Results From An X-band Backward-Wave Oscillator Experiment

We have performed a backward-wave oscillator (BWO) experiment using a slow-wave structure which consists of a non-sinusoidal corrugated-wall waveguide. The period of the structure is z
₀
= 1.67 cm, with r
_min
= 1.17 cm, r
_max
= 1.97 cm, and length L = 15.03 cm (nine periods). We injected an annular electron beam with the following parameters: 0.8 MV ≤ V<>beam ≤ 1.2 MV, 1 kA ≤ I
_beam
≤ 7 kA, r
_beam
= 0.7, 0.8, or 0.9 cm, and T
_pulse
≈ 50 ns. The guiding axial magnetic field is varied from 0.6 T to 3.0 T. The BWO operates at f ≈ 8.6 GHz in the lowest-order TM mode of the coupled beam-structure system. We used various diagnostics to assess the performance of this microwave source. These included both waveguide couplers and electric field probes connected to diode detectors which measured the inband and wideband signals. In addition, we recorded a direct measurement of the microwave signal using a high-bandwidth oscilloscope. This diagnostic was used to determine the operating frequency of the device and as an additional measurement of the microwave power. The maximum output power observed was approximately 200 MW.