PLASMA-DENSITY MEASUREMENT IN A GAS-FILLED X-BAND BACKWARD-WAVE OSCILLATOR WITH A DOUBLE CONVERSION HETERODYNE MICROWAVE INTERFEROMETER

Plasma density was measured with a heterodyne microwave interferometer in both a gas-filled X-band backward wave oscillator (BWO) and in a s mooth tube. Plasma is generated by impact ionization of a 650 kV, 2 kA electron beam. For fixed gas pressure we found that the plasma densit y rise in the operating BWO was much faster than in a smooth tube, ind icating that Trivelpiece-Gould modes, or high power microwaves, increa se plasma generation. Additional plasma enhanced BWO microwave output power. Measured plasma density at optimum power levers was n(cr) appro ximate to 6 x 10(12) cm(-3) at onset of emitted microwaves.