Time-Resolved Spectroscopic Measurements of Plasmas after Pulsed Discharges in Liquid Helium

The densities and electron temperatures in transient plasmas produced by high-voltage pulsed discharges in liquid helium below 4.2 K, at 1 atm, and cryogenic helium gas near 4.2 K, at 1 atm, have been measured by time-resolved emission spectroscopy up to 3.8 µs after the discharges. The densities are measured using Stark broadening of line profiles. The electron temperatures are deduced from the ratios of total spectral line intensities to continuum intensities. Gaseous plasmas with relatively high density and low electron temperature are obtained in cryogenic environments. Typical density and electron temperature at 0.2 µs after breakdown are 1018 cm-3 and 35,000 K, respectively. Mechanisms of the dominant spectral line broadening at an early stage after the discharges in liquid helium are discussed in detail.