Transition of Absolute instability from global to local modes in a gyrotron traveling-wave amplifier

This study systematically characterizes the local absolute instabilities and their transitional behavior. The axial modes were characterized from the perspective of beam-wave interaction, which share many characteristics with the global modes. The transition from global modes to local modes as the distributed loss increases was demonstrated. The electron transit angle in the copper section, which determines the feedback criterion, governs the survivability of an oscillation. In addition, the oscillation thresholds predicted using this model are more accurate than those obtained using a simplified model.