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

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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.