Microwave breakdown of air at low pressure with a 5 ns, 35 GHz pulse

Summary form only given. This research was inspired by the need for a microwave source powerful enough to lead to enhancement in the air breakdown volume, when synchronized in time and space with a small region of pre-ionized air created with an intense laser pulse. We used the compact and portable RADAN pulser that drives a backward wave oscillator to produce microwaves for this purpose. The microwave pulse duration and power are 5 ns and 2 MW, respectively. The frequency of operation is 35 GHz. It is well known that air breakdown at atmospheric pressure requires an electric field amplitude of 30 kV/cm. Thus, an output power of 2 MW simply radiated in air does not have sufficient electric field to assist in air breakdown. For this reason, a parabolic dish was used to focus the microwaves into a small spot, thereby enhancing the electric field. The 2-dimensional, fully relativistic and fully electromagnetic code MAGIC was used to simulate the amplitude of the electric field and the axial position of the focal point of the parabolic dish. The parabolic dish used for this purpose was 30 cm across and 4.5 cm deep. It was observed in the simulations that the focal spot was located 15 cm from the dish depth and the radial electric field was 20 kV/cm. Despite this promising value of the electric field, experimental verification was necessary due to the following arguments that 5 ns was too short a pulse and that 35 GHz was too high a frequency for breakdown. In order to confirm the fidelity of the simulation results a low pressure chamber transparent to the 35 GHz was manufactured. The parabolic dish was placed opposite the radiating horn antenna with enough distance between them to allow breakdown to occur within the low pressure chamber. Time integrated photographs were captured for every microwave pulse. A very well defined breakdown was observed at 180 Torr of air. These results together with some theoretical development of the breakdown mechanism will be presented.