Spectroscopic study of plasma evolution in runaway nanosecond atmospheric-pressure He discharges.

Time- and space-resolved visible-emission spectroscopy measurements are applied to study plasma parameters in nanosecond electrical discharges in He gas at pressure of 10(5) Pa, using a 150 kV, 5 ns duration high-voltage pulse. The plasma evolution during the discharge is investigated by applying line-shape analysis of several He I spectral transitions, with the Stark and opacity effects accounted for. The analysis shows that the discharge plasma is not in equilibrium and that significant electric fields of several kV/cm are present in the plasma during the discharge. Regions of plasma with significantly different electron densities are identified and a qualitative model of the plasma formation and evolution is proposed.