Propagation of an intense relativistic electron beam through a plasma region into vacuum

An intense relativistic electron beam (IREB) is injected through a narrow plasma region into a vacuum drift tube at current levels far above the space-charge limit of the drift tube. Experiments show that accelerated co-moving positive ions from the plasma enable the beam to propagate to a collector at the end of the 15 cm diameter drift tube. In one set of experiments, the IREB (1 MeV, 27 kA, 30 nsec) passes through a hydrogen gas cloud with an effective width of approximately 2 cm. The net current measured by a 7 cm diameter collector at the end of a 55 cm long drift tube is 70% of the peak injected electron current. In another configuration, the IREB is injected through a laser-produced carbon plasma. A model is proposed linking electron beam propagation with collective ion acceleration which results in the generation of charge- and current-neutral plasmoids capable of free space propagation. Possible implications of this work to astrophysical processes (cosmic ray acceleration) and laser fusion (observation of high-energy positive ions escaping from target plasma) are discussed.