Index of content:
Volume 64, Issue 1, 01 July 1988

Probe beam size effects in photothermal deflection experiments
View Description Hide DescriptionImportant effects of the finite size of the probe beam occur in optical beam deflection experiments when the probe beam radius is of the order of magnitude of the thermal diffusion length. The calculation of this effect is checked by experiments in air at frequencies around 10 kHz and in liquids around 100 Hz. In this last case, for certain values of the modulation frequency and of the probe beam‐sample surface distance, the amplitude of the deflection exhibits a sharp minimum and its phase varies drastically.

Propagation of wiggler focused relativistic sheet electron beams
View Description Hide DescriptionA recent design concept for millimeter‐wave free‐electron lasers [J. Appl. Phys. 6 0, 521 (1986)] would require the stable propagation of a sheet electron beam through a narrow waveguide channel. Experimental results reported in this article support the feasibility of such a configuration by demonstrating the stable propagation of relativistic sheet electron beams through a narrow waveguide gap (3.2 mm) using focusing by a short‐period electromagnetwiggler. 90% of the electron current in a 100‐keV sheet electron beam was transmitted through a 5‐cm‐long channel with peak wiggler fields of 800 G. Almost 80% of a 400‐keV beam was similarly confined with a 1600‐G wiggler field. The data were consistent with single electron trajectory models, indicating that space‐charge effects were minimal. No evidence of beam breakup or filamentation instabilities was observed.

Reverse field injection for the gradient B drift transport of a high‐current electron beam
View Description Hide DescriptionReverse field injection was used to inject a 0.9‐MeV electron beam produced in a ring diode into a drift tube. The annular beam then was transported distances of up to 183 cm by gradient B drift in an applied magnetic field. The electron beam struck a tantalum target at the end of the drift tube, producing bremsstrahlung x‐rays. An electron beam current of 1.1 MA was inferred from measurements of x‐ray production in the target. This implies an injection efficiency of 69%.

A model for pixellation of nonlinear interference filters
View Description Hide DescriptionWe present a theoretical model for the effect of material pixellation on the operating characteristics of optical logic elements using nonlinear interference filters. From our calculations we predict pixel packing densities of 250×250 per cm^{2} (an increase of three orders of magnitude over optical pixellation) and power levels of 100 μW. We define a figure of merit which describes the effects of pixellation of thermo‐optic logic elements. We also show numerically the effect on cooling rate.

Two‐wave mixing with gain in liquid crystals at 10.6‐μm wavelength
View Description Hide DescriptionSignal‐beam amplification is achieved in degenerate two‐wave mixing experiments in liquid crystals at the wavelength λ=10.6 μm of the cw CO_{2} laser. The origin of the dynamic grating is the large thermal index change that arises when the cell operates near the phase transition temperature. Maxwell’sequations are analytically solved for the calculation of the amplitude variations of the incident waves (I _{ p }:pump, I _{ s }:signal) and of the self‐diffracted wave (I _{−} _{1}). It is shown that energy transfer arises from the contribution of the π/2 component of the index modulation generated by the interference of (I _{ p },I _{−} _{1}). The resulting three‐wave model yields good agreement with the experimental results.

Phase locking and stability properties for two coupled semiconductor lasers
View Description Hide DescriptionIn this paper, a theoretical investigation of the locking of two weakly coupled semiconductor lasers is presented. The analysis begins with a set of rate equations for the electric fields and carrier densities inside each laser diodecavity. From these, the dynamic stability of the solutions as a function of the detuning between the lasers and the linewidth enhancement factor of each laser is determined. It is seen that when the linewidth enhancement factor is the same for both lasers, the entire locking bandwidth is dynamically stable. When the linewidth enhancement factor is different for the two lasers, asymmetric locking occurs, and only a portion of the locking bandwidth is dynamically stable.

A thermodynamic phenomenology for ferroelectric tungsten bronze Sr_{0.6}Ba_{0.4}Nb_{2}O_{6} (SBN:60)
View Description Hide DescriptionThe tetragonal tungsten bronze ferroelectrics in the strontiumbariumniobate system have been extensively studied over many years. As for many of the bronzes, a crude interpretation of the experimental data has been attempted in the past using the simple Landau‐Ginsburg‐Devonshire expansion of the Gibbs free energy as a Taylor series in powers of the polarization, lumping all the temperature dependence into the lowest order term. In this paper new measurements are presented for the temperature dependence of dielectric polarization,permittivity, and the E‐field dependence of the permittivity. It is shown that for a realistic fitting of the data, the Taylor expansion must be taken to at least the eighth power term, and that the coefficients of terms up to the sixth power must be taken as functions of temperature. Since the phenomenology describes equilibrium behavior, it is the total static polarizability that is being explored in this treatment. The nature of this temperature dependence strongly suggests that the phase transition from a macropolar to a macrononpolar state is tetracritical.

Liquid‐metal flows and power losses in ducts with moving conducting wall and skewed magnetic field
View Description Hide DescriptionFully developed, laminar liquid‐metalflows, currents, and power losses in a rectangular channel in a uniform, s k e w e d high external magnetic field were studied for high Hartmann numbers, high interaction numbers, low magnetic Reynolds numbers, and different aspect ratios. The channel has insulating side walls that are skewed to the external magnetic field. Both the perfectly conducting m o v i n g t o p w a l l with an external potential and the stationary perfectly conducting bottom wall at zero potential act as electrodes and are also skewed to the external magnetic field. A solution is obtained for high Hartmann numbers by dividing the flow into three core regions, connected by two free‐shear regions, and Hartmann layers along all the channel walls. Mathematical solutions are presented in each region in terms of singular perturbation expansions in negative powers of the Hartmann number. The free‐shear layers are treated rigorously and in detail with fundamental magnetohydrodynamic theory. Numerical calculations are presented for the total current carried by the core region between top and bottom electrodes, Joulean and viscous power losses, and channel resistance at different skewed external magnetic field angles. With the high external magnetic field, the current through the central core region between the electrodes must be parallel to the external magnetic field lines. The two side core regions carry no current to the zeroth order. The two free‐shear layers carry less current than the central core region. The theoretical magnetohydrodynamic model derived here was developed to provide data to help in the design of liquid‐metal current collectors.

Experimental verification of cyclotron absorption of polarized electromagnetic waves in a plasma waveguide
View Description Hide DescriptionThis paper presents the results of an experiment dealing with the absorption of electromagnetic waves in a longitudinally magnetized plasma waveguide at electron cyclotron frequency. Our results demonstrate, under different operating conditions, the importance of the polarization of the incident waves as well as the linear behavior of the plasma density as a function of the absorbed power. The gas used in this study was argon.

Ion beam propagation in a transverse magnetic field and in a magnetized plasma
View Description Hide DescriptionPropagation of a charge‐neutralized ion beam, in a transverse magnetic field (B _{ z } <400 G) and in a magnetized plasma, has been studied. Measurements indicate that the beam propagation mechanism is due to the E×B drift in the region of high β (1<β<400), where β is the ratio of beam kinetic energy to transverse magnetic field energy. Diamagnetic measurements, both internal and external to the propagating beam, confirm the fast diffusion of B _{ z } into the beam on a time scale much shorter than the beam rise time of 10^{−} ^{7} s. When the beam is injected into a magnetized plasma the electric field is shorted to a degree that increases with increasing background plasma density. When the plasma density reaches 10^{1} ^{3}/cm^{3} (∼200×the beam density) complete shorting occurs and the beam is deflected by the transverse magnetic field.

Aspects of energy transport in a vortex‐stabilized arc
View Description Hide DescriptionBy measuring the axial profiles of thermal wall loading and radiation output in a dc vortex‐stabilized arc, it has been shown that the vortex significantly influences the energy transport in the arc. The arc was operated at 225 A in an argon vortex at a pressure of 5.5 atm. The measurements show that there is a rapid onset of thermal wall loading immediately adjacent to the upstream electrode. Furthermore, there is a significant amount of thermal wall loading upstream of the arc column, which is found to scale with the flowrate through the vortex. This fact, taken together with observations of the tracks produced by tungstendroplets on the arc wall, and photographs of the electrode regions indicate that reverse axial flow exists in the core of the arc vortex. Near the electrodes there is a region of locally increased thermal wall loading and radiation output, but between these regions there exists a region where the wall loading and radiation are axially invariant. In this region the measured wall loading is as much as 100% larger than that predicted using laminar heat transport. The scaling of this wall loading with gas flowrate in the vortex has been measured, and it has been shown that if the data are extrapolated to the no‐flow condition, the wall loading is in much better agreement with the laminar heat transportmodel. The existence of turbulence or Taylor–Görtler vorticies in the stabilizing gas flow are proposed as mechanisms for the enhanced thermal wall loading.

Collisional‐radiative ionization and recombination model for high‐temperature air and its application to discharges
View Description Hide DescriptionA collisional‐radiative (C‐R) model for high‐temperature air is developed. Ionization and recombination coefficients are presented for electron temperatures between 1.0 and 3.0 eV and electron densities between 10^{1} ^{6} and 10^{1} ^{9} cm^{−} ^{3}. For an assumed airlike mixture of 80% nitrogen and 20% oxygen, these results are compared with C‐R coefficients of nitrogen and oxygen plasmas developed separately and show little difference. The inclusion of charge‐exchange effects does not alter the coefficients. Saha decrements for air are used to delineate the onset of local thermodynamic equilibrium (LTE) for the above conditions. These thresholds are compared to other criteria and show some difference. Finally, ratios of spectral line intensities are presented in terms of the Saha decrements as a means of diagnosing high‐temperature air discharges, such as lightning channels, without requiring LTE.

Evolution of the electron acoustic signal as function of doping level in III‐V semiconductors
View Description Hide DescriptionThe evolution of the electron acoustic signal has been measured for Be‐ and Si‐doped GaAs and Ga_{0.28}Al_{0.19}In_{0.53}As layers with doping levels from10^{1} ^{7} to 10^{2} ^{0} at. cm^{−} ^{3}. The samples have also been analyzed by cathodoluminescencespectroscopy for near‐band‐edge transition and deep level emission. The results are explained by the reduction of the mean free path of phonons, giving rise to a lattice thermal conductivity decrease. Meanwhile, the electronic part of the thermal conductivity of these compounds is found to be nearly negligible.

The influence of annealing on the density and mechanical properties of amorphous Se_{100−x }Sb_{ x }
View Description Hide DescriptionThe density and microhardness of the system Se_{100−x }Sb_{ x } (where x=5, 10, and 15 at. %) were determined at room temperature using the stepwise method. The values of the density and microhardness increase with time of annealing, temperature of annealing, and Sb content. The energy required for the crystallization processes was calculated from the density as 31.82, 39.8, and 49.7 K cal/mol for the samples Se_{9} _{5}Sb_{5}, Se_{9} _{0}Sb_{1} _{0}, and Se_{8} _{5}Sb_{1} _{5}, respectively. On the other hand, these energies are given by 43.76, 49.72, and 54.7 K cal/mol, using the microhardness data. The dimensional nature of the growth process has been looked for through the determination of the n exponent of the Avrami rate equation. The value of the ‘‘n’’ exponent of the Avrami rate equation indicates that the crystals grow in a one‐dimensional way.

X‐ray topographs of silicon crystals with superposed oxide film. A theoretical study by means of simulations
View Description Hide DescriptionStresses induced in a silicon wafer by mask edges or recessed oxides may be studied by the means of x‐ray topographs. The contrast of such topographs has been previously studied using the Blech and Meieran theory [J. Appl. Phys. 3 8, 2913 (1967)] for the stresses and Kato’s theory [Acta Crystallogr. 1 6, 282 (1963)] for the diffraction. Simulations using Takagi’s equations [S. Takagi, Acta Crystallogr. 1 5, 1131 (1962)] show that this model is not correct: the magnitude of stresses is too large and should lead to additional fringes in the image. This could not be predicted using Kato’s theory where the interaction between the wavefields and the most deformed areas of the crystal is underestimated. It suggests that a relaxation occurs along the surface to decrease the value of the stresses.

Dislocation loops and precipitates associated with excess arsenic in GaAs
View Description Hide DescriptionDislocation loops and precipitates in an arsenic‐rich GaAs crystal have been studied by transmission electron microscopy to investigate their formation mechanism and establish their relationship to point defects. The precipitates are identified to be hexagonal arsenic phases having a simple orientation relationship with the GaAs matrix. Detailed analyses of the loops indicate that they are composed of two separate defects lying on (111) planes: a faulted loop and a perfect loop. It is proposed that the loops are formed by condensation of excess arsenic interstitials followed by clustering of excess Ga vacancies and subsequent generation and movement of Shockley partial dislocation(s). The faulted loop is interpreted as an hcp arrangement of arsenic atoms. This model supports the hypothesis that arsenic interstitials and Ga vacancies coexist in GaAs at high temperatures although arsenic interstitials initiate the formation of arsenic‐related dislocation loops. Implications concerning the formation process of the EL2 deep‐level defect are also discussed.

A moving dislocation kink as the soliton on a background of quasiperiodic process in unbounded sine‐Gordon system
View Description Hide DescriptionThis paper deals with an extension of the well‐known case of the harmonic oscillation of a nonkinked dislocation line to the case of both harmonic and anharmonic oscillations of a dislocation line in the presence of a moving kink. The one‐kinked dislocation is considered as the unbounded, nondissipative sine‐Gordon system, and the mixed, one‐soliton one‐periodic solutions of the sine‐Gordon equation, recently obtained by applying the method of Riemann theta function, are used for this purpose.

Oxygen bubbles along individual ion tracks in O^{+} implanted silicon
View Description Hide DescriptionSilicon wafers with and without a protective thermal oxide were implanted with oxygen at 150 keV with doses of 1.6–2.0×10^{1} ^{8} cm^{−} ^{2}. Transmission electron microscopy and secondary‐ion mass spectroscopy were used to study the silicon layer above the implanted buried oxide. A regular array of spheroidal bubbles, postulated to be filled with oxygen gas, was observed only in the samples that were not protected by the oxide. The bubbles were aligned in individual columns whose orientation matched the direction of the implantation. The origin and kinetics of their formation are discussed. A model for column formation involving the overlap of ionization thermal spikes is proposed. It is also proposed that the observed phenomenon is a solid‐state analog of the bubble chamber effect.

The elastic interaction between screw dislocations and cracks emanating from an elliptic hole
View Description Hide DescriptionBased on the solution of an internal crack, we have investigated the elastic interaction of a screw dislocation and cracks emanating from an elliptic hole by using a conformal mapping technique. We have derived the stress field, the image force on the dislocation, the stress intensity factor at the crack tip, and the crack extension force. From the image force, we find the unstable equilibrium positions of dislocation. It is possible to form a plastic zone from the region containing the unstable equilibrium position by collecting dislocations. By extending the concept of the unstable equilibrium position, we also develop the dislocation emission criterion. It is found that the critical applied stress for dislocation emission is dependent on the geometry of the crack. In addition, we also find that the elliptic hole prefers to emanate double cracks. Finally, it is worthwhile to mention that the mapping function cannot be arbitrarily chosen.

Lattice theory of fracture of solids with layered structure
View Description Hide DescriptionThe lattice trapping effect of a crack in solids with layered structure has been studied with a double‐chain model. It is shown that the difference in physicomechanical properties between different atomic planes has significant influence upon this effect, and the stress range for stability of a crack can be enhanced by increasing the difference. This perhaps, provides a way for the lattice trapping effect to be made observable by appropriately synthesizing solids with superlattices of special design.