Interaction of an intense relativistic electron beam with a plasma-filled waveguide in a magnetic field

Analytic computations and particle-in-cell (PIC) code simulations for the interaction of an intense relativistic electron beam (REB) and a plasma have been carried out. In the simulations, a fast risetime ( approximately 5 ns) 10-kA REB (1 MeV) was injected into a plasma-filled waveguide immersed in an axial magnetic field. Beam transport and microwave generation by beam-plasma instabilities were investigated in both the infinite- and finite-B-field cases. In the finite-B-field case, both the two-stream and cyclotron instabilities were important. Calculations of charge and current neutralization of the REB were performed in the intense beam regime. These calculations provided the appropriate parameters for the linear dispersion relation of the system, which was solved to determine the nature of the instabilities. For large magnetic fields the linearly unstable waves on the lower branch of the dispersion curve can backscatter off the accumulation of plasma electrons at the beam front produced in the charge neutralization process. These backscattered waves can then mix with the original unstable wave in a three-wave process to produce a wave on the upper branch of the dispersion curve at a higher-frequency. Still higher frequencies can be produced by a cascading of wave-mixing processes.<>