V. L. Granatstein

Wave Propagation along Warm Plasma Columns

Dispersion relations for waves propagating along homogeneous, isotropic, warm plasma columns surrounded by a dielectric medium are obtained. A steady‐state linearized model with temperature introduced through a scalar pressure is analyzed both quasistatically and more rigorously from Maxwells equations. The results show effects of temperature on the cold plasma surface wave and predict a new series of propagating modes above the plasma frequency related to the Tonks‐Dattner scattering resonances near cutoff and to the Bohm‐Gross longitudinal plasma waves for large wave numbers. Warm and cold plasmas, large and small columns, and quasistatic and rigorous predictions are contrasted. Regions of the Brillouin diagram corresponding to different bound and radiating wave types are delineated and related to their physical properties.

SCALAR RADIATIVE TRANSPORT MODEL FOR MICROWAVE SCATTERING FROM A TURBULENT PLASMA.

The scalar radiative transport equation is solved in an interative manner for the backscattered reflected intensity. A model for the direct and cross‐polarized cross section is derived in terms of the first two terms of the iterated series. Model calculations agree to good approximation with an experiment where microwaves are scattered from a turbulent plasma column. At very low plasma density fluctuations (less than 0.01 of the critical plasma density) a single scatter regime exists where the direct cross section is proportional to the square of the plasma density fluctuations and the cross‐polarized cross section is much less than the direct cross section. As the fluctuations increase, this single scatter regime is followed by a regime where multiple scattering makes a significant contribution to the backscattered signal; here, the magnitude of the cross‐polarized cross section is approximately 0.1 of the magnitude of the direct cross section. Finally, when the plasma density fluctuations are about 0.1 ...

Multiple Scattering of Laser Light from a Turbid Medium

Through a controlled laboratory study, theoretical modeling was developed that accurately relates the reflectance of turbid water to the concentrations of suspended and dissolved materials; this modeling will be useful in quantitatively mapping pollutant concentrations in lakes and rivers through aerial photography. Laser light illuminated water containing both Teflon particles and black dye. Over a large range of concentrations of these scattering and absorbing materials, measured reflectance was successfully predicted by a multiple scattering analysis. (Single scatter analysis produced serious errors.) The theoretical development involved solving the radiative transport equation and accounted for correlated scattering from closely spaced particles.

Observation of Nonquasistatic Plasma Surface Waves

Surface waves have been launched from a waveguide onto a glass‐clad plasma column of moderate size (2πR≈λ0) in order to test the validity of quasistatic analysis when the small‐column condition (2πR≈λ0) is not satisfied. Resonances were observed in an open microwave cavity, and dispersion of both the circularly symmetric surface wave and the dipole surface wave has been determined from experimental data. The two waves exhibit regions of backward‐wave and forward‐wave propagation. The observed positive slope of the dipole mode dispersion curve near ω/kz=c is contrary to the predictions of quasistatic analysis and is shown to agree quantitatively with the results of a more rigorous analysis.

Laser Probing of a Weakly Ionized Turbulent Gas: Comparison of Neutral and Plasma Fluctuations

The structure of a turbulent gas flow may be probed by analyzing scintillations of a laser beam which has traversed the turbulent medium. This technique is used to measure the spectrum of neutral gas turbulence in a pipe flow where the gas is nonuniformly heated and weakly ionized by an electrical discharge. First, the usual isotropic analysis relating the spectrum of light scintillations to the fluid turbulence is extended to include strong anisotropy as is appropriate in the present case. Second, the gas turbulence spectrum revealed by the scintillation, as well as by a hot film anemometer, is compared with the spectrum of plasma density fluctuations. It is found that the scale size of the neutral gas turbulence is larger than the scale size of the plasma density fluctuations by a factor of 2 for a case in which the plasma column is constricted to a considerably smaller diameter than the pipe. This indicates that whenever the plasma production process is constricted to dimensions smaller than the gas tu...

HIGH-FREQUENCY SURFACE WAVES ON INHOMOGENEOUS PLASMA COLUMNS.

Dipolar waves are launched onto the surface of a positive column afterglow. Dispersion characteristics are obtained over a more extensive range of parameters than previously examined; phase velocity, ω/kz, ranged from 0.94c to 0.27c and signal frequency is high (λ0≈2πR). The waves are forward near the light line (ω/kz≈c), but become backward waves as ω/kz decreases. Experimental results are compared with an analysis that uses the full set of Maxwells equations (nonquasistatic) but which adopts a dielectric‐clad homogeneous plasma cylinder as a model of the discharge tube. This analysis qualitatively predicts the major dispersive features. In particular, as circumference to free space wavelength ratio, 2πR/λ0, increases, the magnitude of group velocity in the backward wave region decreases; also the peak in the dispersion curve becomes less sharp and its position recedes farther from the light line. However, lack of quantitative fit between experimental results and analytical predictions indicates the nee...

LIMITS OF VALIDITY OF BORN APPROXIMATION IN MICROWAVE SCATTERING FROM TURBULENT PLASMA.

Microwave scattering from a turbulent weakly ionized plasma column is studied. The scattering cross section is proportional to the mean‐square plasma density fluctuation, 〈n2〉, as predicted in the Born approximation for 〈n2〉½ up to 5% of the critical density.

Frequency Spectrum of Microwaves Scattered by Turbulent Plasma

Microwaves are scattered from plasma fluctuations produced by forcing gas in turbulent flow through a discharge tube. The frequency broadening of the scattered wave is studied as a function of several experimental variables including the magnitude and orientation of the wave vector and the extent of the scattering volume. The experiment is compared with a theoretical result that expresses the frequency broadening in terms of three distinct contributions: a Doppler broadening due to random convection of the plasma turbules; transit time broadening; and a non‐Gaussian broadening due to spatial gradients in the mean convection velocity. It is shown that measurement of the frequency broadening can be used to estimate the level of gas velocity fluctuations with about 25% accuracy.

Remote sensing of gas concentrations in smokestack emissions.

A measurement technique is discussed that allows for remote sensing of polluting gases that are emitted together with the water droplets in a steam plume. Infrared laser radiation is backscattered from the droplets. As wavelength of the radiation is varied, resonant absorption in the gases diminishes the backscatter signal; this may be quantitatively related the Concentration of specific gases. The backscatter arrangement makes possible the construction of a compact, single-ended, remote sensing instrument. Feasibility of this concept has been demonstrated in a laboratory experiment where a laser beam at 3.391 microm and 4.217 microm was scattered from a steam plume containing controlled amounts of CH(4) and CO(2).

Effect of Neutral Impurity Levels on the Oscillation Spectrum of a Magnetoplasma

Neutral gases were added to a chamber containing a fully ionized magnetoplasma. Above certain critical background pressures, naturally occurring drift waves were quenched and a new low‐frequency wave appeared. The wave quenching is attributed to ion‐atom collisions.