Theory, Design, and Operation of Large-Orbit High-Harmonic Gyroklystron Amplifiers

The high-harmonic gyroklystron is a compact RF amplifier which relies on the synchronous interaction between large-orbit axis-encircling electrons and high-order azimuthal cylindrical cavity TEn11 modes. Because of this unique structure, it offers the promise of moderate- to high-millimeter-wave output power. Experimental results for a fifth-harmonic two-cavity device are compared to both small-signal analytic theory and large-signal numerical calculations which assess gain, bandwidth, sensitivity to longitudinal velocity spread, self-oscillation, and saturation characteristics. Principal theoretical results include the transition to infinite gain as the start-oscillation current threshold is reached as well as the demonstration of the insensitivity of the small-signal gain to parallel velocity spread. The required high-energy rotating electron beam is provided through gyroresonant RF acceleration. To date, over 20 dB of small-signal gain has been achieved at 11.3 GHz in a 1.3-kG magnetic field. The design of a four-cavity configuration is also presented along with simulation of its gain, bandwidth, amplitude modulation (AM) and phase modulation (PM) sensitivity, dependence upon guiding-center spread, and large-signal saturation characteristics. This device has been constructed and initial tests have yielded a gain of 30 dB. Gain in excess of 50 dB is predicted.