Particle-In-Cell Simulation of a Tapered Cavity for a Millimeter Wave Gyrotron

A waveguide structure, which includes a downtaper, a straight-midsection, and an uptaper waveguide sections with two parabolic-corrections at transitions of waveguide sections along with a raised cosine nonlinear taper, was considered as an interaction structure for the gyrotron. The particle-in-cell (PIC) simulation and large-signal analysis were performed using CST Studio Suite and GYRO-K, respectively, with the proposed structure in nine combinations involving three radius and three magnetic field profiles. The time signal and electric field growth were monitored and the output powers were estimated for the TE 6,2 and other competing modes. One of the radius and magnetic field profile combinations leads to 135-kW power in the PIC simulation at ~95 GHz with 49.1% beam-to-wave conversion efficiency. For the same profile combination, GYRO-K estimates ~7.9% power degradation with a 45-MHz lower side frequency shift than that of the PIC simulation.