Millimeter Wave Generation by a Relativistic Electron Beam in a Plasma Filled Sheath Helix Loaded Waveguide

A relativistic electron beam propagating through a plasma filled sheath helix, a slow wave device, excites a Trivelpiece‐Gould (TG) mode around 10 GHz with high efficiency. A large amplitude TG mode undergoes stimulated Compton backscattering off the electron beam parametrically driving an electromagnetic wave at millimeter wave length. The growth rate of the instability scales as one third power of the beam density and decreases with the operating frequency of the free electron laser (FEL). The FEL frequency increases with γ02, γ0 is the relativistic gamma factor. As the beam mode looses energy, the phase matching gets detuned, resulting in the saturation of the instability.