Phase characteristics of klystron and gyroklystron

A study of RF phase characteristics in a four-cavity klystron and in a three-cavity gyroklystron has been carried out experimentally. The RF phase in the penultimate cavity is studied to determine its influence on electron bunching for efficiency enhancement. In the gyroklystron, maximum output power is measured when the resonant frequency of the penultimate cavity is detuned to the low frequency side of the input frequency which is opposite to the effect of klystron detuning; this is due to the opposite energy dependence of the bunching process. The phase sensitivity to the beam voltage is measured as 13 and 4 degrees/(% change in beam voltage) in the 7.5 kV klystron and 30 kV gyroklystron respectively, and compared with theory. The 30 kV gyroklystron showed less phase sensitivity to the beam voltage than the 7.5 kV klystron due to the inherent differences in gun dynamics and interaction mechanism. However, for high beam voltage, it is predicted that the phase stability of a klystron becomes better than that of a gyroklystron. It is verified that the magnitude and frequency of the phase jitter is well correlated with the droop and ripple on the beam voltage implying that the voltage droop and ripple is the main source of the phase jitter. Phase jitter can be reduced by employing a fast feedback circuit with a voltage-controlled phase shifter. Both the klystron and gyroklystron showed a phase jitter reduction by a factor of 3 by adding an integrating circuit to the feedback loop.