Coupling of symmetric and asymmetric modes in a high-power, high-efficiency traveling-wave amplifier

A three-dimensional model has been developed for the investigation of the coupling of symmetric (TM01) and asymmetric (HEM11) modes in a high-power, high-efficiency traveling-wave amplifier. In the framework of a simplified model it is shown that the coupling between these two modes is determined by a single parameter that depends on the beam characteristics. For a specific set of parameters corresponding to operation at 35 GHz, simulations indicate that an initial HEM11 power of 0.5 MW at the input end is sufficient to deflect electrons to the wall. The build-up of this parasitic mode is investigated over many round trips of the wave in the structure and a threshold criterion for self-sustain oscillation is established. Finally a way for suppressing the HEM11 mode is analyzed.