Investigations of high-harmonic gyrotrons with frequency-doubled prebunched beams

There is currently considerable interest in operating gyrotrons at the second and higher cyclotron harmonics in order to access the near-THz regime and reduce magnetic field requirements. High frequency gyrotrons have successfully operated at the second harmonic [1]; however, competition from the fundamental harmonic limits operation and essentially precludes higher harmonic operation. Bandurkin and Savilov [2,3] recently proposed a scheme for frequency-doubled bunching of gyrating electron beams in a waveguide resonator formed from Bragg reflectors and with the drive frequency equal to the cyclotron frequency. Advantages of prebunching at twice the cyclotron frequency include suppression of the fundamental harmonic, enhanced second harmonic operation, and increased likelihood of fourth harmonic operation. We have investigated the use of this bunching technique to enhance higher-harmonic operation in gyrotron oscillators with annular beams. We compute the frequency-doubled bunching produced by a Bragg-type prebunching cavity and use a large-signal, multimode, multi-harmonic gyrotron oscillator code to simulate the effect of this bunching on a highly overmoded output cavity. Regimes of stable operation are predicted for second and fourth harmonic point designs.