Quasi-Optical Output-Cavity Design for a 50-kW Multicavity W-Band Sheet-Beam Klystron

In this paper, a quasi-optical (QO) output coupler, consisting of a tapered waveguide coupling to WR-10 waveguide, was designed to serve as the multigap output cavity of a 50-kW peak-power W-band sheet-beam klystron. In this structure, TE₁₀ mode radiation from the gap ports is superimposed in the tapered waveguide coupling out to the external waveguide. In our numerical design, the circuit dimensions were systematically optimized by parametric finite-difference time-domain simulations; the 2 pi mode of the optimized QO output cavities (three, five, and seven gaps) have an external Q(Qe) of 650, an ohmic Q(Q ₀) of 1650, and a total Q(Qt) of 460 at 95.4 GHz. The measured transmission coefficient (S ₂₁) through the two QO external waveguide ports is in reasonable agreement with the transmission predicted from simulation. MAGIC3-D particle-in-cell simulation predicts that the QO output cavity stably produces more than 50-kW output power from a 70-kV and 3.6-A electron beam in the eight-cavity klystron circuit.