Effect of Gas Flow on the Microwave Dielectric Breakdown of Oxygen

Pulsed microwave power is used to produce breakdown in a constricted 3-cm waveguide containing oxygen gas in the pressure range from 15 to 25 mm Hg. The breakdown characteristics are studied as a function of the flow velocity of the gas. The power necessary to maintain the discharge after it has been initiated increases with flow velocity up to a critical velocity because the electron density at the constriction is reduced by the flow. Beyond the critical velocity the necessary power decreases with flow velocity. This latter phenomenon is believed to be due to ``back flow'' of the gas.