Volume 55, Issue 7, 01 April 1984
Index of content:

A new metal resistor bolometer for measuring vacuum ultraviolet and soft x radiation
View Description Hide DescriptionA new metal resistorbolometer has been developed by applying thin‐film technology. It is composed of three layers, a 4‐μm‐thick radiation absorber made of gold, a 7.5‐μm‐thick kapton dielectric, and a 0.1‐μm‐thick 5‐kΩ goldresistor. This detector with the appropriate electronics shows a linear response to radiation power, including both neutral‐particle emission and electromagnetic radiation from the soft x‐ray part of the spectrum to the infrared. The bolometer has a very high operating reliability and sufficient suppression of ambient interference under extreme environmental conditions, such as high neutron and gamma radiation fluxes, high temperatures, mechanical vibrations, and strong electromagnetic fields. In plasma discharges in the ASDEX tokamak a radiationdetection limit of 100 μW/cm^{2} was obtained at a time resolution of 10 ms. The bolometers of an array can be calibrated i n s i t u; the calibration data are reproducible and stable in time within ±10%. Measurements in ASDEX which demonstrate the capability of the method are discussed.

Fluorescence line narrowing in neodymium laser glasses
View Description Hide DescriptionPolarized and time‐resolved resonant laser‐excited fluorescence spectra were recorded for the ^{4} F _{3} _{/} _{2}→^{4} I _{1} _{1} _{/} _{2}transition of Nd^{3} ^{+} in several commercial silicate and phosphate glasses. Excitation sources were pulsed Nd:LiYF_{4} (1053 nm) and Nd:Y_{3}Al_{5}O_{1} _{2} (1064 nm) lasers. Measurements of line‐narrowed spectra yield linewidths ranging from 35 to 64±10 cm^{−} ^{1}, depolarization ratios from 0.81 to 0.93, and spectral energy transfer rates ≲3×10^{3} s^{−} ^{1} for Nd^{3} ^{+} concentrations≲2×10^{2} ^{0} cm^{−} ^{3}. The variations of these properties with wavelength and glass composition are discussed. Hole burning and gain saturation of glass laser amplifiers due to spectral and polarization inhomogeneities can be predicted from these results.

Matrix formulation of Slater’s cavity perturbation theorem
View Description Hide DescriptionIn this modification of Slater’s cavity perturbation theorem the perturbed magnetic field is expressed as a Fourier series based on the unperturbed cavity modes. The discontinuity in the field (hence the current) at the perturbing surface is two times the normal mode sum. Mode coupling terms are defined, leading to a matrix whose eigenvalues are the perturbed resonance frequencies of the cavity. Further manipulations of this matrix yield the normalized wave function as the normal mode sum. This technique is illustrated by a calculation for ridge waveguide.

Space‐charge‐Cerenkov and cyclotron‐Cerenkov instabilities in an electron‐beam dielectric system
View Description Hide DescriptionA relativistic electron beam propagating along a constant magnetic field has four normal modes: the slow and fast space charge and the slow and fast cyclotron modes. When propagating through a dielectric where the beam velocity is greater than the velocity of light in the dielectric, the slow modes may become unstable. We call these the space‐charge‐Cerenkov and cyclotron‐Cerenkov instabilities. In this paper we use the cold fluid model in the small signal limit to obtain a general dispersion relation for propagation at an arbitrary direction with respect to the magnetic guide field. This describes both of the above instabilities as special cases. This result is then generalized to include the effects of a thermal spread. Implications for possible application for generation or amplification of microwaves, where the beam passes near the dielectric, are also considered. We conclude that the cyclotron‐Cerenkov instability is a strong candidate for the short wavelength microwave regime.

High‐energy optically pumped iodine laser. I. Kinetics in an optically thick medium
View Description Hide DescriptionThe operation of a high‐energy atomic iodine laser pumped by high‐intensity broadband radiation is described theoretically. Nonlinear transport of pump radiation into an optically thick medium is specifically taken into account. Integrodifferential formulations are made for both planar and cylindrical geometries. The model obtains the temporal and spatial dependences of the various chemical species, gas temperature, pump radiation, and stimulated emission in the laser medium. For an active gas of n‐C_{3}F_{7}I, relevant temperature‐dependent rate constants, absorption cross sections, and line‐broadening coefficients were selected after a critical review of the literature. The model is used to interpret recent laser experiments performed in the nonlinear regime using an intense (∼22 000 K) flash lamp to pump a n‐C_{3}F_{7}I medium with and without buffer gas. The theoretical results are in excellent agreement with the experimental measurements. The relative importance of the various kinetic processes is evaluated, and the spatial and temporal dependence in the optically thick medium is exhibited. Heating of the medium leads to major changes in the lasing kinetics. It is also observed that a gas‐dynamic perturbation wave from the flash‐lamp surface significantly limits laser output in some cases. Under some conditions this source drives a pyrolysis wave into the medium, but it is not strong enough to drive a faster photolysis wave.

Thermal‐anneal wavelength modification of multiple‐well p‐n Al_{ x }Ga_{1−x } As‐GaAs quantum‐well lasers
View Description Hide DescriptionData are presented showing that ordinary thermal annealing can be used to modify GaAs square wells into rounded Al_{ x }Ga_{1−x }As quantum wells and shift the continuous 300‐K laser operation of a p‐n multiple‐well Al_{ x }Ga_{1−x }As–GaAs heterostructure laser to higher energy. Transmission electron microscopy is used to show that thermal annealing at 900 °C for 10‐h changes, for example, well sizes from 85 to 105 Å and coupling barriers from 95 to 75 Å, which results in a change of laser photon energy of Δℏω∼50 meV. Bandfilling is minimal in multiple quantum‐well lasers, thus making thermal annealing a useful method to ‘‘tune’’ a continuous 300‐K quantum‐well laser to shorter wavelength as shown here. These thermal annealing experiments indicate that the Al‐Ga interdiffusion coefficient at a heterointerface is D(900)∼10^{−} ^{1} ^{8} cm^{2}/s.

Ray theory of the Love wave interactions
View Description Hide DescriptionA ray theory is developed for the treatment of three types of Love wave problems. First, the characteristics of the Love wave are analyzed by the ray technique. Second, for a film with an appropriate weak harmonic corrugation on one of its surfaces, a ray treatment of the first‐order coupling between the two Love waves is given. Third, for a film with a suitable weak harmonic corrugation of its free surface, the second‐order coupling between the Love wave and the bulk elastic wave in the substrate is investigated using the ray procedure. In all cases, the ray method yields asymptotically exact results for small relative corrugation depths.

Optogalvanic detection of acoustic resonances in a high‐pressure sodium discharge
View Description Hide DescriptionIn high‐pressure discharges, arc instabilities may occur due to acoustic resonances inside the discharge vessel. We have observed experimentally the presence of longitudinal acoustic resonances in a high‐pressure sodium discharge well below the threshold above which the arc is unstable. A dye laser, tuned to about 17 140 cm^{−} ^{1}, proved to be able to excite the acoustic standing waves, and their galvanic effects have been measured. A local thermodynamic equilibrium discharge model is presented, which qualitatively describes the measurements. It is shown that the instability threshold is correlated to an acoustic pressure amplitude in the ventral segments of about 6 kPa.

Analysis of the skin current in a finite‐length cylindrical plasma with external electric field
View Description Hide DescriptionTheoreticalanalysis and numerical calculations are carried out of the skin current in a finite‐length cylindrical plasma when a step‐like electric field is applied externally, within the framework of classical theory. The spatial and temporal evolutions of the skin current are numerically calculated for two cases of plasmas with uniform and parabolic density distributions.

Propagation of intermediate frequency waves in a bounded magnetoplasma
View Description Hide DescriptionThe paper presents a study of the propagation of intermediate frequency waves in a plasma column magnetized axially and enclosed in a conducting cylinder. The general dispersion relation for the waves, obtained by the dynamic method, is found to reduce to four distinct equations associated with different forms of the field components. The ω‐β plane is divided into several regions of four types so that within each region only one of the above four equations would apply. Dispersioncharacteristics are obtained for all possible types of intermediate frequency modes in a hydrogen plasma for a typical set of ω_{ c e }/ω_{ p e } and aω_{ p e }/c, where ω_{ c e } is the electron cyclotron frequency, ω_{ p e } is the electron plasma frequency, a is the plasma radius, and c is the velocity of light in free space. It is discussed how the dispersioncharacteristics of the different modes would change with variations in the plasma parameters. The investigations show that, contrary to the predictions of the usual quasistatic approximation, propagation of all intermediate frequency modes is possible at frequencies below the lower hybrid frequency ω_{ l h }. Moreover, the I ^{−} _{ n1} modes with n≠0 and the superscript indicating the polarization form a special category of modes in the sense that they have a resonance at the ion cyclotron frequency ω_{ c i } and that their propagation below the frequency ω_{ l h } is confined to the domain 0<ω<ω_{ c i }. In the case of all other modes the phase coefficient β increases gradually to a finite value as ω increases from its cutoff value to ω_{ l h } ; this value of β lies within two specified limits, β_{1} and β_{2} ( β_{2}>β_{1}), which are functions of ω_{ p e }, ω_{ c e }, and ω_{ c i }. The dispersioncharacteristic of each of these modes has a sharp discontinuity at ω=ω_{ l h }. If ω is increased beyond ω_{ l h }, the characteristic of the I _{0} _{1} mode starts off from the point (ω_{ l h },0), while the corresponding point for all other modes is given by (ω_{ l h }, β_{1}) . However, all the modes have a resonance at ω_{ p e }(ω_{ p e }<ω_{ c e }). It is interesting to note that if the mode number is increased or the plasma radius is reduced, keeping the other parameters fixed, the phase coefficient β at any given frequency increases in the frequency domain ω_{ l h }<ω<ω_{ p e }, while in the domain 0<ω<ω_{ l h } it falls until it becomes zero. Finally, it is shown that there is considerable discrepancy between the results given by the quasistatic approximation and those by the dynamic analysis even in the most favorable case of the I _{0} _{1} mode in the frequency domain ω_{ l h }<ω<ω_{ p e } if the plasma radius a is relatively large, compared to c/ω_{ p e }.

Ion dynamics of rf plasmas and plasma sheaths: A time‐resolved spectroscopic study
View Description Hide DescriptionA new plasma diagnostic technique, time‐resolved laser‐induced fluorescence (TRLIF) spectroscopy, has been developed to monitor ion dynamics in rf glow discharges. Time‐dependent ion concentrations are measured by pulsing a dye laser synchronously with the applied rf potential and then detecting laser‐induced fluorescence. In this initial work, Cl_{2}/N_{2}plasmas have been investigated. In the electrodesheaths, the ion response to the applied potential was measured directly. This response governs ion transport to microelectronic device surfaces in plasma etching and deposition. At frequencies below the ion plasma frequency (∼1 MHz), we found that the electrodesheaths expand and contract with period f ^{−} ^{1}. The sheath front travels at a speed which depends linearly upon both frequency and sheath thickness, but the sheathexpansion begins only after a critical voltage is applied. Characterization of the sheath in terms of its thickness, expansion speed, and critical expansion voltage promises to be a useful means by which ion transport can be modeled. In the discharge center, TRLIF was used to measure ground‐state ionic lifetimes, which are essential input for kinetic models of plasma reactors. In pure N_{2} discharges the N^{+} _{2} loss rate increases linearly with pressure but the loss mechanism is not clear. In N_{2}/Cl_{2}plasma mixtures, N^{+} _{2} rapidly charge exchanges with Cl_{2} (k≥6×10^{6} sec^{−} ^{1} Torr^{−} ^{1}) to form Cl^{+} _{2} .

Finite geometry effects on the stability of a charged beam propagating through a relativistic annular electron beam
View Description Hide DescriptionFinite geometryeffects on the stability properties of a charged beam propagating through an intense relativistic annular electron beam have been studied. The stability of the system under transverse oscillation has been examined in detail in a parameter domain pertinent to the collective particle accelerator, currently under development at the Naval Research Laboratory. Both the normal mode and the convective aspects of this instability have been investigated. Despite a substantial temporal growth rate as predicted by the normal mode approach, this instability does not prevent successful acceleration of a portion of the axial beam. Thus the transverse oscillation is not fatal to the collective particle accelerator operation.

Measurement of electron loss rates in high‐pressure mercury
View Description Hide DescriptionThe rates of electron decay in high‐pressure mercury/helium discharges have been measured using a novel two‐photon ionization technique. The rate coefficient of electron loss in this system was found to be well characterized by a two‐body rate coefficient of k _{ d }=1.05(±0.25)×10^{−} ^{8} cm^{3} sec^{−} ^{1}, assuming dissociative electron‐ion recombination. The electron temperature during these measurements is calculated to be 0.4(±0.1) eV. We also report a free‐bound optical absorption coefficient at 222 nm of σ̂_{ A }=(2.0±0.5)×10^{−} ^{3} ^{9} cm^{5}.

Dielectric results of polymeric glasses formed under hydrostatic pressure: Is the densified sample closer to thermodynamic equilibrium?
View Description Hide DescriptionThe energy state of densified polymericglasses is examined by using the Lennard–Jones model. We question the proposal that densified glasses are closer to equilibrium than glasses formed at the same cooling rate at atmospheric pressure. The origin of an internal stress quenched into the sample during densification treatment is shown. Some dielectric results on polyethylene terephthalate, polyvinyl chloride, and polystyrene are discussed in terms of the free volume decrease or the quenched energy. Our conclusion is that the densification treatment induces two concurrent phenomena: (1) a decrease of fractional free volume, which is preponderant on the energy state of the material when the densification pressure is small; (2) an internal stress which increases the internal energy and develops quickly for high pressures.

Determination of the crystal structure of Pb_{2}CrO_{5}
View Description Hide DescriptionThe crystal structureanalysis of Pb_{2}CrO_{5} is determined by comparison with powder x‐ray and electron diffraction data. The structure is monoclinic: c2/m. All of the atom positions are determined by considering the calculated values of d spacings, line intensities, bond lengths, and bond angles. A new set of lattice parameters are a=14.018, b=5.683, c=7.143 Å, and β=115.23 °. A preliminary result on photoconductivity in a Pb_{2}CrO_{5}ceramic disk is also given.

Dynamics of defect creation by ion implantation in thermal SiO_{2}
View Description Hide DescriptionE ^{′} _{1} (bridging oxygen vacancy) defect densities have been measured in thermal SiO_{2}filmsimplanted with ions having various energies and masses. From a plot of the defect production per unit volume versus energy deposited into elastic processes per unit volume we observe a linear slope yielding one E _{1} defect per 2000 eV deposited in displacement energy. Comparison with the measured average displacement energy per ion of 4 eV suggests that up to 500 defects of other forms might be created for every E ^{′} _{1}. The predictions of a simple model for E _{1} creation/annihilation are further tested and the magnitude of the microscopic creation and annihilation cross sections found to be of the order of 10^{−} ^{1} ^{3} cm^{2} per ion in both cases. The ratio of the creation and annihilation cross sections is found to decrease monotonically with decreasing implant ion mass.

The effect of material strength on determining pressures ‘‘on’’ and ‘‘off’’ the Hugoniot
View Description Hide DescriptionIt has been traditional to use the fluid approximation when using solid Hugoniot data to construct a constitutive equation for the pressure as a function of volume and energy. The tradition rests on the assumption that material strength is negligible at large pressures, and the unavailability of Hugoniot shear stress data. However, material strength may be non‐negligible, even at large Hugoniot stresses, and a method for calculating shear stresses along the Hugoniot has been developed, so that in principle, P(V,E) can be constructed without making the fluid approximation. Using beryllium as an example, we show how to construct P(V,E), taking material strength into account. We also show that the calculated shear stresses along the Hugoniot are less than the actual values, due to assumptions that are made to reduce the data. Thus, the resulting ratios of the Hugoniot shear stress to the Hugoniot pressure underestimate the actual values. In beryllium, this ratio varies from 0.07 at 5 GPa to 0.015 at 35 GPa.

Physical and electrochromic properties of the amorphous and crystalline tungsten oxide thick films prepared under reducing atmosphere
View Description Hide DescriptionThe physical and electrochemichromic properties of the amorphous and crystalline tungsten oxide films prepared by thermal evaporation of WO_{3}powder under a reducing atmosphere have been investigated. The oxide films with 850–24 500 Å thick were deposited on the substrates maintained at a temperature ranging from 50 to 500 °C under a vacuum below 1×10^{−} ^{5} Torr. Properties of the oxide films formed depend considerably on the depositiontemperature. The as‐prepared oxide films formed at a temperature higher than 400 °C have a resistivity of 10^{−} ^{3}−10 Ω cm, and are blue colored, and mainly composed of crystalline WO_{3}. The oxide films formed at a temperature lower than 350 °C have a resistivity of 10^{5}–10^{9} Ω cm, and are transparent and amorphous. Both of the amorphous and crystalline oxide films have good electrochemichromic properties. From x‐ray analysis, the crystalline WO_{3} thick films were found to change to hydrogen tungsten bronze films with a chemical composition of H_{0.33}WO_{3} during electrochemical coloration, and the H_{0.33}WO_{3}films were found to be reversed to the WO_{3}films during bleaching. In the electrochemical coloration using the cells composed of a crystalline or amorphous oxide film, the transmittance (λ=0.5 μm) and current were found to reach simultaneously an electrochromic steady state when a sufficient charge was injected. The density of the tungsten oxide films with 5000–24 500‐Å thickness was also measured, and found to be 5.3–6.6 g/cm^{3} depending on the depositiontemperature. The optical properties and the results of observations by scanning electron microscopy are also described.

Stabilization of positive charge in fluorinated ethylene propylene copolymer
View Description Hide DescriptionStabilization of positive charge in fluorinated ethylene propylene copolymer (Teflon FEP) is demonstrated using coronacharging through a controlling grid at elevated temperatures. The extrapolated lifetime of the equivalent surface potential is 800 yr for samples charged at 150 °C and deteriorates for lower charging temperatures. Obtained results for the trap structure confirm published data on the same material.Activation energies, determined by the initial rise of thermally stimulated current measuremnts, are 0.63, 1.16, 1.43, and 1.5 eV for samples charged at 30, 60, 90, and 150 °C, respectively. A uniform charge distribution throughout the bulk of the material is found in samples charged above 100 °C. The total density of occupied traps decreases from 2.2×10^{1} ^{4} cm^{−} ^{3} to 1.0×10^{1} ^{4} cm^{−} ^{3} when charging at temperatures ranging from 100 to 180 °C.

Propagation properties of fluxons in a well‐damped Josephson transmission line
View Description Hide DescriptionFluxon threshold properties and propagation velocities have been studied with regard to Josephson transmission line (JTL) having an external shunt resistance parallel to the Nb/Nb‐oxide/Pb junction. Stable and separated multifluxon waveforms have been observed in the well damped JTL. Experimentally obtained thresholds for the number of propagated fluxons and interval times between each respective fluxon agree with numerical simulations. The dependence of fluxon propagation velocities on the bias current level can be interpreted in terms of the McLaughlin–Scott theory.