MEASUREMENTS OF HIGH-DENSITY PLASMAS AFTER PULSED DISCHARGES IN LIQUID-HELIUM

High density plasmas created by high-voltage pulsed discharges in liqu id helium below 4.2 K, 1 atm, and cryogenic helium gas near 4.2 K, 1 a rm, have been investigated by spectroscopic measurements in the visibl e spectral range. The well known atomic lines exhibit strong and broad ened profiles with underlying continua at early stages, Several experi mental findings indicate that this broadening is dominated by Stark br oadening 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 10(18) cm(-3) at 0.2 mu s after the breakdown. The el ectron temperatures are measured from the ratios of total spectral lin e intensities to continuum intensities. It turns out to be about 35,00 0 K at 0.2 mu s. The measured plasma parameters suggest that the obser ved plasmas are moderately strongly coupled plasmas with Coulomb coupl ing coefficients around 0.1. In addition, some strong spectral feature s are observed near 430 nm, which can be assigned to He-2 transition ( g(3) Sigma(g)(+)-a(3) Sigma(u)(+)). The reaction mechanisms are discus sed in this paper.