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

1998年和1999年，对浸没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间和空间分辨测量。观察到中性氦原子在等离子体中发射的587.6nm谱线的Stark展宽，从而推断出局域等离子体密度。在电子显微镜束源中使用尖端曲率为50微米的钨电极，我们发现脉冲放电的重复性比以前有很大的改善。电极间距为0.3mm，并且向电极施加20 kV、电流50 A和时间宽度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个光学台，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。当光源在狭缝上形成焦像时，谱线光强最大。让我们假设三维笛卡尔坐标; x轴朝向光源的狭缝，z轴朝向光斑。在y方向上观察到等离子体密度的最敏感变化。在z方向上，另一方面，密度的半值是为0.8 mm的位移获得的。等离子体被估计为一个形状的一个扁平的椭圆体的旋转轴相同的电极柄。最后，我们将阐明在世界上首次观测到的液氦中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.