ION OSCILLATIONS IN AN ELECTRON BEAM GENERATED PLASMA.
STANFORD UNIV CA INST FOR PLASMA RESEARCH
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The work reported here is concerned with the oscillatory phenomena associated with an electron beam passing through partially ionized, low pressure gas. Three distinct physical regimes are defined, namely the very low pressure regime where the residual ion density is a negligible fraction of the electron beam charge density, the low to moderate pressure region where the residual ion density is a significant fraction or even equal to the electron beam charge density, and the third regime where the moderate gas pressure allows the ion density to be equal or significantly exceed the electron beam charge density. In those regimes where the ion density is significant various workers have observed the presence of low frequency, typically in the megacycle region, oscillations in the electron beam current. These oscillations have been generally assumed to be associated with the ion system and appeared to be motions where the charged particles moved in a predominantly perpendicular direction to the electron beam drift direction. It was the principal goal of this work to obtain a theoretical and experimental explanation of this often-observed phenomenon. The finite, three-dimensional theory presented in this work is a cold electrostatic theory of a two-component plasma which consists of equal number densities of initially stationary ions and drifting electrons. This is the ion-neutralized electron beam model. The theory is examined in some detail to obtain the dispersion relations of a pseudo-surface wave nature for both the monopole and dipole interactions which lead to particle motions which are predominantly perpendicular to the electron beam drift direction and a static magnetic focusing field.
- Plasma Physics and Magnetohydrodynamics