Observation of Quasi-Coherent Electron Oscillations and Spatial Bunching in a Converging Beam–Plasma System

Abstract

A spatially periodic modulation of the plasma density is observed which is attributed to nonlinear bunching of counterstreaming electron beams leading to locally enhanced ionization. Microwave bursts (~1–5 GHz) strongly polarized in the direction of the beams are detected. Nearly coherent oscillations are found at frequencies about one order of magnitude lower than the electron plasma frequency but above the ion plasma frequency. The oscillations can be described by a theoretical cold beam–cold plasma model which takes into account the finite size of the plasma. The experimental results are in qualitative agreement with this model. A negative voltage pulse of 500 V and ~10 μs duration is applied between two concentric cylindrical electrodes. The outer electrode, the cathode, consists of heated tungsten filaments; the inner one, the anode, is perforated. The electrons emitted by the filaments are accelerated into the space enclosed by the anode where a plasma of a density [Formula: see text] and an electron energy of 1.5–10 eV is formed in argon at a pressure of 5 × 10−4 Torr.