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Journal of Applied Physics

Issue 9 • Date May 1985

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Displaying Results 1 - 25 of 29
  • Scalar multipole expansions and their dipole equivalents

    Page(s): 4301 - 4308
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    If the source of a field that satisfies Poisson’s equation can be written as the divergence of a vector s, then a scalar multipole expansion of the source can be evaluated in terms of s, which is a dipole density. A multipole expansion in terms of the dipole density can be computed about different origins. This allows us to evaluate the expansion of a dipole displaced from the origin and find a method of approximating some multipole expansions by displaced dipoles. In many physical applications it is known that the source is a displaced dipole, and we can find its location from a multipole expansion at some convenient location. It is possible to derive pictures in terms of dipole densities that in the proper limit become the individual multipoles. There are, however, ambiguities in that for some multipoles more than one picture gives the proper field. View full abstract»

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  • An advanced kinetic model of electron‐beam‐excited KrF lasers including the vibrational relaxation in KrF*(B) and collisional mixing of KrF*(B,C)

    Page(s): 4309 - 4322
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    Computer models developed so far on electron‐beam‐excited KrF(B–X, 248 nm) lasers that include the vibrational relaxation process in the upper lasing B level at the finite rate could not predict the high intrinsic laser efficiency which was experimentally reported. This is attributed to the reduction of the laser extraction efficiency. We have developed a four‐level KrF laser model that includes the vibrational relaxation process and also the collisional mixing of the KrF*(B) and the KrF*(C) levels. The collisional quenching rates for KrF*(B,C) that we used and the vibrational relaxation rate were carefully estimated by using the effective spontaneous lifetimes for KrF*(B,C). As a result, the model prediction was in quite good agreement with many experimental results for a saturation behavior of KrF*(B–X) fluorescence, for small‐signal gains, for small‐signal absorptions, and for intrinsic efficiencies. Estimated rate constants in this model for the vibrational relaxation and the KrF*(B,C) mixing are 4×10-11 and 5×10-10 cm3/s, respectively, for a two‐body collision rate with argon gas. View full abstract»

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  • Shock‐wave initiation of heterogeneous reactive solids

    Page(s): 4323 - 4334
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    Shock‐wave initiation of solid explosives depends on localized regions of high temperature (hot spots) created by heterogeneous deformation in the vicinity of various defects. Current mathematical models of shock initiation tend to fall into two broad categories: (1) thermodynamic‐state‐dependent reaction‐rate models, and (2) the continuum theory of multiphase mixtures. The level of generality possessed by (1) appears to be insufficient for representation of observed initiation phenomena, while that of (2) may exceed necessary requirements based on present measurement capabilities. As a means of bridging the gap between these two models, we present an internal‐state‐variable theory based on elementary physical principles, relying on specific limiting cases for the determination of functional forms. The appropriate minimum set of internal‐state variables are the mass fraction of hot spots  μ, their degree of reaction  f, and their average creation temperature θ. The overall reaction rate λ˙, then depends on  μ,  f, and θ in addition to the usual macroscopic thermodynamic variables (current state as well as their history). Two specific forms of this set of equations are applied to time‐resolved shock‐initiation data on PBX‐9404. Numerical solution is achieved by the method of characteristics for rate‐dependent chemical reaction. Additional questions such as the effect of thermal equilibrium between phases (solid reactants and gaseous products) on the theoretical results are discussed quantitatively. View full abstract»

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  • Stimulated Raman scattering from a relativistic electron beam

    Page(s): 4335 - 4341
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    A plane transverse electromagnetic wave propagating antiparallel to a cold relativistic electron beam extending over all space is backscattered from a negative energy space‐charge wave into a frequency up‐converted plane transverse electromagnetic wave. Parametric and nonlinear three‐wave interactions are analytically examined in a systematic manner taking account of all the nonlinearities. With the inclusion of appropriate phenomenological collision terms, and frequency and wave‐number mismatches, the amplitude threshold, the growth rate, and the saturation values are determined. A heuristic estimation reveals wave amplification to be limited by the beam velocity. An expression for maximum amplification is obtained. Representative numerical results are presented to display the characteristics of the nonlinear three‐wave interaction. View full abstract»

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  • Effect of positive and negative ion energies on H- destruction by mutual neutralization in low‐pressure plasmas

    Page(s): 4342 - 4348
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    The effect of plasma potential profile and collisions with neutrals and charged particles upon positive and negative ion energies has been studied in a low‐pressure (2–13.6 mTorr) hybrid magnetic multicusp hydrogen discharge. When the plasma density is lower than 1011 cm-3, the negative ion temperature is always close to the gas temperature, while the positive ion temperature goes down from 1 to 0.1 eV when the pressure goes up. The observed reduction of the relative negative ion density when the pressure is increased is shown to be related to the reduction of positive ion velocities and to the associated increase in the mutual neutralization rate coefficient. View full abstract»

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  • Radiative properties of puffed‐gas mixtures: The case of optically thick plasmas composed of two elements with similar atomic numbers

    Page(s): 4349 - 4353
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    Using detailed atomic and radiative transfer calculations, we consider the K‐shell radiative properties of optically thick plasmas composed of two different elements with nearly the same atomic number. A sodium‐neon mixture is chosen for specificity, and the plasma conditions considered are similar to those achieved in puffed‐gas or exploding‐wire Z pinches. The emissivity of an individual K‐shell line is parameterized in terms of an ‘‘ultimate’’ photon escape probability which is a function of both the single‐flight photon escape probability and the collisional quenching probability. Holding the total ion density (sodium plus neon) constant, we have evaluated the total K‐shell emissivity as a function of sodium‐to‐neon ratio. In contrast to the optically thin case the emitted K‐shell power peaks at values of this ratio intermediate between pure neon and pure sodium. The opacity effects responsible for this phenomenon are analyzed using the escape parameterization. View full abstract»

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  • Thermal transport measurements in six‐beam, ultraviolet irradiation of spherical targets

    Page(s): 4354 - 4359
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    Thermal transport, mass ablation rates, and preheat have been studied in spherical irradiation at λ=351 nm, using six of the 24 beams of the OMEGA spherical irradiation laser system at the Laboratory for Laser Energetics. Mass ablation rates are higher at 351‐nm than at 1054‐nm irradiation, even when compared at the same absorbed irradiance. Similar to the case of 1054‐nm irradiation, very deep burnthrough was found at 351 nm. However, the shallow‐gradient temperature profile at the heat front, characteristic of the experiments at 1054 nm, was not observed here, nor was the large difference between uniform and tight focus irradiation of spherical targets. Ablation pressures derived from charge‐collector data rise from 10 to 100 Mbar for absorbed irradiance in the range of 4×1013 to 9×1014 W/cm2. View full abstract»

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  • Electron attachment to H2O in Ar, N2, and CH4 in electric field

    Page(s): 4360 - 4367
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    The attachment of electrons to H2O in Ar, N2 or CH4 is investigated using a parallel‐plate drift‐tube apparatus. Electrons are produced either by irradiation of the cathode with ArF laser photons or by two‐photon‐ionization of a trace of trimethylamine in a buffer gas. The transient voltage pulses induced by the electron motion between the electrodes are observed. The electron attachment rate of H2O is determined from the ratio of transient voltage with and without H2O added to the buffer gas. The measured electron attachment rate constants of H2O in Ar increase with E/N from 2 to 15 Td. Electron attachment due to the formation of ‘‘temporary’’ negative ions in the H2O–N2 and H2O–CH4 mixture were observed. The lifetime of the negative ion was determined to be about 200 ns, whose nature is discussed. The ‘‘apparent’’ electron attachment rate constants for the formation of ‘‘temporary’’ negative ions in the H2O–CH4 gas mixture are measured for E/N from 1 to 20 Td. The electron drift velocities for the gas mixtures of H2O in various buffer gases are measured. View full abstract»

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  • Collisional cross sections of CCl2F2 and transport coefficients of CCl2F2 and N2‐CCl2F2 mixtures

    Page(s): 4368 - 4376
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    A set of effective electron collision cross sections, including momentum transfer, vibration, and electron excitation collisions has been determined for CCl2F2 gas: the momentum transfer is approximately 2×10-15 cm2 with the highest maximum about 4×10-15 at 6 eV, two vibrational maxima with thresholds of 0.11 and 0.14 eV are about 9 and 4 (10-16 cm2) at 0.2 and 0.25 eV and two excitation maxima with thresholds of 7.0 and 9.8 eV are 2.8 and 5.1 (10-16 cm2) at 11 and 18 eV, respectively. The calculated transport coefficients are in satisfactory agreement with measured values, except for the attachment coefficient, which is too high at low (below 90 V cm-1 Torr-1) and high (above 180 V cm-1 Torr-1) reduced fields. The possibility of comparison in the case of N2‐CCl2F2 mixtures is limited and agreement is less satisfactory than in the case of pure CCl2F2. Deviations for medium fields are within about 30% for the D/μ ratio and ionization coefficients. The attachment coefficients agree well at high partial density of CCl2F2 (above approximately 50%) but the experimental values are much higher than predicted at low CCl2F2 content. Calculated values of the limit fields agree well with the values obtained from breakdown potential measurements available in the literature. View full abstract»

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  • Pressure‐dependent electron attachment and breakdown strengths of unary gases and synergism of binary gas mixtures: A relationship

    Page(s): 4377 - 4385
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    The relationship between the pressure‐dependent electron attachment rate constants which have been observed in 1‐C3F6 and in several perfluoroalkanes and the uniform field breakdown strengths (E/N)lim in these gases is discussed. Also discussed are the various types of synergistic behavior in (E/N)lim which have been observed in binary dielectric gas mixtures. For the latter, a new mechanism is outlined which explains the synergism observed in several gas mixtures where the (E/N)lim values of the mixtures are greater than those of the individual gas constituents, which we call positive synergism. Model calculations are presented which support this mechanism and can be used to explain the pressure‐dependent synergistic effects which have been reported in 1‐C3F6/SF6 and other gas mixtures. Experimentally observed ion–molecule reaction processes for several gases are discussed which support the proposed mechanism. Based on this mechanism, we outline several conditions which must be fulfilled in order to observe positive synergistic behavior in dielectric gas mixtures. View full abstract»

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  • Microwave spectroscopic measurement of the electron density in a planar discharge: Relation to reactive‐ion etching of silicon oxide

    Page(s): 4386 - 4390
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    It is demonstrated that microwave cutoff can be used to measure absolute electron densities for CHF3, Ar, and O2 plasmas in a batch‐mode planar reactor excited at 13.56 MHz. The densities obtained are between 1015 and 1016 m-3 for all gases and a large set of conditions (pressure, rf power, electrode spacing). A comparison with selective etching of SiO2 over Si in the same reactor shows a qualitative agreement between the substrate ion flux calculated from the electron density and the etch rate of silicon oxide; the apparent sputter yield ranges from 0.4 to 1.4 molecules per ion. View full abstract»

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  • Collective acceleration of laser‐produced ions

    Page(s): 4391 - 4395
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    The collective acceleration of heavy ions (C, Al, Fe) from a laser produced plasma to energies in excess of 10 MeV/amu has been observed experimentally. In the experiments, an intense relativistic electron beam (1.2 MeV, 30 kA, 30 ns) is injected through an anode aperture into a downstream drift region. Ions to be accelerated are produced by firing a Q‐switched ruby laser (0.3–15 J, 15 ns) at a target material located immediately downstream of the anode aperture. Accelerated ion energies are measured by time of flight, range‐energy, and Thomson spectrometer techniques. View full abstract»

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  • A pulsed thermoelastic analysis of photothermal surface displacements in layered materials

    Page(s): 4396 - 4405
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    A pulsed photothermal method is proposed for the detection of delaminations in layered materials. The surface of the material is heated by a focused laser pulse to a few degrees above the ambient temperature, and the induced thermoelastic displacement is detected by a laser interferometer. Unbonded areas are characterized by their thermoelastic bending behavior, in addition to their thermal expansion, because of the induced temperature gradients both in the radial and axial directions. A theoretical analysis and an experimental investigation of this behavior are described in order to point out the most significant parameters. An application of the nondestructive inspection of Al‐epoxy honeycomb panels is presented. View full abstract»

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  • Theory for shot noise in submicron semiconductor diodes using two‐point correlations of noise sources

    Page(s): 4406 - 4408
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    In this paper the shot noise process is reformulated in terms of spatial correlation functions. In ‘‘ballistic’’ devices local noise sources remain correlated for separation distances on the order of device dimensions. The effect of randomizing collisions, is introduced as a decay of this correlation. The velocity distribution of carriers is taken into account. The effect of parameters such as injection velocity, scattering probability, and device length on noise in near‐ballistic devices could be calculated using this method. View full abstract»

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  • Solar thermophotovoltaics: An assessment

    Page(s): 4409 - 4414
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    A general model of a solar thermophotovoltaic device is discussed both for improving the efficiency of one‐band‐gap photovoltaic cells by matching the photon energy to the band gap and for concentrating diffuse radiation. First we assume ideal components to calculate theoretical maximum efficiency. It corresponds to that of a perfect selective absorber in conjunction with a Carnot‐engine ranging from 53% for 1 sun to 85% for the highest possible irradiance of 5×104 suns. The improvement over an ideal one‐gap device is roughly a factor of 2. Consideration of available materials shows that any improvement in efficiency can be expected only for high irradiance of 1000 suns. The sensitive parameter is the selectivity of the absorber‐emitter. Concentration of diffuse solar radiation is not feasible. Perspectives appear not much better than for existing technologies such as photovoltaics or solar tower applications. View full abstract»

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  • Enhanced high‐field current carrying capacities and pinning behavior of NbTi‐based superconducting alloys

    Page(s): 4415 - 4420
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    High‐field critical current densities Jc and pinning behavior are discussed for Nb‐63a/oTi, Nb‐61.7a/oTi‐3a/oHf, and Nb‐64.7a/oTi‐7.2a/oTa superconducting alloys. Jc properties for 8–12 T in these alloy superconductors under superfluid helium environments can be even superior to those in Nb3Sn at 4.2 K, when they are heavily cold‐worked after final heat treatments. A temperature scaling law of the flux pinning force Fp is found to hold for these alloys in the form of FP=K [Bc2(T)]nbp(1-b)q, where Bc2 is the upper critical field and b≡B/Bc2. The peak reduced field, bm, in the Fp-b curve depends upon how the alloys have been processed. The origins of n, p, and q are discussed in terms of the processing condition. View full abstract»

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  • Theory of photopyroelectric spectroscopy of solids

    Page(s): 4421 - 4430
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    Light absorption by a solid material and conversion of part, or all, of the optical energy into heat due to nonradiative deexcitation processes within the solid can give rise to an electrical signal in a pyroelectric thin film in contact with the sample. This effect forms the basis of a new spectroscopic technique for the study of condensed phase matter. A one‐dimensional theory is presented, which describes the dependence of the pyroelectric signal on the optical, thermal, and geometric parameters of the solid/pyroelectric system. Specifically, the theory examines the conditions under which the photopyroelectric signal exhibits a linear dependence on the optical absorption coefficient of the solid. Thus a theoretical basis for the technique of photopyroelectric spectroscopy is established. Qualitative comparisons between predictions of the theory and preliminary experimental observations are used to test the applicability of the theory to experimental configurations of practical interest. View full abstract»

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  • Optical absorption measurements in a new composite material by combined photoacoustic and beam transmission techniques

    Page(s): 4431 - 4436
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    The absorption coefficient of a highly absorbing (β≫105 cm-1) new composite produced in our laboratory, namely manganese dioxide surface‐impregnated polyethylene (MnO2–PE) film, has been measured in the visible range. We have employed a combination of concurrently taken photoacoustic and beam transmission data. This combination proved to be an effective means for the optical evaluation of highly absorptive samples which dispenses with the need for prior knowledge of the sample reflectivity. Theoretical interpretation of MnO2–PE absorption profile, with due account of the processes underlining the photoacoustic signal generation, reveals a semiconductor behavior with an estimated effective gap wavelength of 8050 Å. Technical implications of the availability of these highly absorptive flexible films are briefly considered. View full abstract»

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  • Measurement of the deuterium concentration profile in a deuterium‐exchanged LiNbO3 crystal

    Page(s): 4437 - 4440
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    We have measured the concentration of deuterium as a function of depth in a deuterium‐exchanged LiNbO3 sample using nuclear analysis. From the shape of the concentration profile we calculate the concentration dependent effective diffusion coefficient for deuterium‐lithium counterdiffusion. Comparison of the concentration and refractive index profiles for this waveguide confirms that the index increase in exchanged material is a nonlinear function of concentration. We discuss implications of these results for optical waveguide fabrication. View full abstract»

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  • A model for the plasma‐activated chemical vapor deposition process

    Page(s): 4441 - 4446
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    This paper presents the results of a theoretical study of the plasma‐activated chemical vapor deposition (PCVD) process used for the preparation of optical fibers. We show how the static deposition profile of silica can be computed by taking into account convection, reaction, and diffusion processes inside the glass tube. The parameters describing these processes are estimated. We examine how the deposition profile depends on the gas velocity, the reaction, and the diffusion constants. Theory and experiment are in good agreement. The calculations confirm the idea that in the PCVD process the deposit is formed from single molecules and not from clusters. View full abstract»

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  • A computational model of exploding metallic fuses for multimegajoule switching

    Page(s): 4447 - 4460
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    A new model for determining the time‐dependent behavior of exploding metallic fuses is formulated. The model draws on an atomic data base and gives insight into the temporal behavior of the material density and temperature of the fuse as well as the nonlinear electrical circuit interaction. The model includes an embedding insulating tamper and leads to a plausible explanation of fuse ‘‘restrike.’’ The model predicts time‐scale compression of 500 for inductive store systems powered by explosive driven magnetic flux compression generators. A scenario for achieving multimegajoule foil implosions is predicted. View full abstract»

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  • AK‐cut quartz resonators

    Page(s): 4461 - 4473
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    We calculated the turnover temperatures Tto of crystallographically doubly rotated quartz plates vibrating in the thickness mode. In addition to the known cuts situated along the loci of zero value of the first‐order temperature coefficient of frequency, there is another family of orientations, with Tto≥75 °C, which, compared to other singly or doubly rotated cuts, are relatively insensitive to crystallographic orientations. We designate these orientations as the AK cut. We report experimental static frequency as a function of temperature characteristics f(T) for 14 AK‐cut orientations, with angular combinations 30°≤ϕ≤46.1° and 21° ≤θ≤28.44°. Tto values are between 80 and 150 °C, in good agreement with computations. The f(T) curves have been fitted with third‐order polynomials. Compared to the fundamental mode of vibrations, the 3rd overtone shows a larger number of modes, and resonance amplitudes are weaker. The frequencies of the harmonic and anharmonic modes are in fairly good agreement with calculations based on a trapped energy resonator model, including effects of coupled thickness shear and thickness twist. View full abstract»

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  • The flow of heat and the motion of the weld pool in penetration welding with a laser

    Page(s): 4474 - 4479
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    Lasers are well suited to penetration welding as they can supply large amounts of power to a very small volume far from the surface of the workpiece. This paper reports the results of calculations performed with the aid of a mathematical model for the flow of heat and the motion of the molten metal surrounding the keyhole formed by the laser. The equations were solved with the aid of an approximate analytical solution for low welding speeds, and a computer program for higher speeds. A number of different materials are considered and graphs of the temperature distribution and the stream function describing the fluid motion are shown for a typical case. Relationships between the weld width, the power absorbed per unit thickness of the workpiece, and the speed of welding are investigated in detail. Over the range of welding speeds considered, the relationships given by the approximate analytical solution were found to be close to those obtained by numerical integration of the exact equations, even though the fine details of the solution, such as the ratio of the weld width to the total length of the weld pool at a given instant, or the details of the fluid motion, are substantially different. View full abstract»

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  • Electromagnetic wave propagation in a planar helix with a metal shield

    Page(s): 4480 - 4481
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    Dispersion properties of the electromagnetic (em) waves propagating through a planar helix are investigated. A dispersion relation is obtained which includes influence of the outer metal shield on the em wave propagation. It is shown that the phase velocity of em waves reduces as the metal shield is brought closer to the planar helix. View full abstract»

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  • Cylindrical hollow‐anode discharge

    Page(s): 4482 - 4484
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    Hollow‐anode discharge represents a new type of discharge which is realized in a diode consisting of a cylindrical cathode and special type of hollow anode whose inner surface only is conductive. The cylindrical cathode focuses electrons toward the hollow anode placed in the center of curvature, increasing the density of high‐energy electrons and efficiency of excitation and ionization in the hollow aperture of the anode. Optical spectrum consists of intense ion lines of the operating gas without anode or cathode material lines. 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