Analysis of spurious oscillations in klystron due to backstreaming electrons from collector

A new type of spurious oscillation was observed in a newly developed ultra high frequency (UHF) klystron at High Energy Accelerator Research Organization (KEK). The oscillation occurred in regions of high beam voltage with a frequency very close to the operation frequency of the klystron. After experimental investigations, it was found that the oscillation was caused by backstreaming electrons from the collector. The process of production of backstreaming electrons was investigated using the Electron Gamma Shower (EGS4) code. The amount and energy distribution of backstreaming electrons were simulated for different beam voltages, collector sizes, and collector materials. The oscillation mechanism was analyzed by introducing a feedback loop comprising the main beam and backstreaming beam. The feedback coefficient was calculated as a function of rf frequency and beam voltage. The calculation results agreed well with the experimental results with respect to the beam-voltage ranges and rf frequencies of the spurious oscillation for the first three prototype tubes. Hence, the oscillation was considered to be attributable to the feedback loop due to the backstreaming electrons. By applying this result, the oscillation was suppressed completely in succeeding tubes. The present analysis would be useful for suppressing similar oscillations for high-power klystrons in the future. (author)