The excitation of coherent electromagnetic radiation in plasma waveguides by relativistic electron beams

A theoretical analysis is presented of the interaction between a monoenergetic relativistic electron beam and a bounded plasma and the conditions for the generation of electromagnetic radiation in a single mode by such a system are derived. Two specific geometries are considered: (a) a metallic waveguide of radius B, completely filled with plasma which is penetrated by a beam of the same radius and (b) a plasma cylinder of radius r0, with a hollow beam flowing along it and filling a metallic waveguide of radius R > r0. In the first of these systems the excitation of the electromagnetic waves is only of a coherent nature provided that a sufficiently strong longitudinal magnetic field is applied, when the fundamental mode of axially symmetric quasi-longitudinal electromagnetic waves is excited. In the second system, which has the hollow electron beam, the excitation of an axially symmetric mode of electromagnetic surface waves is in a single mode when certain conditions are satisfied regarding the magnitude of the magnetic field, the plasma density and the beam density. In both cases, the spectra and growth rates of the excited oscillations are established and the generation efficiencies calculated. (author)