Microwave absorption and emission from magnetized afterglow plasmas

[摘要] The microwave scattering properties of an axially magnetizedafterglow plasma column in an S-band waveguide have been investigatedexperimentally. The column axis is perpendicular to the electric fieldand the direction of wave propagation in the H_(10)-mode waveguide.Strong absorption is found in the range of upper hybrid frequencies, ω_c ≤ ω ≤ [ω^2_c + ω^2_p(r,t)]^(1/2) where ω_c is the electron cyclotron frequency and ω_p is the locally and temporally varying electron plasma frequency. With the high absorption the noise emission approaches theblackbody limit.A microwave radiometer has been used to measure thenoise power and with a comparison and null-technique the electron temperature.As emission and absorption are largely confined to a resonant layer, spatially resolved temperature data are obtained.Time resolution is obtained by gating the radiometer. The peakelectron density is derived from the emission or absorption onset atthe maximum upper hybrid frequency and confirmed by independent measurements. With this diagnostic technique the electron density and temperature decay has been studied under a variety of experimentalconditions.Ambipolar diffusion and collisional cooling essentiallyaccount for the plasma decay, but impurities and metastable ionsplay an important role. The diagnostic method is successfully appliedin a microwave heating experiment. The existence of absorbingresonant layers is shown by a peak in the radial temperature profilewhere the local upper hybrid frequency equals the heating frequency.The knowledge of the plasma parameters is important in the study of hot plasma effects.Buchsbaum-Hasegawa modes are investigated in a wide range of magnetic fields (.5 < ω_c/ω < .985).

[发布机构] University:California Institute of Technology;Department:Engineering and Applied Science

[关键词] Electrical Engineering