Observation of the beam-plasma interaction below threshold

Abstract
The wave spectrum of longitudinal electrostatic fluctuations of a beam‐plasma system has been studied in the regime below the threshold for exponentially growing waves. The observed wave spectrum consists of a series of triplets, one set for each natural mode of the magnetized, cylindrical, axially bounded plasma column. The lineshape and intensity of the triplets are in good agreement with a simple theory based on the linearized dispersion relation.