Experiments on high-power microwave transmission through a belljar

The ability to radiate the microwave pulse generated by a high-power microwave source can be limited by the interface from the vacuum to the ambient atmosphere. We report here on experiments that use geometric changes to control the tangential electric field amplitude applied to the vacuum side of the belljar. These geometric changes have resulted in observed changes in the base vacuum levels obtained with various sized belljars. In addition, the presence of a diffuse discharge on the vacuum surface of the belljar did not alter the magnitude or mode pattern of the measured microwave pulse. This discharge appears to have a resonant nature because it only exists only for a limited range of electric field values due to a limited range of total microwave power generated.