The multimode beam wave interaction behavior in a tapered, cylindrical cavity RF interaction structure of a 42 GHz gyrotron operating in the TE03 mode has been investigated through nonlinear analysis and PIC simulation. A technique for producing the annular gyrating electron beam in PIC simulation code CST Particle Studio has been described. An energy transfer phenomenon from electron beam to RF has been demonstrated. The performance of cavity has been monitored to ensure the device operation in the desired mode and frequency. In the PIC simulation, the effect of beam velocity spread on the power output has been discussed. Using multimode behaviour, the effect of presence of nearby modes on the cavity performance has been observed. The simulation results have been compared with the results obtained from self-consistent single-mode analysis and time-dependent multimode analysis. It has been found that output power is well above the desired 200 kW level for the designed 42 GHz gyrotron operating in TE03 mode.
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