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Journal of Applied Physics

Issue 7 • Date Jul 1982

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Displaying Results 1 - 25 of 127
  • Issue Table of Contents

    Page(s): toc1
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    Freely Available from IEEE
  • Computational methods for solving the Dirichlet problem via Fredholm integral equations

    Page(s): 4567 - 4570
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    The numerical solution of potential problems of the Dirichlet type based on Fredholm integral equations is considered. The method is illustrated for the case of a unit sphere. In solving the integral equations there are advantages in extrapolating in the order of Gaussian quadrature, and this is done by introducing an empirical formula for curve fitting purposes. An iterative solution which overcomes the peaked nature of the kernel in the integral equations is also considered, together with a discussion of convergence and error accumulation. View full abstract»

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  • Isolation of instability in the Fredholm integral equation of the first kind: Application to the deconvolution of noisy spectra

    Page(s): 4571 - 4578
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    The numerical solution of the Fredholm integral equation of the first kind is formulated so that the source of instability is identified and can be isolated. The problem is regarded as a minimization with no constraints on the solution. It is shown that the introduction of instability depends on the path taken from the initial to the final estimate of the solution. The initial estimate is the given data, and the path which avoids instabilities is that which requires monotonic variation of these data. The method is applied to the deconvolution of noisy spectra and a more objective procedure results than has been previously obtained. Detailed applications and comparisons with previous methods are given. View full abstract»

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  • A radial ion diode for generating intense focused proton beams

    Page(s): 4579 - 4596
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    A magnetically insulated light ion diode which has produced an ion beam with total current exceeding 400 kA for 25 ns and generated a proton current density approaching 500 kA/cm2 is described. This intense beam current is achieved in a noncurrent neutralized mode via geometric focusing and a balance between self‐magnetic field and space‐charge forces. A number of techniques are described which have been used to diagnose the beam production, transport, and focusing. These include observation of Kα emission due to beam‐induced atomic excitation, prompt‐γ radiation due to beam‐induced nuclear reactions, and thermal emission due to beam‐target heating. View full abstract»

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  • The effect of charge exchange on ion guns and an application to inertial‐ electrostatic confinement devices

    Page(s): 4597 - 4601
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    In 1967, R. L. Hirsch [J. Appl. Phys. 38, 4522 (1967)] reported neutron production rates of 1010 neutrons per second from an electrostatic inertial confinement device. The device consisted of six ion guns injecting deuterium or a mixture of deuterium and tritium ions into an evacuated cathode chamber at 30–150 keV. No previous theoretical model for this experiment has adequately explained the observed neutron fluxes. A new model that includes the effects of charge exchange and ionization in the ion guns is analyzed. This model predicts three main features of the observed neutron flux: Neutron output proportional to gun current, neutron production localized at the center of the evacuated chamber, and neutron production decreasing with increasing neutral background gas density. Previous analysis modelled the ion guns as being monoenergetic. In this study, the ion gun output is modelled as a mixture of ions and fast neutrals with energies ranging from zero to the maximum gun energy. Using this theoretical model, a survey of the possible operating parameters indicates that the device was probably operated at or near the most efficient combined values of voltage and background pressure. Applications of the theory to other devices are discussed. View full abstract»

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  • The emission characteristics of an aluminum liquid metal ion source

    Page(s): 4602 - 4605
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    A study of the composition and energy distribution of a beam of ions obtained from an Al liquid metal ion source supports the field evaporation mechanism for ion formation at low currents. The beam composition varies as Al+≫Al++≳A+ 2≳ Al+ 3≳ Al+ 4≳ Al+ 5 and the energy spread ΔE varies as Al++≪Al+≪Al+ 5≪ Al+ 4≪ Al+ 3 ≪Al+ 2. Both ΔE and the ratio of Al+n/ Al+ increase with total current. Because of the strong mass dependence of ΔE, Al+ exhibits an angular intensity twice as large as Ga+ for ΔE values between 5 and 10 eV. View full abstract»

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  • A coordinate‐free approach to wave propagation in anisotropic media

    Page(s): 4606 - 4609
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    This paper considers of a coordinate‐free approach to wave propagation in anisotropic media. Based on the direct manipulation of vectors, dyadics, and their invariants, the approach eliminates the use of coordinate systems. It facilitates solutions and provides results in a greater generality. The paper contains the following results in coordinate‐free forms: The formulation of eigenvalue problems, the dispersion equation, the polarization of waves in magnetic crystals, and the determination of the directions of field vectors. View full abstract»

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  • Theoretical study of 16‐μm laser action from CO2 at low temperatures under conditions of pulsed discharge and externally applied 9.6‐μm transfer laser

    Page(s): 4610 - 4615
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    An earlier model developed by others to study the 10.6‐μm laser transition in CO2 has been extended for studying the 16‐μμ (0200)→(0110) laser transition in cold CO2. The rate equations take into account the rotational relaxation and collisional transfer of population among (0200), (1000), and (0220) levels of CO2. The model has been used to study the effect of various parameters on the 16‐μm output. It has been found that at higher pressures the collisional relaxation of the (0200) level can be overcome by application of a more powerful 9.6‐μm pulse to achieve an inversion between the (0200) and (0110) levels of CO2. It is predicted that if transverse excitation is used in these cold mixtures, pulsed powers in excess of 100 kW could be obtained on 16 μm. View full abstract»

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  • Theoretical modeling of a 16‐μm CO2 gasdynamic laser in the intracavity cascading mode

    Page(s): 4616 - 4620
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    A theoretical analysis based on a conventional CO2 gasdynamic laser (GDL) revealed the possibility of 16‐μm cascade lasing, in which a 9.4‐μm stimulating beam is internally generated, and subequently a 16‐μm emission is produced in a cascade manner. In this simulation, cascade‐ type 16‐μm lasing of 3–10 μJ/cm3 energy and 1–1.5 W/cm3 power is achievable on the P(15) line of the (0200)—(0110) transition at a position 15 cm downstream from the nozzle throat. Because of its high average power capabilities, without requiring a separate CO2 laser as a stimulating beam source, the 16‐μm cascade operation of a CO2 GDL will be practically usable. View full abstract»

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  • The effect of initiator duration on pulsed chemical‐laser performance

    Page(s): 4621 - 4630
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    The effect of initiator pulse duration on the performance of a pulsed chain reaction chemical laser is investigated using a two‐level vibrational model. Analytic results are presented for a saturated laser in the limits of weak and strong initiation. The initiator is assumed to provide a uniform (e.g., electron beam), parabolic (e.g., flash‐lamp), or power‐law variation of F‐atom production rate F˙B with time. Laser performance is presented as a function of tB /te, where tB and te are initiator and laser pulse times, respectively. In the weak initiation regime, an increase of tB /te from zero to one results in a decrement in laser output energy of 20 and 33 (1)/(3) % for a flash‐lamp and electron‐beam initiator, respectively. In the strong initiation regime, an increase of tB /te from zero to one results in an energy decrement of only 5 and 10% for a flash‐lamp and electron‐beam initiator, respectively. In each case, the laser‐pulse time te is increased by a factor of 2 as tB /te increases from zero to one. For strong electron‐beam initiation, peak laser intensity occurs at the end of the initiation pulse when tB /te?2/3, and occurs in the time interval 0.630?t/te?2/3 when tB /te≳2/3. View full abstract»

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  • Longitudinal mode self‐stabilization in semiconductor lasers

    Page(s): 4631 - 4644
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    A general mechanism of self‐stabilization of longitudinal modes in semiconductor lasers is presented. The stabilization is due to the modulation of the inverted population by the beating of the fields of lasing and nonlasing modes, thereby modulating the rate of stimulated emission. This leads to two optical nonlinearities: one causing gain suppression of nonlasing modes and the other causing coupling of pairs of nonlasing modes that are equally separated from the laser line. The two nonlinearities nearly cancel, but their net effect is a weak suppression of the nonlasing modes and stabilization of the lasing mode. Buried optical guide lasers were stabilized in a single longitudinal mode for currents greater than 6mA above threshold. The mode intensity spectra of the lasers were measured over 5 decades and converted to gain spectra, which could be compared with the theory. The gain spectrum is parabolic at threshold. At 20 mA above threshold it remains continuous at the laser line but narrows and becomes structured. The structure is characterized by a dip in the gain and in the mode intensities occurring about 3 modes from the laser line. The laser line also causes a pronounced dip in the gain spectra of the nonlasing first order transverse modes. All of these features can be approximately accounted for by the theory. View full abstract»

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  • The coherent and incoherent pumping of a gamma ray laser with intense optical radiation

    Page(s): 4645 - 4651
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    Both the coherent and incoherent pumping with intense optical radiation of electromagnetic transitions in nuclei were modeled in this work. In the first case the anti‐Stokes upconversion of conventional laser radiation was shown to be a potentially viable means for pumping a γ‐ray laser. With the most favorable possible arrangement of nuclear energy levels, the threshold for stimulated output at 10 keV might be reached at a few tens of Joules for isomeric media with lifetimes of the order of tens of years. Whether such an arrangement of excited states actually exists will require additional experimentation. In the case of the incoherent pumping of nuclear material, the resonant excitation of energetic states of nuclei by x radiation from laser plasmas or exploding wires was modeled with results similar to those for the case of coherent pumping. Thresholds for stimulated output could be as low as a few hundred Joules into the primary laser pulse producing the x‐ray flash. Again the possibilities were found to be critically dependent upon unknown parameters, particularly upon the efficiency for the conversion of input energy into x rays. The overall possibilities for the excitation of a γ‐ray laser with optical pumping were concluded to be reasonably encouraging and the need for additional spectroscopic data was clearly defined. View full abstract»

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  • Laser‐initiated vacuum plasma shutters using a dielectric aperture for retropulse isolation

    Page(s): 4652 - 4656
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    It is demonstrated that sintered LiF spatial‐filter apertures may be used in a vacuum environment of 10-6 Torr as laser‐initiated plasma shutters for retropulse isolation in high‐energy laser fusion systems. In the experimental data presented, a 1.1‐ns duration pulsed CO2 laser at 10.6‐μm wavelength with an energy of 3.0 J is used for plasma initiation. A chopped cw CO2 laser tuned to 9.6 μm is used as a probe laser in determining the time the plasma blocks transmission of a CO2 laser beam. Both the 10.6‐ and the 9.6‐μm transmission were measured as a function of pulsed laser fluence on the aperture edge, up to 240 J/cm2. Blocking times in excess of 1.0 μs were observed for an aperture diameter of 800 μm, for a Gaussian beam with a diameter of 1.2 mm determined at the 1/e2 intensity points. View full abstract»

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  • Copper‐cadmium halide photochromic glasses: Evidence for a colloidal darkening mechanism

    Page(s): 4657 - 4672
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    Optical absorption and electron paramagnetic resonance (EPR) spectra of darkened photochromic glasses containing both copper halides and cadmium halides are presented. Optical absorption spectra, calculated on a colloidal copper model with mean‐free path (MFP) and quantum‐size effect (QSE) corrections to the bulk metal dielectric constant, are fit to the data. Particles of the size derived from the MFP correction are found to be too large to be produced by photolysis of the phase‐separated droplets, presumed to be CdCl2:CuCl decorated with CuCl specks, observed in these glasses. The QSE indicates the presence of oblate spheroidal copper particles with aspect ratios between 1.8 and 3.5 and major diameters ≊8 nm, depending on the heat treatment which the glass has received. The uv‐induced EPR spectra show a peak due to Cu2+ ions with halide ions as ligands superimposed on another, broad‐winged peak which is attributed to pure or nearly pure CuCl2. These data are consistent with a darkening mechanism in which colloidal copper particles are formed by disproportionation of Cu+ in the CuCl specks and the CdCl2:CuCl droplets. View full abstract»

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  • Ultrasonic anomalies in the spectrum of acoustic waves diffracted by periodic interfaces

    Page(s): 4673 - 4680
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    The energy distribution of elastic waves reflected from corrugated periodic solid surfaces is investigated. Broadband pulse echo experiments using ultrasonic spectroscopy are carried out for surfaces with a triangular profile. Sharp discontinuities observed in the received spectrum are interpreted as anomalies first introduced by R. W. Wood [Philos. Mag. 4, 396 (1902)]. These minima are identified as the mode‐converted signals along the interface with bulk or surface wave velocities. The observed anomalies may be used to characterize interfaces and quality of bonding. View full abstract»

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  • Lattice attenuations in CdS measured by acoustic domains in a conventional square pulse application

    Page(s): 4681 - 4687
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    This paper describes a measurement of CdS lattice attenuation αl with an acoustic domain caused by a conventional square pulse. Observing the current oscillations and the intensities of 0.4 and 2.0 GHz acoustic fluxes in the initial and successive generations of the domains, we could find the shock‐excited fluxes in addition to the fluxes amplified from thermal phonons. It has been found that the αl∝ f2.0 (f:frequency) is, as Akhiezer expected, obtained in the frequency range 0.4–2.0 GHz when the domains of the successive generations are used to measure the CdS lattice attenuation. The frequency spectra of the domains’ acoustic fluxes are compared experimentally between those with or without the shock. The fluxes caused by the shock are composed of frequencies 2 GHz or lower, although the fluxes evaluated from a small signal theory are close to 3 GHz. The frequency spectrum of the acoustic fluxes is also discussed regarding the domain reflected from a central electrode between a generation region and a measured region. View full abstract»

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  • Some characteristics of pulsed nozzle beams

    Page(s): 4688 - 4694
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    Some characteristic properties of pulsed nozzle beams produced with an automobile fuel injector were observed. A commercial fuel injector was modified to produce a beam pulse of a regular square shape. The beam pulse of 5 ms is again chopped by a slotted disk chopper to make a shorter burst at an arbitrary portion of the original pulse. Rotation of the disk chopper and actuation of the injector are synchronized by a microcomputer. Time‐of‐flight measurements were performed using beams of 15.6‐μs width formed in this way. Beam fluxes were measured with a flow‐through detector. The maximum beam fluxes and terminal speed ratios obtained for several gases in this work are comparable to the values published by other workers. Recoils of beam molecules from the skimmer and its vicinity significantly affect the quality of the pulsed beams. View full abstract»

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  • Simulation of high‐voltage breakdown in the post‐arc column

    Page(s): 4695 - 4703
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    A simple model of the nonequilibrium discharge in molecular gas has been developed based on the assumption that partition of heavy species is governed by the common kinetic temperature of heavy particles. The model has been used to simulate high‐voltage breakdown in the post‐arc column in nitrogen at a pressure of 20 atm. Axial homogeneity has been assumed and electrode effects were neglected. The initial conditions were given by the arbitrary radial distributions of the electron and heavy‐particle temperatures. The electric‐field impulse was applied to the column after a period of forced cooling, which lasted approximately 30 μs. The gas sustained fields of up to 18 kV cm-1 and breakdown occurred at 24 kV cm-1. View full abstract»

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  • Transverse electron guns for plasma excitation

    Page(s): 4704 - 4710
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    We report a plasma gun, which generates on a continuous basis, kilowatt electron beam discharges (e.g., 0.4 A at 4 kV) in an ambient pressure 0.1–1 Torr without differential pumping. Gun design characteristics, operating parameters, and measured beam profiles are given. Electron beam generation on a pulsed basis has also been studied. More than 100 A of beam current has been measured with a Faraday cup biased to -100V. A trapped electron beam scheme for achieving efficient deposition of the electron beam energy in a gas medium is described. View full abstract»

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  • Spectroscopy and atomic physics of highly ionized krypton for tokamak plasmas

    Page(s): 4711 - 4725
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    We present calculations of electron‐impact collision strengths and spontaneous radiative decay rates for krypton ions of the Li I through F I isoelectronic sequences for transitions between levels of the 2s22pk, 2s2pk+1, and 2pk+2 configurations. From these atomic data, excitation rate coefficients are calculated, as well as populations of levels within the three configurations mentioned above. The calculations of level populations are carried out at three electron temperatures—2×107, 3×107, and 5×107 K (1.7, 2.6, and 4.4 keV)—which are expected to be typical for future tokamak plasmas. View full abstract»

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  • Continuous‐wave laser fluorescence spectroscopy of impurities in tokamaks

    Page(s): 4726 - 4733
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    Laser‐induced fluorescence spectroscopy has been applied as an in‐situ diagnostic for impurity atoms in the edge region of the plasma in the Argonne Plasma Engineering Experiment (APEX) tokamak. Zirconium atoms introduced from a moveable probe were excited by a cw single‐mode ring dye laser and monitored on lines of the a3F‐z3F0 manifold. The fluorescence signal from a 0.03 cm3 volume was recorded at 1‐ms intervals with a computer‐controlled 4‐channel 100‐MHz scaler system. Acousto‐optic modulation of the laser beam at 100 kHz allowed subtraction of plasma background light. Absolute calibration by Rayleigh scattering gave a detectability limit ∼1010 Zr atoms/cm3 in this apparatus. The detectability limit was determined by a detailed consideration of power and transit time broadening. The effects of several experimental parameters were examined and suggestions for increasing detection sensitivity are presented. Doppler‐shift experiments indicated a thermal‐velocity distribution for the detected Zr atoms. Intrinsic‐velocity resolution of the experiments, calculated from effective excitation linewidths, was ∼25 m/s. View full abstract»

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  • uv laser triggering of high‐voltage gas switches

    Page(s): 4734 - 4739
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    Two different techniques are discussed for uv laser triggering of high‐voltage gas switches using a KrF laser (248 nm) to create an ionized channel through the dielectric gas in a spark gap. One technique uses an uv laser to induce breakdown in SF6. For this technique, we present data that demonstrate a 1‐σ jitter of ±150 ps for a 0.5‐MV switch at 80% of its self‐breakdown voltage using a low‐divergence KrF laser. The other scheme uses additives to the normal dielectric gas, such as tripropylamine, which are selected to undergo resonant two‐step ionization in the uv laser field. View full abstract»

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  • Experimental observations of cathode‐spot surface phenomena in the transition from a vacuum metal‐vapor arc to a nitrogen arc

    Page(s): 4740 - 4749
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    This paper provides experimental data on the crater dimensions associated with low‐current (50 A) arcs burning for periods of 100 ms on copper cathodes. The cathodes were arced in metal‐vapor and nitrogen ambients, and the resulting cathode surface contamination was measured for each arcing condition. As the ambient pressure increased from 10-6 to 100 Torr, the crater diameters in the heavily arced areas decreased from 48–58 to 7–17 μm, the current carried per cathode spot decreased, and the surface contamination increased. We consider that the smaller crater dimensions are associated with the reduction in the maximum current per spot and that the maximum cathode‐spot current is influenced by the thin layer of surface contamination. We compare the trend of pressure‐related crater‐size reduction with previous observations of crater sizes as a function of current for vacuum arcs. We conclude that an individual cathode spot operating at a given current level in a gaseous ambient has many of the same basic characteristics as an individual cathode spot operating at the same current level in vacuum. An unexpected feature though, confirmed by our measurements, is that the erosion rate at higher pressures can be significantly lower than the minimum vacuum erosion rate associated with the characteristic 8– 10% ion current. Redeposition may be a plausible explanation. View full abstract»

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  • Behavior of cholesteric‐nematic mixtures under compressional deformations

    Page(s): 4750 - 4754
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    The response properties of cholesteric‐nematic mixtures to compressional deformations are studied. The behavior of transmitted and scattered light as a function of frequency and under pulsed operation is considered, looking both at the finger structure and the time response. It is shown that the system is able to oscillate at very low frequencies (to 0.1 Hz). View full abstract»

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  • Crystallization characteristics of Cu‐Zr metallic glasses from Cu70Zr30 to Cu25Zr75

    Page(s): 4755 - 4760
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    The crystallization characteristics of glassy Cu‐Zr has been studied by differential scanning calorimetry, x‐ray diffraction, and measurement of electrical resistance and magnetic susceptibility. Our results allow us to remove many discrepancies between previously published data; they also suggest that the equiatomic Cu‐Zr does not exist as a single phase in the crystalline state. We observe parallel changes on crystallization between the magnetic susceptibility and the electrical resistance, which we interpret through changes in the d density of states. 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