On the electron beam-breakup transit time oscillator

Summary form only given. Three-dimensional particle-in-cell simulation has confirmed an interaction between a relativistic electron beam and nonaxisymmetric cavity modes, which leads to amplification of the cavity modes. When a relativistic electron beam is injected into a cylindrical cavity, its transverse motion excites nonaxisymmetric electromagnetic modes such as TM₁₁₀ or TE₁₁₂. The physical effect exploited and optimized is that of the beam-breakup instability important in high-current accelerator cavity design. Coupling the instability with the transit-time effect of the electron beam in the cavity, rapid exchange of energy between the electron beam and the preselected electromagnetic mode can occur and microwave power can then be extracted. Under optimal conditions, the linear growth time of the cavity mode at 2.44 GHz is expected to be about 3 ns. In addition, the transit-time resonance naturally leads to very narrowband output with typical wavelengths in the range of 1-10 cm depending on the cavity radius. A comparison of the linear growth rate of the TM₁₁₀ mode with a linear theory prediction for different lengths of the cavity has been made. Simulation results show a slight shift in the cavity length for maximum growth