Interaction of an intense relativistic electron beam with preformed channels

Abstract

The interaction of an intense relativistic electron beam (REB) with preformed channels in gaseous atmospheres has been analyzed in order to delineate the effects of reduced density, avalanche ionization, preexisting conductivity, and channel currents. The REB for these experiments was produced from a field emission diode driven by the ≈1.4 MV pulse from a pulse forming line. Relativistic electron beam currents up to ≈16 kA with current densities up to ≈2 kA/cm2 were achieved and the REB current was approximately a half sine wave of width 27 nsec (FWHM). Preformed channels in the atmosphere were created using laser-guided electric discharges. Current-carrying reduced density channels were produced by applying a second discharge to the reduced density channel produced by the first discharge. Reduced density (≤ρ0/80), nonconducting channels were produced by the absorption of radiation from a pulsed CO2 laser in ammonia gas at background pressures of ≈40 Torr (ρ0/20). The results show that reduced density had little effect on REB propagation except for a decrease in scattering until the density within the channel had been reduced to such a low level that the dominant mechanism by which conductivity is generated shifted from direct collisional ionization to avalanche ionization. Avalanche ionization in a uniform atmosphere increases the growth of REB instabilities but when it is limited to the reduced density channel region the REB was always repelled or expelled from the channel. Preexisting channel conductivity (σ≥0.1 S/m) also caused the REB to be repelled or expelled from the channel. The presence of a parallel channel current permitted the REB to be readily injected into the channel and guided along it with minimal losses. All of these effects and the thresholds at which they occurred are consistent with the present understanding of the interaction of intense REB’s with gaseous atmospheres.