Control of sub-critical microwave filamentary plasma in dense gases

Summary form only given. In the present paper, we describe successful initiation and guiding of microwave plasma filaments with a low-energy pulsed laser. A 50 kW, 2.45 GHz microwave power with 1 ms pulse length was directed into a waveguide-based resonant cavity filled with room air. The electric field in the cavity was kept below that needed for self-initiated breakdown. When ArF laser beam (15 ns pulse, 50 mJ/pulse) was focused at a location inside the waveguide, the streamer started and continued to grow long after the laser pulse, reached the waveguide wall and persisted to the end of the microwave pulse. The streamer followed the track of the laser beam rather than the electric field. The paper describes the observed dynamics of the streamer and the theoretical interpretation in detail. The paper also describes the development of a laboratory-scale supersonic wind tunnel with microwave plasma. In the tunnel, a 2D dielectric nozzle is mounted inside the microwave waveguide, so that the flow direction coincides with that of the microwave propagation.