Characterization of forward-wave interaction in a frequency tunable gyrotron

This study proposes a theory to separately diagnose the beam-wave dynamics of backward wave (BW) and forward wave (FW) interactions in the frequency-tunable gyrotron. It is clarified that the BW interaction would dominant the feedback mechanism regardless of the cavity structure resonance. The FW interaction effect is found to be minor in small signal regime but critical in the large signal regime due to the field contraction behavior. At near cutoff operation, gyromonotron region, FW can be in synchronism with electron beam and lead to high efficiency even in the absence of the cavity structure. In backward-wave operation, it can also cause a non-negligible modulation effect through electron bunching stage to degrade or enhance the efficiency that strictly depends on the acquiring phase from the upstream reflection. As a simulation result of a uniform interaction section with an effective total-reflection boundary at upstream, the efficiency of the favored reflection phase 2π could remarkably prior to that of the disfavored phase π.