Volume 57, Issue 7, 01 April 1985
Index of content:

Analysis of fluid light valve control layers for high‐definition television large picture projection
View Description Hide DescriptionHere, proceeding from a previous study, the deformation problem of a viscose fluid is analyzed under simplified conditions as regards the physical model. This viscose fluid is used as the light control organ in the light valve method of large‐screen television projection. By using simple approximate relationship, the important properties, such as dynamic behavior, frequency response, stability behavior, and resolution limit of the control layer and their dependence on material and system parameter are discussed in detail. The theoretical knowledge obtained from this analysis agrees to a large extent with the experimental results.

Submillimeter wave absorption of n‐type InSb at low temperatures
View Description Hide DescriptionThe absorption coefficient of two high‐purity n‐InSb samples is measured in the 10–40 cm^{−} ^{1} range using Fourier transform spectroscopy. The absorption coefficientspectrum is presented for both samples at 4.2 K. It is also shown for the lower resistance sample cooled to 2.2 K and heated by dc bias to elevated electron gas temperatures of 7.5 and 17.9 K. ac Drude theory gives rather poor agreement with experiment at 2.2 and 4.2 K but does much better when the sample electron gas is heated. In contrast, a simple quantum mechanical theory of absorption based on inverse Bremsstrahlung yields promising agreement at the lower temperatures although its applicability is questionable. The non‐Drudian absorption is shown to have a favorable effect on the performance of InSb hot‐electron bolometers.

An application of a statistical model for the calculation of the logarithmic mean excitation energy of molecules: Molecular hydrogen
View Description Hide DescriptionA statistical model, the local plasma approximation, is considered for the calculation of the logarithmic mean excitation energy for stopping power of chemically bound particles by taking into consideration chemical bonding. This statistical model is applied to molecular hydrogen and leads to results that suggest a value for the logarithmic mean excitation energy of molecular hydrogen that is larger than the accepted experimental and theoretical values.

Electromagnetic missiles
View Description Hide DescriptionUnder transient excitation the energy transmitted by an antenna of finite size to a faraway receiver can decrease much more slowly than the usual R ^{−} ^{2}. Instead, by a suitable choice of excitation, this quantity can decrease as slowly as one wishes, under the physical restriction that the total energy radiated by the antenna is finite. Such cases of slow decrease are referred to as electromagnetic missiles. Another way of stating the result is that the product of the following two quantities can approach zero as slowly as one wishes: (1) the energy transmitted per unit area of receiver, and (2) the total area of the receiver. Examples are constructed when either one, but not both, of these quantities remains finite at infinite distance.

Measurement of spatial correlation functions using image processing techniques
View Description Hide DescriptionA procedure for using digital image processing techniques to measure the spatialcorrelation functions of composite heterogeneous materials is presented. Methods for eliminating undesirable biases and warping in digitized photographs are discussed. Fourier transform methods and array processor techniques for calculating the spatialcorrelation functions are treated. By introducing a minimal set of lattice‐commensurate triangles, a method of sorting and storing the values of three‐point correlation functions in a compact one‐dimensional array is developed. Examples are presented at each stage of the analysis using synthetic photographs of cross sections of a model random material (the penetrable sphere model) for which the analytical form of the spatialcorrelations functions is known. Although results depend somewhat on magnification and on relative volume fraction, it is found that photographs digitized with 512×512 pixels generally have sufficiently good statistics for most practical purposes. To illustrate the use of the correlation functions, bounds on conductivity for the penetrable sphere model are calculated with a general numerical scheme developed for treating the singular three‐dimensional integrals which must be evaluated.

Unipotential electrostatic lenses: Paraxial properties and aberrations of focal length and focal point
View Description Hide DescriptionAn experimental study of electrostatic electron lenses as a function of geometrical and electrical parameters is described. The lenses are of the symmetrical three‐electrode unipotential type. The parameters are the thickness of the center electrode and the interelectrode spacing, both relative to the center electrode aperture diameter, and the ratio of lens voltage to cathode voltage. The lens properties are characterized in terms of the focal length and focal distance, and the spherical and chromatic aberrations of these quantities. In general, the principal surfaces of a lens are not plane, and the aberrations of focal length and focal distance are not the same. Expressions are derived relating the focal length and focal distance aberrations to the spherical and chromatic imaging aberration coefficients C _{ s } and C _{ c }, and the magnification aberrations. The advantages of formulating the lens properties in terms of focal length and focal distance and their aberrations, and the usefulness of the data presented here, are illustrated with several examples.

Monitoring the growth of nonuniform gratings written holographically by Gaussian laser beams
View Description Hide DescriptionInterfering Gaussian laser beams are sometimes used to photochemically etch gratings into materials; the gratings which result are nonuniform since the groove depth varies over the grating cross section. The grating growth can be monitored in real time by observing the diffraction of the writing beams into their various orders. In order to relate the maximum groove depth to the observed diffraction efficiency, the grating nonuniformity must be accounted for. We present a short calculation which does this. The results are checked with measurements on actual gratings and good agreement is obtained. The results can help to interpret previously published measurements.

Multidimensional modeling of transverse avalanche laser discharges: Applications to the HgBr laser
View Description Hide DescriptionGeometrical considerations are important with respect to the stability and efficiency of avalanche electric discharge lasers. Parameters such as the electrode contours and the distribution of preionization electrons affect excitation rates through the relative values of the local electric field, local depletion of initial species, and through the response of the discharge circuitry to spatially dependent conditions within the plasma. Constriction of the discharge and subsequent impedance mismatch of the discharge to the pulse forming line result from these spatial nonuniformities. In this paper geometrical effects in the mercury bromide electric discharge laser are examined by comparing the results from a multidimensional discharge and kinetics model with experimental observations. The code models electron and heavy particle kinetics and laser intensity in time and one spatial dimension: parallel to the electrodes and perpendicular to the optical axis. Quantities whose spatial dependence is perpendicular to this dimension, such as the contours of the electrodes, are accounted for through their impact on the local electric field. HgBr laser efficiency and spatialcharacteristics are examined as a function of stored electrical energy, the impedance of the pulse‐forming circuitry, electrode contours, and profile of the preionization electron density. Parasitic discharges within the gas, but exterior to the optical cavity, are examined as a mechanism through which the magnitude of the preionization density can effect laser efficiency.

Thermal analysis of the catastrophic mirror damage in laser diodes
View Description Hide DescriptionA new thermal analysis of a catastrophic mirror damage in laser diodes is presented in the present paper. Three‐dimensional heat spreading and temperature dependence of a thermal conductivity is taken into account in the model. Both an active layer heating and a nonradiative recombination of carriers generated by absorbed radiation in an active area close to a facet mirror are considered as heat sources. The model is strictly correct for times ranging from 6.2 to 500 nsec for a standard stripe laser diode, i.e., for currents not higher than 4.5 A and not lower than 0.8 A. The analytical solution of the thermal conduction equation gives a dependence of a catastrophic‐degradation time, i.e., a permissible length of current pulses from a point of view of the catastrophic mirror damage, on the amplitude of the pulses for the standard stripe laser diode.

Measurement of kinetic parameters relevant to the operation of an electron‐beam initiated atomic iodine laser
View Description Hide DescriptionParameters relevant to the operation of an electron‐beam initiated atomic iodine laser, which would lase on the transition 5 ^{2} P _{1} _{/} _{2} (I*)→5 ^{2} P _{3} _{/} _{2} (I), have been measured by pulse radiolysis. Kinetic data for the parent compound quenching of I* were obtained by variation of the parent compound pressure at constant buffer gas pressure and observing I* decay rates versus time. Deactivation rates for perfluoroalkyl iodides were found to be much lower than for the corresponding alkyl iodides. Values obtained (in cm^{3} molec^{−} ^{1} s^{−} ^{1}) are as follows: CH_{3}I, (2.0±0.1)×10^{−} ^{1} ^{3}; C_{2}H_{5}I, (5.0±0.3)×10^{−} ^{1} ^{3}; CF_{3}I, (8.8±0.3)×10^{−} ^{1} ^{6}; C_{2}F_{5}I, (9.7±1.0)×10^{−} ^{1} ^{5}; n‐C_{3}F_{7}I, (2.5±0.2)×10^{−} ^{1} ^{5}; i‐C_{3}F_{7}I, (1.7±0.1)×10^{−} ^{1} ^{5}; n‐C_{4}F_{9}I, (1.8±0.1)×10^{−} ^{1} ^{4}. The extent of population inversion was investigated by measuring initial excited state and ground state atomic iodine concentrations, which gives the branching ratio [I*]/[I]. Values obtained are as follows: CH_{3}I, 2.7; CF_{3}I, 3.8; C_{2}F_{5}I, 2.7; i‐C_{3}F_{7}I, 3.2; n‐C_{4}F_{9}I, 1.8. Comparing both branching ratios and lifetimes, it is seen that CF_{3}I is the most promising candidate for an electron beam initiated atomic iodine laser.

The effect of slight surface roughness on the scattering properties of convex particles
View Description Hide DescriptionThe average scattering cross section for reflection of electromagnetic waves by an ensemble of randomly oriented convex particles is calculated. The particles are assumed to be large compared to the incident wavelength and to have a slightly rough surface such that the deviations from a smooth surface are smaller than wavelength. A perturbation expansion resulting from the Rayleigh–Rice approach leads to incoherent and coherent corrections to the zero‐order specularly reflected light. The quantity that in this limit completely determines the scattering behavior of the particles is the two‐point correlation function ρ(r) of the roughness structure. The main effect of roughness is a diminution of the reflected intensity for a wide range of the scattering angle θ and a shifting of the maximum of the polarization curve to larger θ. It turns out that measurements of the backscattered intensity in some wavelength limits provide information on the correlation function ρ(r). For conducting material, knowledge of the cross‐polarized backscatter intensity over a wide frequency range even enables one, at least in principle, to calculate the complete shape of the correlation function.

Laser damage studies of polymer oxide coated silica optics at 350 nm
View Description Hide DescriptionA single layer SiO_{2}film with graded porosity applied from a polymer solution derived from organometallicsiliconcompounds on fused silica lenses provides antireflectivity over the transmission range covering the first, second, and third harmonics of neodymium glass lasers. The damage thresholds of initial samples at 350 nm (3ω) varied from less than 1 to 9 J/cm^{2}. The main causes of this large variation in the damage threshold were determined to be substrate polishing, substrate surface cleaning, and the coating solution chemistry and processing, which, under certain conditions, leads to carbon formation during the heat treatment of the coating.

Pyroelectric enthalpimetric detection
View Description Hide DescriptionA pyroelectric enthalpimetric calorimeter for the H‐O reaction was constructed using Pd and Auelectrodes on a 2.5‐cm‐diam, 0.3‐mm‐thick LiTaO_{3} wafer. The counter (or common) electrode was of Au. The detector was placed in a chamber and various gases were admitted at pressures slightly above atmospheric. 100 ppm H_{2} in N_{2} was used as the hydrogen source and pure O_{2} as the oxygen source. A thermal signature was obtained on the Pd electrode that was confirmed by a thermocouple attached to the pyroelectriccalorimeter. A delayed thermal signature was observed on the Auelectrode resulting from thermal diffusion in the LiTaO_{3} substrate of the heat generated at the Pd electrode. The results on the H‐O reaction are in general agreement with the literature on Pd metal oxide semiconductor(MOS) structures. The results demonstrate the utility of the pyroelectriccalorimeter.

Piezoresistance response of longitudinally and laterally oriented ytterbium foils subjected to impact and quasi‐static loading
View Description Hide DescriptionResistance change measurements in ytterbium foils, encapsulated in a polymethylmethacrylate matrix and subjected to impact loading (to 25 kbars) and quasi‐static triaxial loading (to 4 kbars), are presented. Two gauge orientations were examined. Residual resistance measurements were obtained for impact experiments. Both sets of experiments confirm the overall validity of the theoretical analysis described in Ref. 5. In particular, the crossover in resistance change‐stress curves, predicted at low stresses, was measured. The present results show that gauge resistance measurements cannot always be uniquely related to a particular matrix stress component. Restricting the use of ytterbium foils to 20 kbars is inferred from the experimental results. The residual resistance data depend on both foil and matrix response. Finally, analysis of the resistance measurements, along the two orientations in the impact experiments, shows that polymethylmethacrylate retains significant material strength to 25 kbars.

A stretched betatron
View Description Hide DescriptionWe have demonstrated trapping, confinement, acceleration, and dumping of electrons in a betatron which has a geometry similar to the Astron. 76±22 nC were accelerated to 800±100 keV. This corresponds to a circulating electron current of 60 A. The betatron has a 6‐cm orbit radius and is 1 m long. Electrons are injected tangentially from a 70‐kV, 15‐A field emission diode. A toroidal field roughly equal to the betatron field is used.

Thermally stimulated scattering in plasmas
View Description Hide DescriptionA theory for stimulated scattering of a laser beam is formulated where the dominant nonlinearity is the ohmic heating of the plasma. The analysis is carried out with particular reference to experimental investigations of CO_{2} laser heating of a linear discharge plasma. In the conditions characterizing this experiment local heat conduction is of little importance and the dynamic evolution for the electron temperature is dominated by heating and energy exchange with the ion component. These features are incorporated in the analysis. The resulting set of equations gives a growth rate and characteristic scale size for the filaments, which agrees well with the experimental results.

Plasma properties and thin‐film formation in a pulsed electromagnetic inductive silane discharge
View Description Hide DescriptionA new approach to prepare thin films in a pulsed inductive silane discharge is attemped. The time‐resolved spectral lines qualitatively clarified dynamics of decomposition process in the pulsed silane discharge and the local thermodynamic equilibrium in this discharge was investigated by the population density distribution estimated from excitation temperature measurement. The films had peculiar deposition patterns with good adhesion due to electromagnetic effects and main group of infrared spectra was the SiH_{3} complex.

Arc expansion in xenon flashlamps
View Description Hide DescriptionPlasma arcs in large diameter (d>1 cm) xenon flashlamps often do not completely fill the bore of the discharge tube. The arc is usually initiated on one side of the discharge tube, adjacent to the ground plane, and the fraction of the discharge tube filled with plasma varies as a function of axial location. A model is presented that describes, from first principles, arc expansion in xenon flashlamps. The model simultaneously solves a coupled set of one‐dimensional transport equations in different regions of the discharge tube to simulate two‐dimensional effects in hydrodynamics, electron kinetics, and radiation transport. Using this method, expansion of arcs initiated at arbitrary locations within the discharge tube can be studied. Arc filling fractions are found to decrease with increasing filling pressure of xenon, increasing diameter of the discharge tube, and decreasing stored energy in the discharge circuit. The arc filling fraction also decreases as the breakdown filament moves away from the axis of the discharge tube and towards the wall. Arc expansion is slowed and ultimately halted by a lowered E/N (electric field/gas density) in the gas exterior to the arc, rapid conversion of atomic ions to molecular ions and their subsequent recombination, and by the efficient manner in which radiation dissipates energy which might otherwise be available for thermodynamicexpansion of the arc. The asymmetric expansion of the plasma arc results in asymmetric heating of the inside wall of the discharge tube, also calculated in the model. The growth of the plasma arc is also found to be in part responsible for changes in the spectrum of radiation emitted from the arc for various angles of observation. This effect results from wavelength‐dependent absorption coefficients in the plasma.

Point explosion simulation by fast spark discharges
View Description Hide DescriptionIt is shown here that the late hydrodynamic effects of fast spark discharges (d I/d t‖_{ t=0}≥10^{1} ^{0} A/sec), i.e., after the discharge period, can be described by a point explosion simulation. The solution is not self‐similar since counterpressure has to be taken into consideration. This non‐self‐similar model is confirmed experimentally by shock wave position and velocity measurements.

A new design concept for field distortion trigger spark gaps
View Description Hide DescriptionA common field distortion triggered spark gap utilizing geometric field enhancement at sharp edges usually operates in a cascade mode via the trigger electrode. A new trigger concept is proposed allowing strong field enhancement and direct breakdown between the two main electrodes. A test setup was designed to prove the feasibility of this concept. Experimental results on delay and jitter depending on percent breakdown voltage are presented. Best results achieved are a delay of 9 ns and a jitter of 2 ns at a self‐breakdown voltage of 15 kV.