Volume 51, Issue 12, 01 December 1980
Index of content:

Release wave calibration of manganin gauges
View Description Hide DescriptionA calibration curve for the response of commercial Manganin gauges upon unloading in plane dynamic experiments is presented. The data obtained from 18 plane impact experiments cover shock stresses in the 1.5–15 GPa range and involves various materials. The calibration curve was drawn through normalized data points. The normalization is based on the assumption that the ratio between resistances of the gauge at the shock and unloading states depends on the ratio of the respective stresses.

Analysis of the pressure, density, and velocity distributions in a spectroscopic heat‐pipe oven and the resulting limitations on device performance
View Description Hide DescriptionThe pressure, density, and velocity distributions within a cylindrical heat‐pipe oven are analyzed for inviscid, compressible flow in either the radial or axial direction. For sufficiently large but readily attainable heat flows, density variations within the heat pipe as large as 50% which can lead to inaccuracies in quantitative spectroscopic investigations are predicted.

Determination of fuel density‐radius product of inertial confinement fusion targets by neutron activation
View Description Hide DescriptionThe quality of the confinement and thermonuclear burn of inertial confinement fusion targets scales as the density‐radius product ρR of the fuel. Neutron activation of trace elements placed in the fuel can be used to directly measure ρR. This concept and several candidate tracers, ^{14}N, ^{40}Ar, and ^{79}Br, are discussed.

Use of a radioactive tracer to determine the fraction of fusion target debris collected
View Description Hide DescriptionIn order to obtain the pusher attenuation length (ρΔR) of inertial confinement fusion targets by neutron activation, it is necessary to know what fraction of the target debris is counted. One method of measuring this quantity experimentally is by means of a radioactive tracer. We demonstrate the usefulness of this technique by experiments utilizing a ^{24}Na tracer.

Collective acceleration of electrons using an autoacceleration process
View Description Hide DescriptionA means of collectively accelerating an intense electron beam is demonstrated. A long pulse (800 ns) electron beam is propagated through cavity structures which redistributed the energy in the beam such that most of the beamenergy is transferred to a short segment (6 ns) in the tail of the beam. Electrons are accelerated in a two cavity system from 200 keV to energies of 2.4–3.0 MeV at a beam current of ∼8 kA. The extension of this principle to larger number of cavities appears feasible.

Polarization characteristics of dye‐laser amplifiers I. Unidirectional molecular distributions
View Description Hide DescriptionMany practical laser amplifiers exhibit anisotropic gain due to polarization of the pumping fields or to a fixed preferential alignment of the active dipoles. Several specific causes and consequences of gain anisotropy are discussed in detail. In the analysis, the emphasis is placed on dye‐laser systems including arbitrary amplitudes, phases, and polarizations of the pump and signal fields. Analytical results are presented for a unidrectional molecular distribution, and it is found that the polarization states of the pump and signal fields change with distance in the amplifier.

Polarization characteristics of dye‐laser amplifiers II. Isotropic molecular distributions
View Description Hide DescriptionIn a previous study a general semiclassical formalism has been developed for lasers with arbitrary orientational distributions of the active molecules. Here that formalism is applied to the important case of an isotropic distribution of molecules, and this limit is especially relevant to liquiddye lasers. Extensive analytical and numerical results are presented for the polarization properties of such dye lasers. It is found that in general the polarization states of the pump and signal fields are coupled together and both change with distance in a laser amplifier.

Pumping mechanism of mercury‐ion laser
View Description Hide DescriptionIn a pulsed discharge in a He‐Hg gas mixture favorable for laser oscillation of Hg ion at the green line, the temporal and spatial behavior of the population densities of Hg(6^{3} P _{2}) and He(2^{1} S _{0}) atoms has been studied experimentally. These reactants are thought to be responsible for the Penning‐like excitation process. It is found that the depression of the population of reactants coincides in space and time with the occurrence of the laser oscillation.

Calculated absorption, emission, and gain in In_{0.72}Ga_{0.28}As_{0.6}P_{0.4}
View Description Hide DescriptionThe absorption, emission, and gain spectra of In_{0.72}Ga_{0.28}As_{0.6}P_{0.4} (λ_{ g } =1.3 μm), which lattice matches InP, is calculated using a Gaussian fit to Halperin‐Lax band tails and Stern’s matrix element. The calculation is done both for p‐ and n‐type material at various impurity concentrations. The spectral width of emission increases both with increasing doping and impurity concentration. All results are for 297 K. The gain‐versus‐excitation rate is given by the equation g (cm^{−1})=0.057 (J _{nom} −2400), where J _{nom} is the nominal current density in the active layer expressed in A/cm^{2} μm. Also, the photonenergy at maximum gain increases with increasing excitation rate.

Large aperture stark modulated retroreflector at 10.8 μm
View Description Hide DescriptionWe describe the analysis, design, and construction of a large aperture, wide‐field‐of‐view modulator at 10.8 μm using the Stark effect in ammonia (NH_{3}). The modulator configuration incorporates a longitudinal electric field produced between closely spaced, large‐diameter germanium windows. The windows are antireflection coated for high transmission, and their relatively low resistivity allows their direct use as (transparent) Stark electrodes. Enhanced modulation depth and improved electrical breakdown performance are obtained by using multiple interaction regions. The modulator is mounted at the entrance aperture of a corner cube reflector, thus allowing two passes of the signal beam in a configuration which is retroreflecting and polarization insensitive. This completed device has an aperture of 5.5 cm, a field of view of 38°, and a measured modulation depth of 25% at 1.4 MHz.

On the theory of the prism waveguide coupler in the strong coupling regime
View Description Hide DescriptionIt is shown that under practical assumptions both ray and wave theories of the prism waveguide coupler can be applied to the whole range of the coupling strength parameter 0⩽h⩽1, and yield identical results. A numerical example is given, which shows the range of validity of the weak coupling approximation.

Jet formation experiments and computations with a Lagrange code
View Description Hide DescriptionA jet formation routine has been generated that can be used with (Lagrange) explosive cone‐collapse calculations. It calculates the velocities of jet and slug elements, as well as their mass, formation point, and time. The equations and method of operation are presented here. Experimental jet velocities agree with the model to 10%. Jet deformation calculations required yield strengths up to 24 kbar to yield agreement with experiments on copper jets.

The photoacoustic effect at first‐order phase transition
View Description Hide DescriptionThermal properties are essentially involved in the acoustic response of a liquid or solid sample to the absorption of chopped light. Recently, the first experimental observation of the photoacoustic effect at first‐order phase transitions has been published. In the present work we calculate the temperature variation in the gas at the surface of a sample and determine its amplitude and phase angle. Therefore we solve the differential equation of conduction of heat for a medium exhibiting a temperature gradient superposed by a temperature oscillation due to absorption of light with sinusoidal varying intensity. In the medium there exist two regions of different thermodynamic phases separated by an interface at transition temperature. The latent heat for phase transition is taken into account via a boundary condition for the heat flow at the interface. We have assumed the optical‐absorption coefficient to be much larger than the thermal‐diffusion coefficient realized in strongly absorbing materials. The application of our calculation to measured data is discussed with respect to the technique used to detect the acoustic signal. With the model, we present here, the experimental results can be described fairly well.

Hard‐x‐ray spectrum from a single‐electron‐temperature laser plasma model
View Description Hide DescriptionComparisons between hard‐x‐ray experimental spectra and those obtained from a planar one‐dimensional one‐temperature fluid laser‐produced plasma model show good agreement over several orders of magnitude in x‐ray intensity and for Nd‐laser irradiances varying between 2×10^{14} and 8×10^{15} W/cm^{2}. The key to this agreement is the careful modeling of electron‐energy flux limitation by return‐current‐driven ion‐acoustic turbulence. The hard‐x‐ray temperature agrees with a (Iλ^{2})^{1/3} scaling law.

Multichannel discharges in low‐pressure rare‐gas–mercury mixtures caused by anode oscillation
View Description Hide DescriptionStable formation of a multichannel arc is observed in low‐pressure rare‐gas–mercury discharges in a concentric tube where a hot cathode is located in the inner tube and two to six anodes are arranged symmetrically in the space between the inner tube and the outer bulb. The discharge current for each discharge channel is found to be deeply modulated at high frequencies by the anode oscillations which take place in a regular, complex sequence depending on the anode configuration. In a discharge tube with two anodes,anode oscillation occurs alternately, causing discharge transfers between the anodes. With three anodes, the discharge transfers take place in a manner similar to a ring counter, as anode oscillations occur in turn in geometrical order. In the four and five anode configurations, oscillations occur sequentially in time, but not sequentially in the anode configuration. In the case of six anodes, stable formation of a multichannel arc is not observed.

Spherical shock development near laser‐heated microshell targets
View Description Hide DescriptionSpherical expansion of the luminous front around a laser‐heated microshell target in 20 Torr of helium is compared with theory in the mass‐dependent region. Microshell targets from 74‐ to 7970‐ng initial mass irradiated with up to 0.13 TW (Terawatt) of 1.06‐μm light were studied. Effects of the initial mass and changing γ on the front history are discussed. It is concluded that extraction of expansion energy from the front motion is not easily accomplished in this close‐in region.

Cutoff frequencies of intermediate frequency waves in a bounded magnetoplasma
View Description Hide DescriptionThe paper presents a study of the cutoff frequencies of a class of electromagnetic waves, designated here as intermediate frequency waves, which can propagate in a plasma column enclosed in a conducting cylinder and embedded in a static axial magnetic field. These waves reduce to lower hybrid waves or magnetohydrodynamic TE waves under suitable conditions. The cutoff frequencies of the intermediate frequency modes have been investigated, covering all possible values of ω_{ p e },ω_{ c e }, and a, where ω_{ p e } is the plasma frequency, ω_{ c e } is the electron cyclotron frequency, and a is the plasma radius. The study includes all the possible types of modes, namely, circularly symmetric I _{0m } modes and asymmetric I _{ n m } modes with n ? 1, each I _{ n m } mode being split into two, called I ^{+} _{ n m } and I ^{−} _{ n m } modes, corresponding to two different polarizations of the field components. The cutoff frequency ω_{ c 0} of any I _{ n m } mode with n ? 0 is found to be confined to the frequency domain 0 to ω_{ n m }, where ω_{ n m } is the cutoff frequency of the TE_{ n m } mode in an empty waveguide. The zero frequency is approached if ω_{ p e } or a tends to infinity or ω_{ c e } tends to zero, the other parameters remaining constant in each case. The frequency ω_{ c 0} tends to ω_{ n m } if ω_{ c e } is sufficiently large for any given set of ω_{ p e } and a. The study of intermediate frequency modes reveals that these modes can broadly be divided into the following two categories: (1) I ^{−} _{ n1} modes with n ? 1, and (2) modes other than I ^{−} _{ n1} modes. The nature of variation of ω_{ c o } ith ω_{ p e } for fixed values of ω_{ c e } and a for I ^{−} _{ n1} modes is, in general, quite different from that for the other modes. Furthermore, ω_{ c 0} tends to ω_{ c i } as a tends to zero with ω_{ p e } and ω_{ c e } held fixed in the case of I ^{−} _{ n1} modes, while for other modes ω_{ c 0} tends to ω_{ l h } under the same condition, where ω_{ c i } is the ion cyclotron frequency and ω_{ l h } is the lower hybrid frequency. Any I _{ n m } mode with n ? 0 is found to reduce to the magnetohydrodynamic TE_{ n m } mode at or near the cutoff under the condition: ω_{ p i }≫ω_{ n m }, where ω_{ p i } is the ion plasma frequency. Finally, it is shown how the charge density in a bounded magnetoplasma can easily be determined in most experimental situations by using the cutoff frequency characteristic of the dominant I ^{−} _{11} mode.

Low‐current electric discharges in H_{2}‐He mixtures
View Description Hide DescriptionTests are made of the ability of currently available theory to predict experimental ratios of the electric field to gas density E/N required for the steady‐state operation of low‐current, moderate‐pressure discharges in hydrogen‐helium mixtures. In the experiments the fractional concentration of H_{2} in He was varied from 1% to 4% and the dischargecurrent, i.e., 40±10 μA, and total gas density, i.e., 10^{23}–10^{25} atoms/m^{3}, were chosen to avoid gas heating and striations. The experimental E/N values for the 4‐mm‐diam tube varied from 6×10^{−21} V m^{2} for 1% H_{2} in He at a gas density of 7×10^{24} m^{−3} to about 4×10^{−20} V m^{2} for 4% H_{2} in He at 1.5×10^{23} m^{−3}. Theoretical steady‐state E/N values versus gas density were calculated using (a) solutions of the Boltzmann equation for He metastable excitation, He ionization, and H_{2}ionization rate coefficients; (b) published theoretical calculations of the effects of a finite Debye length and a finite ion mean free path on the electron loss rate; and (c) models for the ion‐molecule conversion and Penning ionization processes. The differences between theoretical and experimental values of E/N are less than 10% for total gas densities above 6×10^{23} m^{−3}, but are 20–30% for a gas density of 1.6×10^{23} m^{−3}. We conclude that except at low gas densities the simple theory gives a good description of low‐current discharges in this gas mixture.

Polar and nonpolar contributions to liquid‐crystal orientations on substrates
View Description Hide DescriptionBoth experimental and theoretical evidences are presented for polar and nonpolar contributions to liquid‐crystal orientations on surface‐treated substrates. Polar and nonpolar components of substrate surface energies are determined from contact angle measurements using polar liquids.Liquid‐crystal orientations are characterized using orientation angles and anchoring strength coefficients at substrate surfaces, both of which are measured by the magneto‐capacitance method. Numerical calculations give correlations between liquid‐crystal orientation and substrate surface energy components, which are in good agreement with experimental results. These calculated correlations involve the critical surface tension hypothesis as a limited case when polar interfacial interactions are negligible.

The mechanism of growth of quartz crystals into fused silica
View Description Hide DescriptionIt is proposed that the growth of quartz crystals into fused silica is effected by a mechanism involving the breaking of an Si‐O bond and its association with an OH group, followed by cooperative motion of the nonbridging oxygen and the hydroxyl group which results in the crystallization of a row of several molecules along a crystalline‐amorphous interfacial ledge. This mechanism explains, at least qualitatively, all the results of our earlier experimental study of the dependence of quartzcrystal growth upon applied pressure: large negative activation volume; single activation enthalpy below Si‐O bond energy;growth velocity constant in time, proportional to the hydroxyl and chlorine content, decreasing with increasing degree of reduction, and enhanced by nonhydrostatic stresses; lower preexponential for the synthetic than for the natural silica.