Measurements of High Density Plasmas after Pulsed Discharges in Liquid Helium

Abstract
High density plasmas created by high-voltage pulsed discharges in liquid helium below 4.2 K, 1 atm, andcryogenic helium gas near 4.2 K, 1 atm, have been investigated by spectroscopic measurements in the visible spectral range. The well known atomic lines exhibit strong and broadened profiles with underlying continua at early stages. Several experimental findings indicate that this broadening is dominated by Stark broadening caused by the presence of the transient plasmas. The spectral profiles are therefore used to deduce the plasma densities, which are on the order of 10¹⁸ cm^-3 at 0.2 µs after the breakdown. The electron temperatures are measured from the ratios of total spectral line intensities to continuum intensities. It turns out to be about 35,000 K at 0.2 µs. The measured plasma parameters suggest that the observed plasmas are moderately strongly coupled plasmas with Coulomb coupling coefficients around 0.1. In addition, some strong spectral features are observed near 430 nm, which can be assigned to He₂ transition. The reaction mechanisms are discussed in this paper.