EXPERIMENTAL STUDY OF SCATTERING FROM A PLASMA COLUMN

Experiments in microwave scattering from the positive column of a low-pressure (1.2 mTorr) discharge in mercury under steady-state conditions are described. A free-space environment is simulated with an anechoic chamber and a unique cw cancellation method. Both amplitude and phase of the scattered electric field for both polarizations in the 2.6- to 5.85-GHz frequency range are measured, so that diffraction characteristics, radar cross sections, frequency ratio, and phase shifts are determined. A calibration procedure using silver-plated cylinders provides a higher degree of accuracy than has been attainable heretofore. Diffraction measurements show that, for plane-wave incidence, the scattering is purely dipolar with a 95% confidence interval, except for overdense scattering where a significant monopole term characteristic of conductors is observed. Theoretical comparisons yield an estimate of the electron density distribution function and the damping of the Tonks-Dattner resonances, as well as providing a test of the various theories. The results of some auxiliary experiments to find the average electron density in the column and the effects of wall and mercury reservoir temperatures upon the observed results are also given. © 1970 The American Institute of Physics.