Experimental study of a plasma-filled backward wave oscillator

We present experimental studies of a plasma-filled X-band backward wave oscillator (BWO). Depending on the background gas pressure, microwave frequency upshifts of up to 1 GHz appeared along with an enhancement by a factor of 7 in the total microwave power emission. The bandwidth of the microwave emission increased from ≤0.5 GHz to 2 GHz when the BWO was working at the RF power enhancement pressure region. The RF power enhancement appeared over a much wider pressure range in a high beam current case (10–100 mT for 3 kA) as compared to a lower beam case (80–115 mT for 1.6 kA). The plasma-filled BWO has higher power output compared to the vacuum BWO over a broader region of magnetic guide field strength. Trivelpiece-Gould modes (T-G modes) are observed with frequencies up to the background plasma frequency in a plasma-filled BWO. Mode competition between the Trivelpiece-Gould modes and the X-band TM01 mode prevailed when the background plasma density was below 6×10¹¹ cm⁻³. At a critical background plasma density of ncr≅8×10¹¹ cm⁻³ power enhancement appeared in both X-band and the T-G modes, with mode collaboration. Power enhancement of the S-band in this mode collaboration region reached up to 8 dB. Electric fields measured by Stark-effect method were as high as 34 kV/cm while BWO power level was 80 MW. These electric fields lasted throughout the high power microwave pulse.