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

研究了磁约束对脉冲放电余辉等离子体密度衰减的影响。脉冲放电产生余辉等离子体的针电极嵌入在磁场最大为2 T的超导线圈中，并浸泡在超流液氦中。采用20 kV、电流50 A、时宽1.0μs的高电压脉冲，在LHe的镜面或尖头磁场中烧制一对针状钨电极。在等离子体中性氦原子处观察到587.6 nm谱线的Stark增宽，用于测量局部等离子体密度。放电后时间t=0.5μs，等离子体密度n=10^<18>cm^<-3>，电子温度Te=32,000 K。在放电后的一段时间内，直到t=3.0μs，由等离子体和磁压之比定义的估计β值从0.28变化到0.014。假定外加磁场的影响是明显的。令人惊讶的是，在t=3.0μs之前，镜面和尖头型的密度衰减在有或没有磁场的情况下都没有变化。我们观察总光强直到t=18 ms以检测差异。没有检测到有磁场和没有磁场的差异。降低LHe的压力和温度不会产生差异。

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.