Effect Of Ionization On Intense Electron Beam Propagation In Low Pressure Media

Abstract

One of the major obstacles to the propagation of intense electron beams in low pressure media is the occurrence of the "ion hose instability" which arises when such a propagation takes place in the so called "Ion Focus Regime" or IFR. In the simplest model of this instability, namely, the rigid beam model, the instability is known to be absolute, i.e. it grows at every point in space. When the radial motion of the ions in the propagation channel is included in the simple model, it is also known that the IFR instability transforms from absolute to connective, i.e. the wave grows as it propagates. In this paper we re-examine this problem by incorporating the effects of collisions that take place between the beam particles and those of the channel medium through which they move. Specifically we include the effects of impact ionization by the beam front as well as the avalanche ionization generated by the ions in the channel. We treat both the beam and channel ions as rigid but bendable, and solve the appropriate set of coupled equations that describe the displacements of the beam and the ions. We find the IFR hose instability to remain connective and its spatial growth rate to be reduced by about 50% for the same charge neutralization factor when avalanche ionization is superimposed on impact ionization. When an ad-hoc damping factor that simulates a spread in the betatron frequency is introduced we find that the damping rate varies inversely with the charge neutralization factor.