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

在380 ~ 720 nm波长范围内，对4.2 K以下的液氦（He）高压脉冲放电以及4.2 K附近、1 atm的低温氦气高压脉冲放电进行了光谱学研究。结果表明，由于低温环境中存在较厚的瞬态等离子体而引起的斯塔克效应，众所周知的中性He原子谱线被极大地加宽。脉冲放电后的等离子体密度高达5 × 10 E18/cc。最高电子温度被测量为35,000 K，观察不同谱线周围的线与连续谱强度比。在相同的实验条件下，放电温度比室温下的气体放电温度低几倍。超流氦包围的等离子体在放电后很长时间内都是低温等离子体。根据所得到的等离子体参数，库仑耦合系数约为0.1，是室温下常规气体放电中库仑耦合系数的5倍。在430 nm和395 nm附近，除了中性的He原子谱线外，还观察到两组强的无展宽谱线。前者是由双原子He分子的旋转振动跃迁引起的，而后者在以前的文献中没有报道，其来源目前还不清楚。提出了一个低温环境下的分层等离子体模型来解释实验结果，强调了目前研究的低温等离子体涉及到各种有待于进一步研究的未知现象。

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.