Amplification studies of a two-cavity second harmonic gyroklystron with a mixed-mode output cavity

Abstract

The performance of a two-cavity second harmonic gyroklystron employing a short, mixed-mode output cavity with stepped radial transitions is described. The short cavity was employed to prevent low frequency spurious oscillations that can compete with the harmonic signal. The radial rf field profile in the cavity and the output radiation consisted of both the TE01 and TE02 circular waveguide modes. This device produced a peak output power of 20 MW at 19.782 GHz, with an efficiency of 23% and a gain of 26 dB. The nominal mode mixture of the radiated power during optimal operation was found to be about 60% TE02 and 40% TE01. Variations from this ratio are observed at lower powers and are attributed to shifts in the cavity rf field profiles. Systematic studies of amplifier performance as a function of beam parameters and magnetic field profile are described. The sensitivities of output power production to changes in operating parameters are compared to results from earlier harmonic and fundamental gyroklystrons. The present device is found to be more stable to parasitic oscillations and less sensitive to variations in beam current and output cavity magnetic field than previous harmonic gyroklystrons that employed longer, smoothly transitioned cavities. However, both the peak power and efficiency in the present device are lower than the 30 MW at 28% efficiency obtained with the final smooth-cavity tube. The tradeoffs between the two output cavity concepts will be discussed.