INSTABILITY CAUSED BY BACKSTREAMING ELECTRONS IN KLYSTRON

The mechanism of backstreaming electrons causing spurious oscillations in klystron will be illustrated using the feedback theory. Since a feedback loop is formed inside the klystron due to the backstreaming electrons, the spurious oscillations will occur if the product of the klystron voltage gain and feedback coefficient is larger than unity, and the phase of the product is zero or integral times of 2π. This kind of oscillations has been observed in the 324MHz klystrons at KEK. In order to analyze the oscillations, a simulation code has been made to calculate the backstreaming electrons from the collector and to evaluate their effects on the klystron. The calculation results have shown a very good agreement with the test results of the 324MHz klystrons. The oscillation phenomena and suppression will be discussed. Proposals are derived for suppressing the oscillations completely in the 324MHz klystron.