Upper-Hybrid Resonance Absorption, Emission, and Heating of an Afterglow Plasma Column

Abstract

Microwave absorption and emission and electron temperatures of a nonuniform axially magnetized afterglow plasma column in a waveguide geometry have been investigated experimentally. Frequency omega and magnetic field omegac are chosen to satisfy the upper-hybrid resonance condition omega2=omega_c²+omega_p²(r), where omegap(r) is the local electron plasma frequency. Nearly perfect absorption is observed in the range of upper-hybrid frequencies, while at other frequencies the absorption coefficient is essentially zero. The sharp absorption onset at the maximum upper-hybrid frequency yields an accurate measure for the peak electron density. Density decay and profile in the plasma column are observed—the latter using a new technique. In the range of high absorption the noise emission approaches the blackbody limit. The electron temperature is measured with a radiometer and a reference noise source in a new technique yielding both spatial and time dependence without perturbing the plasma. The time resolution is obtained by a sampling technique. The spatial resolution results from the fact that upper-hybrid resonance absorption and emission are confined to a narrow resonant layer. This property is also used to heat the electrons locally and observe the thermalization process.