Space-Resolved Measurement of Cryogenic Plasma Produced by Pulsed Discharges in Superfluid Liquid Helium

1998年和1999年，对浸在超流液氦中的针电极间脉冲放电产生的余辉等离子体密度进行了时空分辨测量。从被等离子体包围的中性氦原子发出的587.6 nm谱线的Stark增宽被观测到，以推断局部等离子体密度。采用尖端曲率为50微米的钨电极作为电子束源，发现脉冲放电的再现性比以前有了很大的提高。电极间距为0.3 mm，施加电压为20 kV、电流为50 A、时间宽度为1微秒的高压脉冲。为了实现空间分辨率，我们购买并制作了一个光学平台，使我们能够在三维方向上精确地移动0.1 mm分辨率的光谱仪狭缝准直透镜。当在狭缝上形成光源的焦像时，获得光谱线的最大光强。让我们假设三维笛卡尔坐标；x轴指向光源狭缝方向，z轴指向光斑方向。等离子体密度最敏感的变化是在y方向上观察到的。当位移为0.2 mm时，密度变为半值。另一方面，在z方向上，位移为0.8 mm时，密度的一半值。等离子体的形状估计为一个平面椭球，其旋转轴与电极的柄相同。综上所述，我们将厘清世界上首次观测到的液氦局域低温等离子体的空间结构。将继续作出进一步努力，以获得详细资料。

In the term of 1998 and 1999, time and space resolved measurements were carried out to observe the density of afterglow plasma produced by pulsed discharges between needle electrodes immersed in superfluid liquid helium. The Stark broadening of spectral line 587.6 nm emanated from neutral helium atoms surrounded by plasma is observed to deduce local plasma density. Tungsten electrodes with tip curvature 50 micrometers for beam source in electron microscopes are used, and we find that reproducibility of the pulsed discharges is much improved than before. The electrode separation is 0.3 mm, and high voltage pulses of 20 kV, current 50 A and time width 1 micro-sec are applied to the electrodes. In order to accomplish spatial resolution, we bought and prepared an optical bench that enabled us to move the collimating lens for the slit of spectrometer with 0.1 mm resolution precisely in three-dimensional directions. The maximum light intensity of spectral line is obtained, when the focal image of the light source is created on the slit. Let us assume three-dimensional Cartesian coordinates ; x-axis is towards the slit from light source, z-axis is upward the light spot. The most sensitive change in plasma density is observed in y-direction. The density changes to a half value, when the displacement is 0.2 mm. In z-direction, on the other hand, the half value of the density is obtained for the displacement of 0.8 mm. The plasma is estimated to be a shape of an flat ellipsoid with rotational axis identical to the shank of the electrodes. In conclusion, we are going to clarify the spatial structure of the localized cryogenic plasma in liquid helium that is observed first in the world. Further efforts will be continued to attain detailed information.