Echoes and Scattering From Plasma in a Magnetic Field

[摘要] A study of the high frequency properties of a low temperature plasma column in a longitudinal magnetic field is described. The experimental observations give a relatively complete picture of the microwave (extraordinary mode) properties of the plasmas studied, encompassing continuous wave scattering and noise emission measurements, as well as the demonstration of the occurrence of related echo processes. Theoretical developments deal largely with a physical model consisting of a one-dimensional slab of cold (zero temperature) plasma which is nonuniform in the steady state and immersed in a uniform magnetic field. Data on the continuous wave reflection and noise emission from some afterglow rare gas discharges are given. The electron temperature in these plasmas is low, approaching room temperature. The reflection and emission are measured as a function of magnetic field in the vicinity of electron cyclotron resonance with electron density as a parameter. The electron densities are such that (ω_p/ω) ≤ 1 , where ω_p is the electron plasma frequency and ω is the signal frequency.Both types of experiment show the presence of collective effects which yield a normal mode spectrum strongly dependent on the electron density.A broad peak is observed in the region (ω_c/ω) ≤ 1 where ω_c is the electron cyclotron frequency. This peak shifts to lower values of (ω_c/ω) and broadens as the electron density increases. For all values of electron density, a sharp peak is found very close to (ω_c/ω) = 1, the cyclotron resonance condition. The experimental and theoretical results suggest that the phenomena observed involve resonance effects at the upper hybrid frequencies (ω_h² = ω_c² + ω_p²) of the plasma. Data are also given on a new two-pulse echo process which occurs in these after-glow plasmas. The results establish the intimate relation between the echoes and the upper hybrid normal modes studied in the CW experiments, demonstrating the dominant role played by collective effects in the formation of this echo. A weakly nonlinear cold plasma theory yields upper hybrid echoes which are strongest in a narrow band of frequencies near the maximum upper hybrid frequency of the nonuniform plasma, in agreement with experiment. Furthermore, large signal computations of the dependence of the echo amplitude on excitation pulse separation and amplitude show a complex behavior in qualitative agreement with experiment. As a supplementary topic, the properties of echoes from a general collection of anharmonic oscillators are discussed. The oscillators in this discussion are a general mathematical analog of the upper hybrid normal modes of a cold nonuniform plasma. Through emphasis of effects due to the finite width of the excitation pulses, the calculations show explicitly the role of the various characteristics of the oscillators in echo processes, further delineating the general features thought requisite of classical multiple-pulse echo systems.

[发布机构] University:California Institute of Technology;Department:Physics, Mathematics and Astronomy

[关键词] Physics