Oscillations and Noise Generation in a Beam-Generated Plasma

An electron beam at a few kilovolts is used to generate a plasma in helium or argon at 10−4 to 10−2 Torr in a glass tube aligned with an axial uniform magnetic field of 0.3 to 2 kG. At and above a critical beam current and gas pressure, the discharge changes from a glow region the size of the beam to a glow discharge filling or nearly filling the glass tube (total glow). Measurements of the plasma density, the velocity distribution of the beam, and the spectrum of the rf noise generated in the plasma as functions of beam current, voltage, and gas pressure have been made. These data are consistent with an explanation of the effect involving a convective forward-wave beam-plasma oscillation which has a starting current equal to the measured critical current for the transition to total glow and which is determined by the net gain of the system including reflections at the ends. The oscillation, once started, is not a coherent oscillation, but can have a very wide noise bandwidth extending essentially from zero frequency to near the electron plasma frequency.