CYCLOTRON EFFECTS IN RELATIVISTIC BACKWARD-WAVE OSCILLATORS OPERATINGAT LOW MAGNETIC-FIELDS

A small-signal theory of a resonant relativistic backward-wave oscilla tor (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 ca n interaction: 1) cyclotron absorption due to cyclotron resonance ffj between an electron beam and a fast spatial harmonic of forward electr omagnetic wave; 2) complete compensation of cyclotron absorption under certain conditions; and 3) efficiency enhancement at low-focusing mag netic field. The theoretical results are compared with measurements of the output microwave power and frequency of the Maryland X-band BWO a t very low magnetic field (0.8-4 kG), The measured dependence of the m icrowave output power on the strength of the guiding magnetic field is consistent with this model, The measured dip in the output power at a round 2 kG can be explained as a cyclotron absorption by the forward w ave. Design of a BWO operating at low magnetic field has definite adva ntages compared,vith a traditional BWO because it provides considerabl e reduction of volume, weight, and energy consumption of BWO.