Structure of localized plasma produced by pulsed discharges in pressurized superfluid liquid helium

Summary form only given. At a temperature below 2.17 K, liquid helium (LHe) at 1 atm includes a superfluid component (Bose-Einstein condensate) that has no viscosity against impurity ions, namely, electron bubbles and He cluster ions. To produce a distributed ion mixture, i.e., transient cryogenic plasma with rare collisions between ions we apply high voltage pulsed discharges in superfluid LHe (LHeII). LHe has weak affinity to be a solvent of impurity materials, and our current effort is concentrated on how to implant a large amount of ions into LHe. Our preliminary measurement by time-resolved spectroscopy revealed the presence of high-density plasmas just after pulsed discharges (20 kV, 50 A, I /spl mu/s) between a pair of needle electrodes with small separation in LHeII. The measurements of Stark broadening and intensity ratios of spectral lines of neutral He atoms revealed spatial structure of the localized afterglow plasma in a large gaseous He bubble surrounded by LHe. The plasma is characterized by density above 1018 cm/sup -3/ and electron temperature of a few eV. Stark broadening of line 587.6 nm indicates the shape of the plasma to be a flat ellipsoid. The half widths of the ellipsoid are 0.6 and 1.2 mm at the time 0.4 /spl mu/s after the discharge. The spatial structure of the localized afterglow plasma is estimated to have a central part, where rapidly decreasing high-density plasma in gas He is formed, and a peripheral region surrounded by cool LHe, where a large amount of clusters survive for longer time than the core plasma. It is emphasized that the cryogenic plasma in the present study involves various unknown phenomena to be studied in future such as cluster formation, multi-electron bubbles, strongly coupled plasmas in LHeII.