Optical observation of localized plasma after pulsed discharges in liquid helium

A transient hot localized high-density plasma in liquid helium (LHe) after a pulsed discharge with a high voltage (20 kV, 180-360 A, 0.4-7 μs) between tungsten (W) needle electrodes has been studied. Spatial and temporal changes of the plasma density in helium gas surrounded by LHe are measured by observing Stark broadening of 587.6-nm HeI spectral line. The localized plasma is found to have density above 10¹⁶ cm^-3 with an elongated ellipsoid in shape along the common axis of the needle electrodes. At 0.4 μs after the end of discharge, the half widths of the density distribution along and transverse to the axis are typically 0.6 and 1.4 mm, respectively. We have observed strong spectral lines of magnesium (Mg) atoms without Stark broadening, when one of the electrodes is replaced by an Mg rod. After a discharge is over, Mg atoms are confirmed to be incorporated into LHe.