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

Microwave absorption and emission and electron temperatures of a nonuniform axially magnetized afterglow plasma column in a waveguide geometry have been investigated experimentally. Frequency ω and magnetic field ω_c are chosen to satisfy the upper‐hybrid resonance condition ${\omega }^2{\mathrm{=\omega }}_c^2{\mathrm{+\omega }}_p^2\mathrm{(r)}$ , where ω_p(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.