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

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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.