High power operation of an X-band gyrotwistron.

We report the first experimental verification of a gyrotwistron amplifier. The device utilized a single 9.858 0Hz, TED&z cavity, a heavily attenuated drift tube, and a long tapered output waveguide section. With a 440 kV, 200 — 245 A, 1 ps electron beam and a sharply tapered axial magnetic field, peak powers above 21 MW were achieved with a gain near 24 dB. Performance was limited by competition from a fundamental TE&z mode. A multimode code was developed to analyze this system, and simulations were in good agreement with the experiment. Devices based on the cyclotron maser instability [1,2] have proven to be efficient, high power, high frequency rf sources (see, e. g. , [3]). Oscillator configurations include the gyromonotron, which has received the largest effort to date [4 — 6], and the gyrobackward wave oscillator (gyro-BWO) [7]. Gyrotraveling wave tubes (gyro-TWTs) [8] and gyroklystrons [9 — 12] have successfully demonstrated high power amplification. Applications for these gyrodevices include plasma heating and current drive, deep space and conventional radar, drivers for rf accelerators and supercolliders, and materials processing. Some of these applications require high efficiency, high power amplifiers. A possible candidate for a source with these requirements is the gyrotwistron. The gyrotwistron is closely related to the twystron [13], a linear beam device which utilizes the bunching cavities of the klystron with the output waveguide of the TWT. In an analogous fashion, the gyrotwistron utilizes the bunching cavities of the gyroklystron with the output waveguide of the gyro-TWT. In comparison with the gyroklystron, the gyrotwistron has at least one important advantage: it can sustain higher powers. This is because it has significantly better output coupling than the gyroklystron, whose output cavity has a high quality factor. Therefore, for the same amplitude electromagnetic field in the interaction region, the radiated power is significantly higher in the gyrotwistron. Since this field is limited by breakdown, the gyrotwistron is capable of producing higher microwave power than the gyroklystron. The gyrotwistron also has an advantage over gyro-TWTs because its interaction length is shorter. This is important for the suppression of parasitic modes, whose starting current typically scales inversely as the cube of the length. In spite of these advantages, researchers have not paid close attention to the gyrotwistron.