Magnetic Confinement of Cryogenic Plasma Produced by Pulsed Discharges in Superfluid Liquid Helium

研究了磁约束对超流液氦（LHe）中最大磁场为2 T的分裂式超导线圈中针电极间脉冲放电产生的余辉等离子体密度衰减的影响。在LHe中，用20 kV、50 A、时宽1.0μs的高压脉冲在镜面或尖形磁场中点燃一对钨针电极。利用等离子体中性氦原子587.6nm谱线的斯塔克展宽测量等离子体局域密度。在放电后的时间t=0.5μs时，等离子体密度n=10 μ <18>cm-3<-3>，电子温度Te= 32，000 K。在放电后直到t=3.0μs的一段时间内，由等离子体压力与磁压力之比定义的估计β值从0.28变化到0.014。假设外加磁场的影响是明显的。有些令人惊讶的是，对于镜像和尖点型情况，在t=3.0μs之前，在有磁场和没有磁场的情况下都不能检测到密度衰减的变化。我们观察总光强度直到t=18 ms以检测差异。在有磁场和无磁场的情况下未检测到差异。降低LHe的压力和温度不会导致差异。

The effect of magnetic confinement has been investigated on the density decay of afterglow plasma produced by pulsed discharges between needle electrodes embedded in a split type superconducting coils with maximum magnetic field 2 T immersed in superfluid liquid helium (LHe). A pair of needle tungsten electrodes in mirror- or cusp-types magnetic field in LHe are fired by high voltage pulses of 20 kV, current 50 A, and time width 1.0μs. The Stark broadening of spectral line 587.6 nm from neutral helium atoms in plasma is observed to measure local plasma density. At time t=0.5μs after the discharge, the plasma density n=10^<18>cm^<-3>, and electron temperature Te=32,000 K. For a period until t=3.0μs after the dischagges, the estimated beta value defined by the ratio of plasma and magnetic pressures changes from 0.28 to 0.014. Effects of applied magnetic field are assumed to be evident. Somewhat surprisingly, no changes in density decays with and without magnetic field can be detected until t=3.0μs for both mirror- and cusp-type cases. We observe total light intensity until t=18 ms for detecting the difference. No differences with and without the magnetic field are detected. Reducing the pressure and temperature of LHe results in no difference.