Electron Bunching in a Nonlinear Transmission Line-Driven Electron Beam

The nonlinear transmission line electron beam driver at the Air Force Research Laboratory produces a frequency agile modulated annular electron beam [1]. A linear kinetic bunching analysis is performed based on transmission line theory [2]. This analysis reveals that the axial length of the conical Friedman cathode lowers the maximum bunch amplitude, analogous to the effect of axial distance on the opening of a klystron bunching cavity. Additionally, the somewhat broadband nature of the NLTL pulse translates into a saw-tooth profile for the peak frequency of the electron beam as a function of axial distance down the drift tube. Most importantly, the direct relation between applied voltage and cathode current emission leads to in phase bunching in both current and velocity, which enhances the maximum bunching amplitude when compared to just voltage modulation. The purpose of this exploration is to inform the design of a relativistic klystron amplifier that uses the NLTL e-beam driver as a gun.