Numerical and experimental design study of quasi-optical multi-gap output cavity for W-band sheet beam klystron (WSBK)

A quasi-optical (QO) output coupler, consisting of a tapered waveguide coupling to WR-10 waveguide, was designed for a W-band multi-gap sheet beam klystron output cavity intended for 50 kW output. In this structure, TE₁₀ mode radiation from the multiple interaction gap-ports is superimposed in the tapered waveguide coupling out to the external waveguide. In our numerical design, all the circuit dimensions were systematically optimized by parametric FDTD simulations; the 2pi-mode of the optimized QO output cavities (3-, 4-, and 5-gap) has external Q (Q_e) of 650, ohmic Q (Q₀) of 1650, and total Q (Q_t)of 460 at 95.4 GHz. The measured transmission coefficient (S₂₁) through the two QO external waveguide ports is in reasonable agreement with the numerical transmission spectrum obtained from simulation. MAGIC3D PIC simulation predicts that the 3-gap QO output cavity produces about 80 kW output power with the 70 kV and 3.6 A electron beam in the 8-cavity klystron circuit.