Symmetric and asymmetric mode interaction in high-power traveling wave amplifiers: experiments and theory

High-power microwave amplifier operation has been studied for use in a number of applications. The performance of the amplifiers has been marred, in some cases, by pulse shortening of the microwave signal. A possible source of the shortening is the loss of the beam due to hybrid HEM₁₁ mode interaction with the beam. In this paper, we describe experiments which investigate high-power operation and the effects of HEM modes on the amplifier performance. We report the high-output powers (>50 MW) with efficient (>54%) amplification of microwaves in an X-band traveling wave amplifier. In some experiments, peak power levels exceeding 120 MW were measured at an efficiency of 47%. The excitation of the asymmetric hybrid electromagnetic mode was monitored carefully, but does not seem to have a critical impact on the main interaction process in spite of the fact that its dispersion curve almost overlaps that of the symmetric interacting mode. Theoretical analysis of the interaction in a tapered traveling wave structure indicates that, even if the amount of power in the asymmetric modes at the input of the structure is comparable to that in the symmetric mode, the asymmetric modes cause no power reduction in the symmetric mode. For the case of off-axis beams the TM₀₁ output power may drop by about 30% and the power in the hybrid mode reach about one third of that in the symmetric mode. In order to avoid hybrid mode excitation it is necessary to suppress the reflections from both ends of the output structure several decibels below the gain level of the asymmetric mode