Pumping from plates parallel to the flow produces a medium aberration which is dominated by a cylindrical focus perpendicular to the flow, and a tilt in the flow direction. Beam qualities of better than 2 times diffraction limited (TDL) are obtained with a B4C fueled 1‐m‐long amplifier pumped at an average power deposition of 100 W/cm3 with an inlet 0.1‐MPa helium flow at 100 m/s. Amplifiers using UO2 fuel coatings to pump argon under the same flow conditions with fission fragments have similar qualitative laser medium aberrations but with larger magnitude, resulting in beam qualities of ≳20 TDL for a 100 W/cm3 average power deposition.
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42.55.Vc X- and γ-ray lasers
52.50.Dg Plasma sources
42.55.Lt Gas lasers including excimer and metal-vapor lasers
25.40.Fq Inelastic neutron scattering

Characterization of Er3+‐doped glasses by fluorescence line narrowing

S. Zemon, G. Lambert, L. J. Andrews, W. J. Miniscalco, B. T. Hall, T. Wei, and R. C. Folweiler

J. Appl. Phys. 69, 6799 (1991); http://dx.doi.org/10.1063/1.347669 (13 pages) | Cited 14 times

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The Stark levels of the 4I15/2 ground state manifold have been determined for Er3+‐doped fluorozirconate, fluorophosphate, phosphate, and silicate bulk glasses from fluorescence‐line‐narrowing (FLN) measurements at 4.2 K. Splittings between adjacent Stark levels were observed to be 10–140 cm−1 and the total energy spread of the manifold was found to range from 335 to 400 cm−1. The position of a given Stark level varies up to 60 cm−1 depending on the particular Er3+ sites excited. Using the 4.2‐K results, homogeneous broadening is found to be a reasonable approximation for the 300‐K luminescence band of the glasses examined. Results are also presented for silica preforms as well as for a barium‐zinc‐lutetium‐thorium fluoride composition. The FLN studies together with a Judd–Ofelt [Phys. Rev. 127, 750 (1962); J. Chem. Phys. 37, 511 (1962)] analysis of the absorption data suggest similarities in the local environment between the fluorozirconates and the high‐fluorine fluorophosphates.
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42.55.Rz Doped-insulator lasers and other solid state lasers
42.70.Ce Glasses, quartz
78.45.+h Stimulated emission

Elastic scattering of electrons by free and bound zinc and cadmium atoms

D. B. McGarrah, A. J. Antolak, and W. Williamson

J. Appl. Phys. 69, 6812 (1991); http://dx.doi.org/10.1063/1.347670 (5 pages) | Cited 4 times

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Elastic differential and total cross sections are presented for electron scattering by zinc and cadmium using the optical model. The cross sections are determined for scattering by free atoms and by atoms bound in a solid. The phase shifts are obtained from solutions to the reduced radial wave equation based on an optical potential consisting of static, polarization, and exchange terms. Electron energies from 12.5 to 200 eV are considered. Differences in the total cross section from using two different forms of the exchange potential are examined. A simple parametric fit of the total cross section which is suitable for Monte Carlo applications is given.
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34.80.Bm Elastic scattering

Electrostatic charging of two insulating powders

L. B. Schein, M. LaHa, and G. Marshall

J. Appl. Phys. 69, 6817 (1991); http://dx.doi.org/10.1063/1.347671 (10 pages) | Cited 7 times

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The electrostatic charging characteristics of a mixture of two insulating powders with radii R and r such that R≫r are studied using a charge spectrometer which allows complete characterization of the charge and radius distribution function. The data are consistent with a natural extension of the surface‐state theory which assumes, to first order, that the smaller radii powder particles share a fixed amount of charge determined by surface states on the larger radii particles. The challenges this result poses to microscopic theories of insulator contact charge exchange are discussed.
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73.40.-c Electronic transport in interface structures
73.20.At Surface states, band structure, electron density of states

Plasma waveguide: A concept to transfer electromagnetic energy in space

Hao‐Ming Shen

J. Appl. Phys. 69, 6827 (1991); http://dx.doi.org/10.1063/1.347672 (9 pages) | Cited 27 times

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This paper studies the possibility of using a laser‐generated ‘‘plasma waveguide’’ to transfer electromagnetic (EM) energy. The plasma waveguide is a cylindrical vacuum core surrounded by a plasma cladding. The analysis shows that guided‐mode fields do exist inside the core. Like a general dielectric waveguide, the plasma waveguide is characterized by a ‘‘normalized frequency parameter’’ (also called the V number). Although the permittivity of the plasma varies strongly with frequency, the V number surprisingly remains constant over the entire frequency range. Because of this property, the frequency dependence of the plasma waveguide is different; it has a wider high‐frequency response than the general dielectric waveguide. The EM pulse can propagate in the plasma waveguide at close to the speed of light and keep its profile and shape unchanged. When the V number is smaller than 2.4048 (the first root of the zero‐order Bessel function), only the single HE11 mode exists in the plasma waveguide. Unlike the dielectric waveguide, however, there is no high‐frequency limitation for single‐mode propagation. The EM fields outside the core in the plasma decrease exponentially with increasing radius. Thus, practically, a plasma cladding of sufficient thickness is all that is required to confine the EM wave. Such a plasma waveguide can be generated by a hollow laser beam in upper space and used for guiding EM pulses. A brief survey on laser‐generated plasmas is given.
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52.90.+z Other topics in physics of plasmas and electric discharges (restricted to new topics in section 52)
42.79.Gn Optical waveguides and couplers
52.38.-r Laser-plasma interactions

Studies of synchrotron radiation emission from the modified betatron accelerator

T. J. Smith, J. Golden, and C. A. Kapetanakos

J. Appl. Phys. 69, 6836 (1991); http://dx.doi.org/10.1063/1.347673 (8 pages) | Cited 2 times

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Numerical calculations of synchrotron radiation emitted from the modified betatron accelerator show that, for relativistic electron energies up to approximately 2 MeV, the single‐particle intensity spectrum is characterized by a peak at the Doppler‐shifted cyclotron frequency associated with the applied toroidal field. As the electron energy is increased above a few MeV, the calculated spectrum becomes comparable to that of an electron in purely circular motion. Measurements of the radiation using fixed‐frequency heterodyne receivers indicate that the polarization, amplitude, and the temporal evolution of radiated power during the first few hundred microseconds of acceleration are in good agreement with the predicted single‐particle spectrum. These observations have been used to confirm the energy evolution and provide information about the magnitude of the transverse velocity of the beam electrons. Late‐time signal decay suggests that electrons are moving off the minor axis in a manner that is consistent with the excitation of the electron‐cyclotron resonance.
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41.60.-m Radiation by moving charges
41.75.Ht Relativistic electron and positron beams
29.20.df Betatrons

Low‐temperature operating life of continuous 300‐K AlxGa1−xAs‐GaAs quantum‐well heterostructure lasers grown on Si

D. C. Hall, N. Holonyak, D. G. Deppe, M. J. Ries, R. J. Matyi, H. Shichijo, and J. E. Epler

J. Appl. Phys. 69, 6844 (1991); http://dx.doi.org/10.1063/1.347674 (6 pages) | Cited 5 times

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Data are presented on the continuous (cw) 77–200 K operational characteristics of cw 300‐K AlxGa1−xAs‐GaAs quantum‐well heterostructure diode lasers grown on Si substrates. Operation is demonstrated for over 500 h with a junction temperature as high as ∼200 K for a diode previously operated cw 300 K for over 10 h with its junction side mounted away from the heat sink. The data indicate that longer cw 300‐K lifetimes than previously demonstrated (17 h) may be possible. The effects of the optical power level on the degradation rate are examined, and it is shown that the maximum cw 300‐K power output for these devices (∼30 mW/facet) is limited by catastrophic facet degradation. The effects of naturally occurring microcracks on device stability are also considered, and the effect of stress on the output polarization is measured and discussed.
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42.55.Px Semiconductor lasers; laser diodes
42.60.Da Resonators, cavities, amplifiers, arrays, and rings

Theoretical operational life study of the closed‐cycle transversely excited atmospheric CO2 laser

Hirokazu Hokazono, Minoru Obara, Katsumi Midorikawa, and Hideo Tashiro

J. Appl. Phys. 69, 6850 (1991); http://dx.doi.org/10.1063/1.347675 (19 pages) | Cited 30 times

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By using a comprehensive theoretical model that assumes a stable excitation discharge and homogeneous plasma chemical reactions in the discharge plasma, the laser output performance and the variations of the laser gas components during the sealed‐off operation of the high‐power, closed‐cycle transversely excited atmospheric CO2 laser have been investigated. The fractional CO2/N2, molecules decomposition, and the concentration of the various minor impurities accumulated in the laser gas mixture have been theoretically calculated as a function of shots and number of repetitive discharge pulses. According to the results, the gradual reduction of the laser output energy with the successive excitation pulses was mainly due to the depletion of the CO2 molecules and the reduction of the excitation efficiency; the excitation efficiency was decreased in consequence of the increased operational E/N (E is the discharge field strength, N is the total laser gas number density) caused by the accumulation of highly electronegative impurities such as O2 and O3. The nitrogen oxides were found to show little effect on the operational E/N in spite of their large electron attachment cross sections, because these molecules were much less accumulated in the laser gas mixture than O2 or O3. The theoretical model has clarified for the first time that a trace of water (H2O) vapor in the laser chamber effectively acts as a gaseous catalyst to enhance the CO2 reforming reaction in the discharge plasma. Furthermore, this CO2 reforming reaction by H2O, rather than the other backward reactions, predominantly determines the equilibrium CO2 decomposition level in the actual laser chamber. Finally, with regard to the ultraviolet (UV) preionization, it was theoretically shown that the UV absorption depth of the laser gas mixture steeply decreased as the CO2 decomposition increased owing to the contamination of strong UV absorbing species such as O2 and O3.
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42.55.Lt Gas lasers including excimer and metal-vapor lasers
52.80.Hc Glow; corona

Carrier lifetimes and threshold currents in HgCdTe double heterostructure and multi‐quantum‐well lasers

Y. Jiang, M. C. Teich, and W. I. Wang

J. Appl. Phys. 69, 6869 (1991); http://dx.doi.org/10.1063/1.347676 (7 pages) | Cited 13 times

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The Auger and radiative combination carrier lifetimes in HgCdTe bulk and quantum‐well structures, with band gaps in the wavelength range 2–5 μm, are calculated. The Auger recombination rate in a HgCdTe quantum well (QW) is shown to be significantly smaller than that in bulk material. Threshold current densities of HgCdTe double‐heterostructure (DH) and multiquantum‐well (MQW) lasers are calculated. In a HgCdTe DH laser, Auger recombination dominates the carrier loss at threshold. In a HgCdTe MQW laser, on the other hand, radiative recombination dominates the carrier loss. MQW lasers shown improved temperature performance over conventional DH lasers.
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42.55.Px Semiconductor lasers; laser diodes
73.21.-b Electron states and collective excitations in multilayers, quantum wells, mesoscopic, and nanoscale systems
73.61.Ga II-VI semiconductors
71.55.Gs II-VI semiconductors

Oxide glasses for fast photonic switching: A comparative study

M. E. Lines

J. Appl. Phys. 69, 6876 (1991); http://dx.doi.org/10.1063/1.347677 (9 pages) | Cited 60 times

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New developments involving nonlinear optical effects are conditioned by the improvement of pertinent figures of merit involving the characteristics of nonlinear optical materials within the ‘‘transparency’’ regime of their optic windows. This paper discusses, from a global standpoint, the maximum values of nonlinear refractive index n2, and of its ratio n2/α relative to intrinsic attenuation α, attainable for a multicomponent oxide glass. It also locates and analyses those classes of multicomponent oxide glass that, from the standpoint of these two measures, appear to be the most promising candidates for use as fast photonic switching elements in the 1–2.5‐μm wavelength regime.
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42.79.Ta Optical computers, logic elements, interconnects, switches; neural networks
42.79.Gn Optical waveguides and couplers
78.20.Ci Optical constants (including refractive index, complex dielectric constant, absorption, reflection and transmission coefficients, emissivity)

Band‐edge photorefractivity in semiconductors: Theory and experiment

Afshin Partovi and Elsa M. Garmire

J. Appl. Phys. 69, 6885 (1991); http://dx.doi.org/10.1063/1.347678 (14 pages) | Cited 18 times

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At wavelengths close to the band edge, strong photorefractive gratings using the Franz–Keldysh electrorefractive effect can be written in semiconductors. Two‐beam‐coupling exponential gain coefficients as high as Γ=16.3 cm−1 have been obtained in GaAs by combining the electrorefractive photorefractive grating with the conventional electro‐optic photorefractive grating and using the moving grating technique to enhance the photorefractive space‐charge field. A method for calculation of the gain coefficient near the band edge of materials is presented. The method is applied to GaAs and the results are compared to the experimental data. Reasonable agreement with experiment has been achieved. An optimal spectral range (910 nm<λ<930 nm) for near‐band‐edge photorefractivity in GaAs has been found. Conventional theories of photorefractivity based on Kukhtarev’s equations are found to be sufficient for calculation of the photorefractive space‐charge field near the band edge. Predictions of the gain coefficient near the band edge using the moving grating technique are presented. Other methods of increasing the photorefractive gain such as the temperature‐dependent resonance in InP:Fe are also discussed.
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78.20.Jq Electro-optical effects
78.20.Bh Theory, models, and numerical simulation
42.65.Jx Beam trapping, self-focusing and defocusing; self-phase modulation

Light polarization effect in measurement of thin wire diameter by laser diffraction and its explanation with boundary diffraction wave

Jianping Xie, Yanhui Qiu, Hai Ming, and Chuanqi Li

J. Appl. Phys. 69, 6899 (1991); http://dx.doi.org/10.1063/1.347679 (5 pages) | Cited 6 times

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The polarization effect in the measurement of thin wire diameter by laser diffraction has been studied experimentally and theoretically in this paper. The experimental values of a diameter vary sinusoidally with the vibration direction of incident polarized light. The difference between measured diameter values with S and P component of incident light is nearly proportional to the imaginary part of the complex index which represents the optical property of metal wire. Taken into account the phase change of boundary diffraction wave originating from the edges of metal wire, the physical cause of this polarization effect, and its experimental results have been clearly explained.
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42.25.Ja Polarization
42.25.Fx Diffraction and scattering

Experimental study of the scattering of a compressional wave by an elastic wedge

J.‐F. Piet and M. de Billy

J. Appl. Phys. 69, 6904 (1991); http://dx.doi.org/10.1063/1.347627 (6 pages)

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We investigated the diffraction pattern of an acoustic beam produced by the edge of a dihedral angle. The incident beam is perpendicular to the edge, and the angular variations of the scattered field are recorded in a plane which is perpendicular to the edge. The frequency of excitation is 2.5 MHz, and the dimensions of the samples are much larger than the incident wavelength. The experimental measurements of the angular diagrams are discussed in terms of surface waves and conversion modes.
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43.20.Fn Scattering of acoustic waves
62.65.+k Acoustical properties of solids

Ambipolar diffusion theory of the rectangular positive column with quadratic ionization

J. H. Ingold

J. Appl. Phys. 69, 6910 (1991); http://dx.doi.org/10.1063/1.347628 (8 pages) | Cited 3 times

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The ambipolar diffusion theory of the rectangular positive column with ionization rate proportional to the square of the electron density is discussed. In agreement with previous theoretical work and with observation, it is predicted that the positive column fills the rectangular cross section up to an aspect ratio of 3, after which there is no further spreading. The reason for this behavior is discussed. In disagreement with previous work, it is predicted that little (∼20%) increase in the production of resonance radiation at the same efficiency can be obtained by increasing the aspect ratio when ionization is proportional to the square of electron density
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51.10.+y Kinetic and transport theory of gases
52.80.Tn Other gas discharges
52.25.Fi Transport properties

Characteristics of a cylindrical Langmuir probe of finite length

E. F. Jaeger, L. A. Berry, and D. B. Batchelor

J. Appl. Phys. 69, 6918 (1991); http://dx.doi.org/10.1063/1.347629 (5 pages) | Cited 10 times

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The current collected by a finite‐length cylindrical probe is calculated numerically from a self‐consistent solution to the Poisson–Vlasov system of equations. A Boltzmann distribution is assumed for the electrons, and results are compared to the standard ‘‘orbital‐motion‐limited’’ theory. When the probe is very long compared to the sheath thickness and the ratio of probe radius to electron Debye length, Rp/λDe ≲ 1, the collected current agrees with the orbital‐motion theory for cylindrical probes; when the probe is very short compared to the sheath thickness and Rp/λDe ≲ 1, the collected current agrees with the orbital‐motion theory for spherical probes. When the probe length is comparable to the sheath thickness, there is a critical potential above which the probe behaves as a sphere and below which it behaves as a cylinder. Ion charge density, space potential, and ion particle orbits are displayed graphically in the sheath plasma surrounding the probe tip.
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52.65.-y Plasma simulation
52.70.Ds Electric and magnetic measurements
52.70.Nc Particle measurements
52.25.Dg Plasma kinetic equations

Particle thermophoresis in low pressure glow discharges

G. M. Jellum, J. E. Daugherty, and D. B. Graves

J. Appl. Phys. 69, 6923 (1991); http://dx.doi.org/10.1063/1.347630 (12 pages) | Cited 111 times

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Particles in glow discharges are charged negatively and are therefore often suspended electrostatically since the plasma potential is usually more positive than the potential of surfaces bounding the plasma. However, in addition to responding to electrostatic and gravitational forces, particles are sensitive to forces associated with gradients in neutral gas temperature. A series of experiments were performed varying the temperature of water circulated through planar, parallel electrodes. When particles are present and are suspended in the discharge, they move away from a heated electrode and towards a cooled electrode. In the absence of particles, the discharge optical emission spatial profile and electrical characteristics did not change significantly for any combination of electrode heating or cooling. Particles remaining in the interelectrode gap after the discharge is extinguished appear to be uncharged. With particles present, the discharge electrical properties and time‐ and space‐resolved optical emission take on characteristics associated with discharges in electron attaching gases. We report on a simple particle‐scavenging arrangement based on thermophoresis that can be used to remove particles from a discharge.
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52.80.Hc Glow; corona
52.25.Fi Transport properties

Aluminum‐coated optical fibers as efficient infrared timing fiducial photocathodes for synchronizing x‐ray streak cameras

J. A. Koch and B. J. MacGowan

J. Appl. Phys. 69, 6935 (1991); http://dx.doi.org/10.1063/1.347631 (10 pages) | Cited 7 times

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The timing fiducial system at the Nova Two‐Beam Facility allows time‐resolved x‐ray and optical streak camera data from laser‐produced plasmas to be synchronized to within 30 ps. In this system, an Al‐coated optical fiber is inserted into an aperture in the cathode plate of each streak camera. The coating acts as a photocathode for a low‐energy pulse of 1ω (λ = 1.054 μm) light which is synchronized to the main Nova beam. The use of the fundamental (1ω) for this fiducial pulse has been found to offer significant advantages over the use of the 2ω second harmonic (λ = 0.53 μm). These advantages include brighter signals, greater reliability, and a higher relative damage threshold, allowing routine use without fiber replacement. The operation of the system is described, and experimental data and interpretations are discussed which suggest that the electron production in the Al film is due to thermionic emission. The results of detailed numerical simulations of the relevant thermal processes, undertaken to model the response of the coated fiber to 1ω laser pulses, are also presented, which give qualitative agreement with experimental data. Quantitative discrepancies between the modeling results and the experimental data are discussed, and suggestions for further research are given.
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42.81.Bm Fabrication, cladding, and splicing
07.85.-m X- and γ-ray instruments
52.50.Jm Plasma production and heating by laser beams (laser-foil, laser-cluster, etc.)
85.60.Dw Photodiodes; phototransistors; photoresistors

Metal/water chemical reaction coupled to a pulsed electrical discharge

W. M. Lee

J. Appl. Phys. 69, 6945 (1991); http://dx.doi.org/10.1063/1.348931 (7 pages) | Cited 4 times

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Capacitor‐stored energy is transferred to a wire conductor surrounded by a mixture of a reactive metal powder and water. The overload current pulse quickly explodes the small wire conductor, initiating a chemical reaction in the mixture. The discharge after the explosion is made through a highly condensed state undergoing chemical reaction (producing both heat and hydrogen gas). In this type of discharge the heat and gas from the reaction help the reacting medium maintain some conductance necessary for effective dissipation of the electrical energy. The coupling of the discharge to the reaction is found to be crucial to sustain the reaction.
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52.80.Qj Explosions; exploding wires
82.45.-h Electrochemistry and electrophoresis

A model for particulate contaminated glow discharges

Michael J. McCaughey and Mark J. Kushner

J. Appl. Phys. 69, 6952 (1991); http://dx.doi.org/10.1063/1.347632 (10 pages) | Cited 49 times

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Glow discharges are often contaminated by particulates resulting from gas phase nucleation or sputtering of surfaces in contact with the plasma. If these particulates are sufficiently large, they will negatively charge and act as Coulomb‐like scattering centers for electrons. In doing so, rate coefficients for high‐threshold processes such as ionization may be reduced compared to those in pristine plasmas. If the contamination is nonuniform, then the resulting spatial irregularities in the rates of excitation may lead to plasma properties which are also nonuniform. In this paper, we report on the results of a model for argon glow discharges contaminated by dust. Rate coefficients for this model are generated by a separate Monte Carlo simulation of electron swarms in dusty plasmas. We find that under quasi‐steady‐state conditions, current flow and subsequent excitation of the gas are channeled into regions of lower dust density, and that these effects depend on the density, size, and distribution of the dust. In low pressure (<10 Torr) glow discharges having similar dimensions, comparable perturbation in plasma properties due to contamination is obtained when NDP is approximately constant (ND is the dust density, P is the gas pressure). The onset of perturbations may occur when NDP≳105 cm−3 Torr.
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52.25.Fi Transport properties
52.80.Hc Glow; corona
52.40.Hf Plasma-material interactions; boundary layer effects
52.65.-y Plasma simulation

New scaling relation for ion extraction by external electric field on a barium plasma produced between parallel‐plate electrodes

Kimio Yamada, Tsutomu Tetsuka, and Yoshihiro Deguchi

J. Appl. Phys. 69, 6962 (1991); http://dx.doi.org/10.1063/1.347633 (6 pages) | Cited 11 times

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Parameter dependencies of ion extraction time by an external electric field have been investigated by using barium plasmas suddenly produced between parallel‐plate electrodes under a variety of experimental conditions. It is concluded that the ion extraction time can be expressed as the product of exponents of the three controllable parameters, applied voltage, plasma width, and ion density. This scaling relation can estimate the ion extraction time with an accuracy of 15% for a given controllable parameter set, supporting its universality.
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52.25.Fi Transport properties
52.20.Hv Atomic, molecular, ion, and heavy-particle collisions
52.25.Tx Emission, absorption, and scattering of particles
52.50.Dg Plasma sources

Order parameter spatial variation and anchoring energy for nematic liquid crystals

G. Barbero and G. Durand

J. Appl. Phys. 69, 6968 (1991); http://dx.doi.org/10.1063/1.347634 (6 pages) | Cited 15 times

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A spatial variation of the nematic scalar order parameter leads to a surface anchoring energy renormalization. This renormalization is a consequence of torque conservation and of the ‘‘macroscopic’’ definition of the surface orientation. The order change comes from an assumed property of the boundary or from an external curvature constraint inducing surface melting. In both cases, the maximum effect leads to a surface extrapolation length comparable with the nematic‐isotropic coherence length. Strong anchoring data should be remeasured and reinterpretated.
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61.30.Gd Orientational order of liquid crystals; electric and magnetic field effects on order

Defect mechanisms in degradation of InGaAsP long‐wavelength edge‐emitting light‐emitting diodes

S. N. G. Chu, S. Nakahara, L. C. Luther, and H. W. Krautter

J. Appl. Phys. 69, 6974 (1991); http://dx.doi.org/10.1063/1.347635 (5 pages) | Cited 3 times

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We report on degradation mechanisms in 1.3‐μm wavelength edge‐emitting light‐emitting diodes aged for 1000 h at 150 °C and 150 mA studied by transmission electron microscopy. During this degradation, three types of defect structures are generated at the interface between InGaAsP active layer and n‐InP buffer layer along the light emitting stripe: (i) 1/2〈100〉{100} faulted extrinsic dislocation loops formed by condensation of point defects, similar to those previously observed in degraded channeled substrate buried heterostructure lasers, (ii) long 1/2〈101〉 dislocation clusters developed along the active stripe, and (iii) 1/3[111] and 1/3[111] faulted Frank loops developed from condensation of point defects onto the 1/2[101] dislocation through dissociative reactions: 1/2[101]→1/6[121]+1/3[111] and 1/2[101]→1/6[121]+1/3[111]. The driving force for these reactions is discussed.
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85.60.Jb Light-emitting devices
61.72.Nn Stacking faults and other planar or extended defects
78.30.-j Infrared and Raman spectra
78.40.Fy Semiconductors
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