Measurement of the effective length of laser-plasma channels in a laser triggered gas switch by guided microwave backscattering in SF6 and N2/SF6

Backscattering of guided microwaves has been utilized to measure the effective electrical length of laser produced plasma channels (LPPC´s) in a laser triggered gas switch configuration. The LPPC was introduced into the waveguide by focusing the UV trigger laser beam through small holes in the broad wall. By translating the laser-focusing lens, the plasma channel could be scanned through the waveguide. A measure of the channel length was obtained by determination of the limits over which scattering is observed. Backscattering channel length measurements were compared with three other measurements (D-dot probe, visible imaging, Schlieren imaging). It was found that visible and Schlieren imaging significantly underestimate the plasma length. Additionally, the plasma length has been studied as a function of lens focal length, laser energy, and neutral gas fill pressure in both pure SF₆ and an N₂/SF₆ mixture (90%/10%).