Return-current amplification in the interaction of an intense relativistic electron beam with a gas target

Experiments are reported on the injection of a 1.5-MeV, 50-kA (lambda/..pi..< or approx. =1), 20-nsec electron beam into a neutral gas. Under certain conditions, the amplitude of the return current induced by the primary electron beam in the plasma produced in the gas by this beam is higher than the amplitude of the beam current by a factor of 2--3 (this is return-current amplification). On the other hand, a simple model for the excitation of the return current, I/sub 2/, during the propagation of a pulsed relativistic electron beam with a current I/sub 1/ in a finite-conductivity plasma predicts that I/sub 2/ would be smaller than or equal to I/sub 1/.