Multi-time electron density imaging over arc discharges around the current zero point

Framing Shack–Hartmann type laser wavefront sensors were developed for obtaining time evolutions of two-dimensional electron density images over single SF6 and air arc discharges with decaying arc currents. An SF6 arc discharge showed initial drastic shrinking of the diameter of the filamentary arc column and subsequent rapid electron density reduction from 1023m−3 to less than 1021m−3 around the point of arc extinction. On the other hand, an atmospheric arc discharge around the current zero had almost constant electron densities of 1021–22 m−3 regardless of the presence or absence of the arc current. A current-basis study showed that the observed constant electron densities could be residual ones surrounding the arc column with electron densities of 1020 m−3 and contributing less to the arc current. (paper)