Index of content:
Volume 50, Issue 1, 01 January 1979

Electronic cooling of resonant gravity gradiometers
View Description Hide DescriptionElectronic cooling for improving the signal‐to‐noise (S/N) performance of sensors was applied to the mechanically resonant Hughes rotating gravity gradiometer (RGG). An 8.3‐dB improvement in the S/N was obtained. The electronic cooling technique uses a negative feedback damping circuit between the two RGG transducers. The feedback circuit is electrically equivalent to a high‐impedance resistor cooled to cryogenic temperatures.

Prepulse damage to targets and alignment verification
View Description Hide DescriptionWe measured the damage threshold of 10.6‐μm light incident on glass microballoon laser fusion targets. The threshold is several dozen microjoules, depending on target size and laser pulse width, and the damage mechanism appears to be thermal heating and rupture. Perforating glass microballoons proves to be a useful alignment verification technique.

Photoionization of excited atoms in dc gas discharges by low‐intensity light and its analogy to gas‐breakdown with high‐intensity lasers
View Description Hide DescriptionSpectral effects and nonlinearities that characterize illumination of dc discharges by low‐intensity light and illumination of gas cells by high‐intensity lasers are described. The similarities suggest an analogy between both cases. This analogy can be used to deepen the understanding of the low‐intensity EM radiation interaction with gas discharges and as a possible tool for further study of gas breakdown by high‐intensity lasers, including further investigation of the ’’effective photon’’ concept.

An integral equation method for the analysis of magnetic deflection yokes
View Description Hide DescriptionAn integral equation method is developed for the computer modeling of the magnetic fields produced by color television deflection yokes. Saddle or toroidal yokes with either radial or nonradial winding distributions can be analyzed using this new technique. The integral equation method is shown to yield magnetic field calculations that are at least as accurate as those produced by conventional finite‐difference analysis. In most cases computer algorithms using the integral equation approach have been found to be more computationally efficient than existing finite‐difference programs.

The magnetostatic field of a periodic cylindrical array of perfect conductors of arbitrary x‐y cross section
View Description Hide DescriptionThe magnetic field in an imploding wire array is studied. After the initial explosion of the wires the array starts imploding. The process can be considered quasistatic electromagnetically and the assumption of perfect conduction is made. Under these conditions the driving magnetostatic forces generated by the wires need to be calculated in a series of snapshots, which are stages of the magnetohydrodynamics process. For that purpose a Green’s function technique is developed to solve for the magnetostatic field generated by an array of perfectly conducting wires of arbitrary x‐y cross section, carrying current in the z direction. The wires are enclosed in an outer cylinder, inside of which they are arranged with angular periodicity. This outer cylinder carries all the returning current, and ideally does not permit any magnetic lines to escape from its enclosure. The resulting equations for the magnetic vector potential (A=A?,∇^{2} A=0 in vacuum, A=0 on the outer cylinder, A=const≠0 on the inner conductors) are implicitly solved by writing the potential as a linear superposition of point (z lines) current densities around the boundaries of the inner conductors. The point current densities are determined by imposition of the boundary conditions. For the case of small round wires not overlapping and sufficiently far from the center, an explicit solution is found in the form of an angular Fourier series, having as expansion parameter the ratio between the radius of the inner conductors and the distance to the center, times the number of wires periodically arranged. For the general case the problem is discretized by considering the boundary of each of the inner conductors as a connected set of segments of either circumferences or straight lines, each segment carrying a different amount of current. This procedure yields a set of Nlinear equations with N unknowns; the N unknowns are the currents carried by each of the N segments by which the boundary is approximated. Knowledge of these currents solves the problem completely because with them and using the Green’s function and its complex derivative, the vector potential and magnetic field, respectively, can be obtained anywhere in the region of interest. In particular the magnitude of the magnetic field just on the boundary of each of the inner conductors is related to the magnitude of the segment currents through a proportionality constant. An algorithm to solve the linear equations is discussed and representative cases shown. The method is seen to be accurate and fast.

Generation of far‐infrared radiation by frequency mixing in insulating GaAs
View Description Hide DescriptionThe generation of far‐infrared radiation by frequency mixing has been studied for the case of phase matching by means of the noncollinearity of the generating beams. An exact theoreticaltreatment has been performed for the case where the laser beams propagate linearly in the sample and for the case where the laser beams follow a zigzag path resulting from multiple internal reflections; the profile of each laser beam entering the sample is taken realistically to be a truncated Gaussian; sample absorption of the generating and generated radiations is taken into account. Previous treatments for the two geometries were approximate; in addition, square profile beams were assumed and the effect of absorption was neglected. Measurements have been made on insulating GaAs; the effects of sample length and sample absorption have been investigated. In contrast to previous work where two separate lasers were used, a single TEA CO_{2} laser has been modified to provide both generating beams, resulting in a more consistent temporal overlap of the pulses in the two beams. Furthermore, an SF_{6} saturable absorption cell has been used to select a single longitudinal mode, thereby eliminating the fine structure in pulses of one beam and making the overlap of the pulses from the two beams tractable. The coherence of the generated radiation was measured. The ratio of the theoretical prediction to the experimental results is a factor of ∼6, in comparison with the smallest factor of ∼400 for previous work with pulsed lasers, indicating the significance of the improvements in the experiment and the theory.

Preliminary experiment for stabilizing unmodulated He‐Ne laser to an iodine hyperfine component
View Description Hide DescriptionFirst‐ and third‐derivative signals of saturated absorption in iodine 127 molecular vapor are detected by using a frequency‐modulated 190‐cm‐long 633‐nm He‐Ne laser with a Fox‐Smith–type resonator and an iodine absorption cell which is set outside of the resonator. Aiming to stabilize the laser with unmodulated output, a retroreflector‐type optical modulator is added to the above system, and the first‐derivative signal is obtained by external modulation. Comparing the obtained signal with those obtained by the frequency‐modulated laser, possibilities to detect the third‐derivative signal by using the optical modulator and to stabilize the unmodulated laser are found.

Scattering of an offset two‐dimensional Gaussian beam wave by a cylinder
View Description Hide DescriptionThe scattering of a two‐dimensional Gaussian beam wave by a perfectly conducting or dielectric cylinder centered off both the beam axis and the beam waist is treated by the Fourier‐series‐expansion method. The incident‐beam fields are expanded in a Fourier series with respect to the polar angle in a cylindrical coordinate, and the expansion coefficients are numerically evaluated. Several scattering patterns are obtained by the use of these expansion coefficients, and the dependence of the scattering characteristics on the offset of the incident‐beam wave is examined.

A dispersive phase shifter for use in nonlinear optical interference experiments
View Description Hide DescriptionIt is shown that a pair of counterrotating fused‐quartz plates may be used to vary the phase between the second harmonic and the square of the fundamental in two‐crystal optical interference techniques. The advantages are simplicity of design and ease of use.

Free‐electron lasers using the Smith‐Purcell effect
View Description Hide DescriptionThe theory of linear and stimulated free‐electron lasers using the Smith‐Purcell effect is presented. These lasers depend upon the resonant interaction of an electron beam with the longitudinal field of a surface‐harmonic wave on a diffraction grating. Detailed design criteria for the linear Smith‐Purcell‐effect laser are determined. Numerical estimates show that infrared lasers are feasible at beam currents smaller than 1 A and that single‐mode operation may be achieved. Saturation estimates determine the power output in the range 10^{2} W. An expression for the gain of the stimulated Smith‐Purcell laser is derived. The frequency up‐conversion factor for the stimulated interaction may be very large. Its possible use as a soft x‐ray laser is limited by the available intensity of the pump wave.

Laser power variation and time dependence of populations in a burst‐mode CuBr laser
View Description Hide DescriptionThe time evolution of ground‐ and metastable‐state copper atom concentrations and the laser energy in a bust‐mode CuBr laser were measured as a function of temperature, buffer gas species, and repetition rate within the burst. The concentrations were measured by monitoring the absorption of the 244.2‐ and 510.6‐nm atomic copper lines. The optimum time separations of the electrical pulses within the burst were found to be 80 and 235 μ for helium and argon buffer gases, respectively. A correlation was found between the time dependences of the laser energy and ground‐state density within the burst. Accumulation was found to occur in the copper atom ground state but not in the metastable ^{2} D _{5/2} level.

Twofold joint time interval distribution for chaotic light with Lorentzian spectrum
View Description Hide DescriptionThe correlation structure of the lengths of two disjoint time intervals of the stationary point process obtained by detecting a Gaussian field with Lorentzian spectrum is studied for some special cases.

Initial rate of decomposition of CO_{2} in volume discharges: An experimental study
View Description Hide DescriptionQuantitative data on the plasma and chemical changes in a pulsed glow discharge in CO_{2} have been obtained using a geometrically stable, chemically inert discharge cell. With resistively‐ballasted electrodes it was possible to achieve uniform glow discharges over a relatively large range of pressure and the yield of decomposition products was measured as a function of energy deposited at various initial charging voltages. These experimental results were compared to those predicted by numerical solution of the Boltzmann transport equation by integrating a derived yeild per unit energy, Y (E/N), over the duration of the excitation pulse. Good agreement between experiment and theory was obtained for pressures over 400 Torr but significant deviations were noted at lower pressures.

Rayleigh‐wave reflection from a groove in Y‐Z LiNbO_{3}: A perturbation approach
View Description Hide DescriptionA field theory for the reflectivity of a generalized Rayleigh wave from a single shallow groove etched on a LiNbO_{3} substrate is presented. The scattering properties are studied by using a perturbation approach. Induced equivalent acoustic and electric sources which produce the reflected and scatteredwaves are determined. Calculated reflection coefficient agrees well with known experiments.

Comparison of the grating and meander‐line transducers for magnetoelastic surface‐wave excitation
View Description Hide DescriptionThe gratingtransducer, the dual of the meander line, is studied and its performance compared with a meander line for the excitation of magnetoelasticsurface waves. A LiNbO_{3}‐YIG‐LiNbO_{3} epoxy‐bonded composite is used to experimentally study both transducer geometries at 50 MHz. It is found that the gratingtransducer offers no significant improvement in performance over the meander line. Both types of transducers have large conversion losses because of large Ohmic resistance to radiation resistance ratios. Practical considerations of an allowable metalization thickness for the transducers and spatially nonuniform demagnetizing fields in the substrate permit only modest reductions in the conversion losses of either type of transducer situated on materials such as YIG.

Analysis of parameteric mixing and harmonic generation of surface acoustic waves
View Description Hide DescriptionThe parametric mixing and harmonic generation of surface acoustic waves(SAW) has been treated by a new formalism in which the nonlinear waveequation and boundary conditions are rigorously satisfied at the sum and difference frequencies. The key features are analytical expressions for the spatial structure of modes which can be synchronously driven by volume forces and surface‐boundary conditions. After a propagation distance of a few acoustic wavelengths, the acoustic fields evolve into a linearly growing normal‐mode SAW.Equations are derived from which the nonlinear interaction cross sections can be evaluated and numerical results are given for a number of materials. Good agreement was obtained between experiment and theory for y‐x and x‐y α‐quartz in the limit that the piezoelectricity is neglected.

Surface‐wave generation by nonlinear mixing of bulk waves
View Description Hide DescriptionThe nonlinear interaction between bulk acoustic waves at a surface to produce surface acoustic waves is analyzedtheoretically. In this analysis both the nonlinear waveequation and boundary conditions are rigorously satisfied by the total acoustic fields. Shear‐wave incidence is treated in detail for isotropic fused quartz and numerical cross sections are evaluated for different angles of incidence. Maximum surface‐wave generation is predicted for geometries corresponding to bulk waves, shear or longitudinal, traveling parallel to the surface.

Simultaneous mass and temperature determination using a single quartz wafer: An optimized crystal cut
View Description Hide DescriptionRecently we have reported a method to measure mass and temperature simultaneously using a single rotated Y‐cut quartz crystal coated with a pair of electrodes and a tab. In this paper we will discuss some modifications by which the performance of the method may be strongly improved. A first improvement is achieved by choosing a particular crystal cut (y x l, −20°<φ<20°), and a second improvement is achieved by the introduction of a small gap between the front electrode and the tab. We will present an analysis of the errors in the determination of mass and temperature, which may result from calibration and experimental errors, as a function of the crystal cut and the thickness of the crystal. The results from this analysis are compared with experimental results for three different crystal cuts (y x l, φ=−15°, 0°, +15°). We found that in the presence of a temperature change of 0.1 °C, a mass change of 5 Å (copper) could be measured with an accuracy of 3%, while in the presence of a mass change of 500 Å (copper) a temperature change of 0.005 °C could be measured within 11.3%. Some suggestions are given to increase the relatively small mass range, i.e., 3000 Å (copper), of the method.

Absorption measurements by laser calorimetry
View Description Hide DescriptionA solution to the heat equation is given which describes a cylindrical body uniformly heated along its axis. The application considered is the determination of bulk and surface absorption coefficients of the sample material from temperature‐vs‐timemeasurements when the heat source is a weakly absorbed narrow light beam. Under certain conditions, the full solution can be greatly simplified and the inference of absorption coefficients thus facilitated; these conditions are discussed and specified quantitatively. Equivalent results for rectangular samples are also given. The utility of the analysis is illustrated by several experimental examples.

Ultrasonic assessment of cumulative internal damage in filled polymers
View Description Hide DescriptionA new ultrasonic technique for studying dewetting and cumulative internal damage in filled polymers such as solid rocket propellants has been developed. The ultrasonicmeasurements correlate well with volume‐dilatation data in tension. Ultrasonic assessment of damage has also been performed for uniaxial compression and shear strain fields. Experimental results show that for a filled polymer a universal relation exists between acoustic measures of damage and the maximum principal strain for fairly general states of strain. A quantitative physical model relating the observed ultrasonic effects to specific damage events is presented. Estimates of the vacuole size and number density, arising from dewetting, obtained by applying this model to experimental data are in excellent agreement with independent microscopic observations. Time‐dependent void growth at constant strain has also been monitored by means of the ultrasonic technique.