Dielectric/gas interface breakdown caused by high power microwaves

Physical mechanisms leading to microwave breakdown on dielectric/gas interfaces are investigated for power density on the order of 10 MW/cm² at 2.85 GHz and gas pressures on the order of 10⁻⁴ torr to 10³ torr. The investigation is focused on an alumina/air interface; other gases are considered for reference purposes. A 3 MW magnetron with 3.5 µs pulse width is coupled to an S-band traveling wave resonator with a pressure adjustable test region. The pre-breakdown phase and the breakdown are monitored by recording the traveling and reflected power, and the spatially integrated luminosity. Electric field probes in the vicinity of the interface are included as well to get information about the local field. Furthermore, the light emission was observed with an image intensifier capable of a minimum gate time of 2.5 ns, in temporal correlation to the other phenomena, or with a framing camera having a 20 ns gate time and 100 ns separation between pictures. The pressure dependent breakdown characteristics, such as appearance, breakdown field, and temporal shape of electric signals, are compared to dielectric/vacuum interface breakdown and volume breakdown, all measured utilizing similar setups.