A W-band sheet beam klystron is being developed as a portable coherent radiation source for active denial system application. The interaction circuit design employs eight stagger-tuned cavities (multigap structure) and a 12:1-aspect-ratio sheet electron beam (74 kV and 3.6 A) to produce 50-kW peak power (2.5 kW average) and 40-dB gain with 200-MHz instantaneous bandwidth. The output cavity is designed to have a quasi-optical (QO) external coupler utilizing optical wave superposition. The circuit design has been optimized by using a 1-D disk-model code and a 3-D particle-in-cell (PIC) solver. The iterative simulation analysis predicts that a five-gap configuration is the optimum structure for a QO-output cavity because it provides sufficient output power and stable single frequency operation without mode competition. The 3-D PIC simulation predicts that the designed circuit produces stable 50-kW output power from a 4-W input driving signal, with 40-dB gain, at 94.5 GHz. The frequency sweep predicts a 3-dB bandwidth of 150 MHz in 2π-mode operation. The numerical simulation results agree well with the small-signal analysis, thereby providing confidence in the predicted output performance of the QO klystron amplifier module.