Spatial-filter-installed Shack-Hartmann sensor for two-dimensional electron density visualization of SF6 arc discharge under strong turbulent flow

By using the conventional Shack-Hartmann sensors, it has been impossible to measure the two-dimensional electron density distributions over the -gas-blast decaying arcs exposed to the strong turbulent flow with high spatial frequency variations in the gas density. In order to remove the high spatial frequency components, spatial filters were implemented into the Shack-Hartmann sensors. The novel sensing system was successfully applied to the decaying arc plasmas under current-zero phases generated in a 50 mm-long interelectrode gap confined by a gas flow nozzle. Our experimental results showed that the decaying arcs had large shot-to-shot variations in the electron densities and arc diameters, and these parameters were not always smallest in the upstream nozzle region with the highest blasting gas speed. Such irreproducible arc behaviour was quite different from the previously reported air and arc plasmas and it was predominantly caused by the spatiotemporally irreproducible strong turbulent flow.