EXPERIMENTAL-STUDY OF A PLASMA-FILLED BACKWARD-WAVE OSCILLATOR

We present experimental studies of a plasma-filled X-band backward wav e oscillator (BWO). Depending on the background gas pressure, microwav e 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 less-than-or-equal-to 0.5 GH z to 2 GHz when the BWO was working at the rf power enhancement pressu re region. The rf power enhancement appeared over a much wider pressur e range in a high beam current case (10-100 mT for 3 kA) as compared t o 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 o f magnetic guide field strength. Trivelpiece-Gould modes (T-G modes) a re observed with frequencies up to the background plasma frequency in a plasma-filled BWO. Mode competition between the Trivelpiece-Gould mo des and the X-band TM01 mode prevailed when the background plasma dens ity was below 6 x 10(11) cm-3. At a critical background plasma density of n(cr) congruent-to 8 x 10(11) cm-3 power enhancement appeared in b oth X-band and the T-G modes. Power enhancement of the S-band in this mode collaboration region reached up to 8 dB. Electric fields measured by the Stark-effect method were as high as 34 kV/cm while the BWO pow er level was 80 MW. These electric fields lasted throughout the high p ower microwave pulse.