Production and Spectroscopic Measurement of Dust Plasma in Superfluid Liquid Helium

对超流液氦（LHe）高压脉冲放电产生的尘埃等离子体进行了光谱测量。首先，我们尝试将直径为0.07μm的银（Ag）细颗粒粉末溶解到LHe中。微细粒子的存在促进了LHe的蒸发，并有粘附在杜瓦瓶壁上的倾向，阻碍了光学观察。接下来，我们尝试了各种金属棒作为电极来启动高压脉冲放电，并将其烧蚀成LHe。到目前为止，我们只成功地将镁（Mg）颗粒从固体电极烧蚀成LHe。钨(W)电极和直径1mm的镁棒沿公共轴线固定在充满超流体LHe的杜瓦瓶底部。将源准直像的光强引入光谱仪，利用多通道像增强器（Hamamatsu, PMA-50）测量尘埃等离子体产生的时间分辨谱线强度。测量了室温下1大气压氦气中电极间光的光谱强度。在518.2nm处的MgI线强度表明，与极性相反的情况相比，镁棒为阴极的情况下强度强15倍，光持续时间更长。然后，电极浸入具有前极性的LHe中。在脉冲放电过程中，在587.6nm处观察到强烈的连续光和强HeI三重线，这意味着高密度等离子体的存在。脉冲放电后20μs，等离子体发出的光因环境LHe的冷却作用而熄灭。另一方面，在氦等离子体结束后35μs， 518.2和552.8nm处的MgI谱线可以被观测到。估计在LHe中产生了烧蚀Mg尘埃粒子的等离子体。对局部尘埃等离子体的大小进行了详细的测量。尘埃颗粒的大小有待于将来的研究。

Spectroscopic measurement of dusty plasma produced by high voltage pulsed discharge in superfluid liquid helium(LHe) is carried out. First, we tried to dissolve powder of silver(Ag) fine particle with diameter 0.07μm into LHe. Evaporation of LHe is enhanced by the fine particles, and they have tendency to adhere the wall of Dewar bottle which prevents optical observation. Next, we tried various metal rods as an electrode to initiate high voltage pulsed discharge that is ablated into LHe. So far, we are successful only to ablate magnesium(Mg) particles into LHe from the solid electrode. Tungsten(W) electrode with and Mg rod with 1mm diameter are fixed along a common axis at the bottom of the Dewar bottle filled with superfluid LHe. The light intensity of collimated image of the source is introduced into optical spectrometer and time-resolved spectral line intensities produced dusty plasma are measured by multi-channel image intensifier(Hamamatsu, PMA-50). Spectral intensities of the light between the electrodes in 1 atmosphere helium gas at room temperature are measured. MgI line intensity at 518.2nm shows that the intensities are fifteen times stronger and light lasts for longer time for the case of that Mg rod is cathode rather than the case that the polarity is opposite. Then, the electrodes are immersed in LHe with the former polarity. During the pulsed discharge, intense continuum lights and strong HeI triplet line at 587.6nm are observed that means the presence of high-density plasma. The light from the plasma is extinguished at 20μs after the pulsed discharge because of cooling effect of ambient LHe. On the other hand, MgI lines at 518.2 and 552.8nm can be observed until 35μs, after the helium plasma ends. Plasma of ablated Mg dusty particles is estimated to be produced in the LHe. The sizes of the localized dusty plasma are measured in some detail. The sizes of the dusty particle are remained to be examined in future.