Particle-in-cell (PIC) code simulations have been carried out to investigate the physical mechanism responsible for the wave amplification in the output section of a harmonic-multiplying gyrotron traveling-wave tube. Simulation results demonstrate that the injected signal frequency plays an important role in determining the wave growth process. This is because the initial frequency mismatch in the input section influences the extent of bunching at the entrance of the output section and the slope of the electron phase trajectory. For a small frequency mismatch, the amplification arises entirely from the second harmonic generation due to the bunching at the fundamental frequency. As the mismatch is increased, the bunched electrons start to disperse early in the output section, and the wave amplification is then dominated by the cyclotron maser instability. The electron axial velocity spread is observed to substantially degrade the device performance.
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Lin et al. (2002) studied this question.
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