Particle-in-Cell Simulation and Optimization for a 220-GHz Folded-Waveguide Traveling-Wave Tube

The folded-waveguide slow-wave structure (SWS) is a robust beam-wave interaction circuit for millimeter-wave traveling-wave tubes. We applied a particle-in-cell solver (Computer Simulation Technology Particle Studio) to analyze and optimize a 220-GHz folded-waveguide SWS. The beam-wave interaction and the electron beam dynamics were studied based on simulation results. The influence of the beam tunnel, with respect to its transverse shape and size, on the circuit performance was investigated in detail. A beam tunnel with a square cross section exhibits lower gain and efficiency and similar bandwidth compared with a beam tunnel with a circular cross section. With a proper phase-velocity taper employed in the nonlinear region of the circuit, the small-signal gain, peak saturated output power, and efficiency were significantly improved, increasing from 21.5 to 27.8 dB, 41 to 70.5 W, and 4.5% to 8%, respectively, for a 36-mm-long loss-free circuit. A simulated 3-dB bandwidth of ~ 15 GHz is demonstrated.