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Volume 50, Issue 4, 01 April 1979

Radiative transfer theory for scattering by layered media
View Description Hide DescriptionRemote sensing of brightness temperatures of a scattering medium composed of spherical scatterers beneath a homogeneous surface layer is studied by using Mie scattering functions. We illustrate the effects of a surface layer by plotting the brightness temperatures as functions of frequency and viewing angle for different layer thicknesses, dielectric constants, and fractional scattering volumes. It is found that the brightness temperature increases when the loss tangent of the surface layer is increased and when the fractional volume occupied by the scatterers is decreased. In the absence of a surface layer the brightness temperature is usually decreasing as a function of frequency. The presence of a homogeneous layer may cause the brightness temperature to increase with frequency. At all viewing angles the vertical polarization is brighter than the horizontal polarization.

Wave amplitude variation and energy flow in autoresonant collective ion acceleration
View Description Hide DescriptionAn expression for the energy of small‐amplitude waves in a radially inhomogeneous cold‐fluid unneutralized relativistic electron beam is obtained in terms of the wave amplitudes. The result is employed together with conservation of wave energy to determine how the axial electric field strength and beam‐edge modulation associated with slow cyclotron waves change with position in an electron beam propagating along the adiabatically spatially decreasing magnetic guide field of an autoresonant collective ion accelerator. The resulting axial profiles are found to depart markedly from those of earlier radially homogeneous beam calculations. Possible impacts of these findings on a planned autoresonant acceleration feasibility experiment are discussed.

Light scattering by dielectric needles and disks
View Description Hide DescriptionLight‐scattering properties of very thin or flat dielectric particles are approximately calculated by means of a method which combines Rayleigh‐Gans and Rayleigh scattering. For the approximation to be valid the shortest diameter of the particle, R _{min}, must be small compared to the longest diameter R _{max} and to the wavelength λ of the incident radiation. The method is applied to needles and circular disks and yields the scattering matrix and the differential and total scattering cross section in the lowest order of R _{min}/R _{max} and R _{min}/λ. For the disks the total cross section is calculated only in the limiting cases of long and short wavelengths.

Power flow along a 40‐kV multimegawatt neutral beam line
View Description Hide DescriptionThe results of a systematic investigation of the power flow along the Oak Ridge National Laboratory/Princeton Large Torus (ORNL/PLT) neutral beam line, using an intense modified duoPIGatron source with a 22‐cm active grid diameter and 1799 circular apertures that provide 53% transparency, are reported. The variation of power deposited on the different components of the beam line is measured over a wide range of perveance values. The maximum efficiency for beam power transmitted through a 20×25‐cm aperture located 4.1 m downstream was found to be 41%. This efficiency was raised to 53% when a voltage of about 175 V was applied to preaccelerate the ions entering the extraction apertures. Transmission efficiencies approaching 60% were obtained using shaped apertures on the plasma grid. Higher efficiencies should be attainable when used for PLT injection since the acceptance angle for the Princeton tokamak is 11% higher than that of the test facility. Detailed analysis of the results using straight circular apertures without precel voltage suggests that the power‐deposition profiles have broad wings that significantly affect the transmission efficiency. The results allow optimization of the performance of the beam line and provide guidelines for designing future multimegawatt neutral beam lines.

Magnetocardiography in weak dc ambient magnetic fields
View Description Hide DescriptionThe magnetic field detectable outside the chest arising from the heart is analyzed using Maxwell’sequations. It is shown to consist of two contributions: (i) the field arising from the heart’s electrical activity, commonly known as the magnetocardiogram, and (ii) a contribution occurring only in the presence of an external magnetic field arising from the motion of diamagneticmaterial, principally blood. The latter contribution has been studied by means of a superconductinggradiometer in model circulatory systems consisting of plastic or rubber ventricles filled and emptied by pumps and tubing, and has been shown to be readily detectable in magnetic fields comparable to that of the earth. The effect has also been measured with human subjects, suggesting the possibility of clinical application of our results.

An asymmetric electrostatic lens for field‐emission microprobe applications
View Description Hide DescriptionThe properties of a physically asymmetric three‐element electrostatic lens have been calculated numerically for voltage‐symmetric and voltage‐asymmetric cases. The lens has low chromatic aberration,C _{ c } _{ ∞ }/f < 0.5, when operated in the voltage‐asymmetric mode with voltage ratios V _{final}/V _{initial} as low as 2. Calculations of beam diameter versus current with realistic field electron emission parameters of d I/dΩ=1 mA sr^{−1} and ΔE=2 eV at 5 kV emitter voltage show that at 30‐kV final‐beam voltage 250 nA current could be focused into a 0.25‐μm spot at 5 cm working distance, with an emitter‐lens separation of 1.5 cm. It is shown that while the two‐element immersion lens is inappropriate for high‐current microprobe applications, the three‐element asymmetric lens is well suited for this work.

Structures of an electron beam drifting in vacuum and in low‐pressure gases
View Description Hide DescriptionWe performed a series of tests on the propagation of intense electron beams with ν/γ?0.40 and 0.85 in a vacuum (∼10^{−4} Torr) and in a few gases (nitrogen, helium, and carbon dioxide) at pressures ranging from 50 mTorr to 1 Torr. Beam currents were recorded step by step by a Faraday cup movable along the drift‐tube axis up to about (1/2) m from the anode of the field emission diode. Current neutralization and almost complete transport of the electron beam along the drift tube occur at different pressures in different gases. Structures in the transmitted currents appear as a function of the drift distance, markedly at pressures slightly lower than that of complete neutralization of the beam. They are in good agreement with Lawson’s predictions.

Fourier‐Bessel series solution for potential and intensity field of open electron optical systems with rotational symmetry, in terms of I _{0} Bessel functions
View Description Hide DescriptionA Fourier‐Bessel series representation in terms of I _{0} Bessel functions is given for the potential distribution in certain open Einzel‐ or immersion‐type electrostatic configurations with rotational symmetry described by φ (A,z) =F (z), 0<z<L; and φ (A,z) =0 for all z<0 and for all z≳L. F (z) is a given function and A and L are constants. The method is a direct one—no iterations or matrix inversions are required—and a superior rate of convergence in the paraxial region is achieved through the elimination of J _{0} Bessel functions. The precision reached is apparently limited by the computer word length only, programming is of a simple nature, and computer memory requirements are modest enough to allow implementation on small desk‐top computers.

A numerical method of determining propagation characteristics of guided waves along inhomogeneous planar waveguides
View Description Hide DescriptionA simple method is presented of numerically solving inhomogeneous planar waveguide problems. The characteristic mode equation given here to determine rigorous solutions is similar to WKB’s mode equation and is solved by the stepbystep numerical method. Examples are given for the sake of comparison with other results.

Accelerated aging test of Ga_{1−x }Al_{ x }As DH lasers
View Description Hide DescriptionAn accelerated aging test is performed at temperatures of 70, 110, and 140 °C to estimate the activation energy. Samples are operated in the LED mode at high temperatures of 110 and 140 °C. At 70 °C samples are operated in lasing mode of optical output 0.27 mW/μm. The laser lifetime is defined to be a time at which the cw threshold current measured at a standard temperature 25 °C reaches 1.5 times the initial value. The activation energy 0.74 eV is obtained. It is shown that the degradation does not depend on the optical output power below the power level of 0.3 mW/μm and that the activation energy is indifferent to the operation mode, LED mode, or lasing mode, at a low power level. The use of the LED mode operation and the definition of the laser lifetime in terms of the characteristics at low standard temperature enable one to perform the accelerated aging test at high temperatures above 110 °C where cw operation is almost impossible. The reliability is examined at 70 °C using 50 samples and an average laser lifetime is obtained. The application of the activation energy gives the average laser lifetime at the junction temperature of 30 °C to be 3×10^{5} h.

An organic crystal with an exceptionally large optical second‐harmonic coefficient: 2‐methyl‐4‐nitroaniline
View Description Hide DescriptionWe have grown and measured the optical second‐harmonic coefficient d _{ i j k } of the new nonlinear crystal 2‐methyl‐4‐nitroaniline (MNA). We find that the d _{ i j k } are very large with d _{12} being 5.8 times larger than d _{31} of LiNbO_{3} giving a birefringence phase‐matching figure of merit d ^{2}/n ^{3} which is 45 times larger than LiNbO_{3}. The other coefficient d _{11} is 40 times larger than LiNbO_{3}, giving a huge figure of merit which is 2000 larger than LiNbO_{3}.

Laser‐to‐microwave frequency division using synchrotron radiation
View Description Hide DescriptionCalculations are made to demonstrate the feasibility of obtaining one‐step frequency division from optical or infrared laser frequencies to a subharmonic in the microwavespectral region. The cyclotron orbit of a single relativistic electron in a Penning trap is driven with a Gaussian laser beam focused to a spot diameter ∼λ; the laser subharmonic frequency is measured from the electron synchrotron radiation. The uncertainty in orbit dimensions is limited to λ/2 by radiative cooling and the technique of motional sideband excitation.

High‐energy electron acceleration by ponderomotive forces in tenuous plasmas
View Description Hide DescriptionThe results of an experimental study of electrons produced by multiphoton ionization of helium gas by intense 25‐psec‐duration pulses from a neodymium laser are reported. Electrons with energies up to 300 eV have been observed and both the maximum electron energy and the measured electron‐energy spectra are shown to be characteristic of electron acceleration by ponderomotive forces.

Catalytic control of the gas chemistry of sealed TEA CO_{2} lasers
View Description Hide DescriptionSealed operation of TEA CO_{2} lasers is demonstrated using external catalytic control of the laser gas chemistry. With copper catalysts, although the level of oxygen is maintained at very low levels, the level of CO is sufficient that some increase in instability is anticipated. With platinum‐on‐alumina catalysts the levels of both CO and O_{2} are very low and it is anticipated that indefinite laser gas lifetime is feasible even for high‐repetition‐rate high‐power devices.

Frequency‐stabilized 3.39‐μm He‐Ne laser with no frequency modulation
View Description Hide DescriptionThe frequency of the 3.39‐μm He‐Ne laser whose output beam does not contain any artificial frequency modulation was newly stabilized in reference to the saturated absorption line of CH_{4} gas contained in an external cell. Instead of modulating frequency by vibrating the resonator length, the frequency modulation was given by an external vibrating mirror on which the laser beam was reflected. A 3.39‐μm low‐noise optical amplifier was also used for observing saturated absorption in the external cell. A signal proportional to a frequency deviation from the reference absorption line was fed back to the resonator to control its length such that the frequency deviation was minimized. The short‐term stability so achieved under a proportional feedback control was ±1.25×10^{−12}/min, the frequency drift was 2.3×10^{−10}/h, and the long‐term stability under an integral feedback control was ±3.5×10^{−11} over 5 h.

Tripling of CO_{2} laser frequency in deuterium chloride gas
View Description Hide DescriptionThe generation of third‐harmonic radiation from the 9.6‐μm P (22) CO_{2} laser line was investigated in DCl (deuterium chloride). A total of 11 conversion efficiency maxima were observed up to a pressure of 28 atm. The measured index mismatch (n _{3}−n _{1}) between the third harmonic and the fundamental frequencies is 4×10^{−6}/atm. This compares favorably with the calculated index mismatch of 3.6×10^{−6}/atm. It is found that the major contribution to n _{3}−n _{1} arises from the far‐infrared pure rotational transitions. The measured χ^{(3)} was deduced from experimental results and shown to agree with calculated values to a factor of 2.

Loss measurement in p‐type GaAs dielectric waveguides using Raman scattering
View Description Hide DescriptionRaman scatteringmeasurements have been made to determine losses in p‐type GaAs slab waveguides cladded by Al_{0.3}Ga_{0.7}As epitaxial layers. For these measurements, 1.064‐μm laser light is focused onto a cleaved edge of the waveguide layer and the Raman‐scattered photons are detected at 90°. The attenuation of the resulting signal is measured as a function of the propagation distance along the waveguide. Five Ge‐doped waveguide layer samples, ranging in concentration from 3×10^{16} to 3×10^{18} cm^{−3}, were measured. Results are reported for samples 3–5 mm long having layer thicknesses of ∼5–6 μm. Losses from 0.8 to 12.2 cm^{−1} have been measured. Comparison is made with earlier results in n‐type GaAs.

Guided‐radiation mode interaction in off‐axis propagation in anisotropic optical waveguides with application to direct‐intensity modulators
View Description Hide DescriptionA closed‐form solution of the coupled‐mode equations is derived for the guided–radiation mode interaction in optical waveguides by using a Lorentzian approximation to the coupling‐factor distribution. This solution does not neglect reconversion from radiation to guided modes as did the previous perturbation solution. It is thus more accurate for the case of higher‐order guided modes for which the power attenuation due to radiation conversion exhibits an oscillatory form and the ’’effective’’ perturbation assumption that the coupling factor is independent of the phase constant is not valid. Peaks in the mode conversion characteristics at certain eigenvalues of the relevant guided mode can be physically interpreted as the coupling to leaky waves. Illustrative examples and related discussions are given for the polarization‐rotated (TE→TM) guided‐to‐radiation mode conversion in off‐axis propagation in an anisotropic LiNbO_{3} waveguide. Also, an electro‐optic direct‐intensity modulator is described where the angle of propagation is used instead of a dc field to set a bias point for linear modulation.

Theoretical gain‐optimization studies in CO_{2}–N_{2} gasdynamic lasers. I. Theory
View Description Hide DescriptionBased on a method proposed by Reddy and Daum, the equations governing the steady inviscid nonreacting gasdynamic laser (GDL) flow in a supersonic nozzle are reduced to a universal form so that the solutions depend on a single parameter which combines all the other parameters of the problem. Solutions are obtained for a sample case of available data and compared with existing results to validate the present approach. Also, similar solutions for a sample case are presented.

Theoretical gain‐optimization studies in CO_{2}–N_{2} gasdynamic lasers. II. Results of parametric study
View Description Hide DescriptionBased on a method presented in detail in a previous work by the authors, similar solutions have been obtained for the steady inviscid quasi‐one‐dimensional nonreacting flow in the supersonic nozzle of a CO_{2}–N_{2} gasdynamic laser system, with either H_{2}O or He as the catalyst. It has been demonstrated how these solutions could be used to optimize the small‐signal gain coefficient on a specified vibrational‐rotational transition. Results presented for a wide range of mixture compositions include optimum values for the small‐signal gain, area ratio, reservoir temperature, and a binary scaling parameter, which is the product of reservoir pressure and nozzle shape factor.