Backward-Wave Microwave Oscillations in a System Composed of an Electron Beam and a Hydrogen Gas Plasma

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Abstract
A traveling‐wave interaction structure is used to investigate the properties of a low‐density plasma formed by the interaction of an electron beam with hydrogen gas. In this experiment, microwave oscillations near the electron cyclotron frequency are observed as the result of growing waves in a beam plasma interaction. Electron densities are determined by observing the correlation between the measured frequencies of oscillation and the theoretical predictions of [A. W. Trivelpiece and R. W. Gould, J. Appl. Phys. 30, 1784 (1959)] relating to backward‐wave propagation in a beam‐plasma system in a magnetic field. This electron density was verified by observation of the shift in the resonant frequency of a microwave cavity containing the plasma. The shift in the resonant frequency of the cavity gives a direct measurement of the electron plasma frequency and, hence, the electron density.