title: Transition of absolute instability from global to local modes in a gyrotron traveling-wave amplifier.
abstract: The gyrotron traveling-wave amplifier employing the distributed-loss scheme is capable of very high gain and effective in suppressing the global absolute instabilities. This study systematically characterizes the local absolute instabilities and their transitional behavior. The local absolute instabilities are analyzed using a model that incorporates the penetration of the field from the copper section into the lossy section. The axial modes were characterized from the perspective of beam-wave interaction and were found to 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.
authors.affiliation: Department of Physics, National Tsing Hua University, Hsinchu, Taiwan, Republic of China.
authors.forename: T H
authors.initials: TH
authors.lastname: Chang
authors.forename: N C
authors.initials: NC
authors.lastname: Chen