Cyclotron effects in relativistic backward-wave oscillators operating at low magnetic fields

A small-signal theory of a resonant relativistic backward-wave oscillator (BWO) operating at a very low-guiding magnetic field is developed. The theoretical approach is based on the successive iteration method of analytical solution of the exact three dimensional (3-D) equations of electrons motion. It was shown that cyclotron resonance effects in relativistic BWO (400 kV) operating at low-focusing magnetic field can lead to three different regimes of combined Cerenkov-cyclotron interaction: 1) cyclotron absorption due to cyclotron resonance between an electron beam and a fast spatial harmonic of forward electromagnetic wave; 2) complete compensation of cyclotron absorption under certain conditions; and 3) efficiency enhancement at low-focusing magnetic field. The theoretical results are compared with measurements of the output microwave power and frequency of the Maryland X-band BWO at very low magnetic field (0.8-4 kG). The measured dependence of the microwave output power on the strength of the guiding magnetic field is consistent with this model. The measured dip in the output power at around 2 kG can be explained as a cyclotron absorption by the forward wave. Design of a BWO operating at low magnetic field has definite advantages compared with a traditional BWO because it provides considerable reduction of volume, weight, and energy consumption of BWO.