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

1998年和1999年，我们用时间分辨测量和空间分辨测量的方法，观测了超流液氦中针状电极间脉冲放电产生的余辉等离子体密度。观察到中性氦原子在等离子体包围下发出的587.6 nm谱线的斯塔克展宽，从而推算出局域等离子体密度。采用尖端曲率为50微米的钨电极作为电子显微镜的束源，我们发现脉冲放电的重复性比以前有了很大的提高。电极间距为0.3 mm，施加20kV、电流50A、时间宽度1微秒的高压脉冲。为了实现空间分辨率，我们购买并准备了一台光学工作台，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的准直透镜。当光源的焦点像在狭缝上产生时，获得了谱线的最大光强。让我们假设三维笛卡尔坐标；x轴朝向光源的狭缝，z轴向上光斑。在y方向观察到的等离子体密度变化最为敏感。当置换为0.2 mm时，密度将更改为半值。另一方面，在z方向上，对于0.8 mm的位移，得到密度的半值。等离子体形状估计为扁平椭球体，其旋转轴与电极柄相同。总之，我们将阐明液氦中局域低温等离子体的空间结构，这是世界上首次观测到的。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.