Measurements of Cryogenic Plasma in Pressurized Superfluid Liquid Helium Using Pulsed Discharge Method

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High voltage pulsed discharges in pressurized liquid helium (He) below 4.2 K,as well as cryogenic helium gas near 4.2 K,1 atm, are spectroscopically studied in a wavelength region from 380 nm to 720 nm. The results demonstrate that well-known neutral He atomic lines are extremely broadened due to Stark effects caused by the presence of thick transient plasmas in cryogenic environment. The plasma density just after the pulsed discharges attains to be as high as 5X10E18/cc. The maximum electron temperature is measured to be 35,000 K observing line to continuum intensity ratios around various spectral lines. The temperature is several times lower than the gaseous discharge cases at room temperature under identical experimental condition. The plasmas surrounded by superfluid He are concluded to be cryogenic plasmas long after the discharges. According to the plasma parameters obtained, the Coulomb coupling coefficients are around 0.1 that is 5 times larger than the values in conventional gaseous discharges in room temperature. This fact is clarified for the first time in the present reseaech.In addition to the neutral He atomic lines, two series of strong spectral lines without broadening are observed near 430 nm and 395 nm. The former is identified to be from rotaional-vibratinal transitions of diatomic He molecules, whereas the latter was not reported in previous literatures and the origins of the emissions are not understood at this moment. A stratified plasma model in cryogenic environment is proposed to explain the experimental results.It is emphasized that the cryogenic plasma in the present study involves various unknown phenomena to be studied in future.