Shack-Hartmann type laser wavefront sensors were used for the direct
imaging of two-dimensional electron density distributions over SF6
gas-blast arc discharges with damping sinusoidal arc current waveforms in order
to experimentally characterise electron density profiles for the success and
failure of arc interruption in the thermal reignition phase. When electron
densities at the current zero were locally reduced to less than 1021m-3
and the axial distributions were seen to be discontinuous, the arc current
interruption was successful with a probability of 100%. The undetected electron
densities around the tip of the lower electrode were assumed to be 1017～18m-3 from the simultaneous arc
conductance measurement and numerical studies of electrical conductivity. On
the other hand, axially continuous electron densities of more than 1021m-3
bridging the interelectrode gap drastically degraded thermal interruption
performance and caused a failure probability of 86%.