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Description
High-power klystrons play a critical role in microwave power amplification in radio-frequency (RF) systems for large-scale accelerators, and their operational stability directly affects the long-term reliability of the RF system. To investigate output-power fluctuations, electron reflection, and parasitic oscillations that may occur under load-mismatch conditions, this study takes the output cavity as the core and establishes a coupled analysis model incorporating the focusing magnetic field, beam transport, complex loading of the output cavity, and feedback from backstreaming electrons. Based on the relativistic equations of electron motion, the effects of the focusing magnetic field on beam transmission and the fundamental-frequency beam current upstream of the output cavity are analyzed. By combining the equivalent-circuit model of the output cavity with transmission-line theory, the relationships among the voltage standing-wave ratio (VSWR), reflection phase, normalized load admittance, equivalent external quality factor, loaded quality factor, and gap voltage are derived. Electron reflection under strong decelerating fields and the corresponding feedback conditions are also discussed. Calculations using the output-cavity parameters considered in this study show that, at a VSWR of 1.2 and with the fundamental-frequency beam current assumed to remain approximately unchanged, different reflection phases can increase the gap voltage by approximately 19.8% or decrease it by approximately 16.5% relative to the matched condition. This indicates that the same VSWR does not necessarily correspond to the same operating state of the output cavity. Therefore, the influence of load mismatch on stability depends jointly on the magnitude and phase of the reflection coefficient, while the focusing magnetic field further affects the risk of electron reflection by modifying beam transport and bunching conditions. These results provide a reference for the selection of output-cavity loading parameters, adjustment of the focusing magnetic field, and determination of protection thresholds against load mismatch.
