Parametric design of folded waveguide for a 60GHz high efficient traveling-wave tubes

Characterized with full-metal structure, low RF loss, broad bandwidth and relatively high output power, folded waveguide (FW) is considered as a prospective slow-wave structure for traveling-wave tubes (TWTs) at millimeter and sub-millimeter wave range. In this paper, a high efficient FW slow-wave structure is designed for 60GHz central frequency TWT. Firstly, cold-test properties such as phase velocity, beam-wave interaction impedance and cold circuit attenuation are studied and optimized by 3D simulator. Based on the detailed cold-test simulation, the optimized structural parameters for the FW circuit with high interaction impedance, low cold-test attenuation and fairly flat phase velocity dispersion are obtained. And then, the beam-wave interaction simulation is also carried out with the PIC code in CST Particle Studio, to obtain the beam-wave interaction efficiency and saturated output power for lossy FW circuit with constant-period structure. In order to improve the beam-wave interaction and saturated output power further, varying-period structure is adopted. It shows that the saturated output power and beam-wave interaction efficiency are are greatly improved and also the input power at saturation is lower at the 58~62GHz frequency range when we adopt the varying-period FW structure. The detailed analysis will be reported in the full manuscript.