Beam-Plasma Discharge: Buildup of Oscillations

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Abstract
Plasma electron heating was accomplished by exciting plasma oscillations with a pulsed, 10‐kV, 1‐A electron beam drifting in a longitudinal magnetic field of 100 to 1000 Oe. The oscillatory energy gained by plasma electrons leads to an increase in their density and mean kinetic energy. The resulting plasma, which is called the ``beam‐plasma discharge,'' is an rf discharge in which a beam‐plasma interaction generates the rf fields. At the beginning of a beam pulse, beam electrons generate a plasma by ionization of the background gas. Within a few microseconds excited plasma electrons become the dominant ionization source. This paper describes the transient rf oscillations that appear in the first few microseconds of the beam pulse. From a detailed study of this transient it was concluded that a nonconvective instability initiates the oscillations, which first appear at the electron cyclotron frequency, and that a convective instability at the plasma frequency generates the oscillations that sustain the discharge.