RF Coupling and Beam-Wave Interaction Study in a Periodically Loaded ${X}$ -band 25-MW Gyro-Twystron

A megawatt class X-band gyro-twystron using periodically loaded alternate lossy dielectric and metal output waveguide section is studied for its beam-wave interaction behavior using a fully nonlinear, self-consistent, and multimode theory. Nonlinear amplification of the desired TE01 mode and its competing parasites including TE11, TE21, and second harmonic TE02 modes are investigated using multimode code to ensure the device stability. The present amplifier developed a peak RF output of ~25 MW in the desired TE01 mode that is validated with a 3-D commercial particle-in-cell code. Furthermore, an input coupler and an output RF window are simulated for their transmission and reflection characteristics. The return and transmission losses of input coupler are calculated as ~28 and ~0.13 dB, respectively. Similarly, the RF window is designed to be transparent for the output with a transmission loss of ~0.075 dB and a return loss of ~63 dB.