PROPOSAL OF CRYOGENIC PLASMAS IN LIQUID-HELIUM-II

Novel possibilities to create cryogenic plasma states in liquid helium II below 2.15 K at 1 atm are proposed. Below 0.4 K, a mixture of posi tive and negative ions in the superfluid can be collision-free in the sense that the momentum relaxation collision frequencies between ions and neutral atoms, impurity atoms and excitons such as phonons, rotons and vortices are less than the plasma frequencies. In particular, pos itive ions in the plasmas are analogous to free electrons in conventio nal gaseous discharges at low pressures, although the plasma parameter s are quite different from each other. A possible cryogenic plasma is proposed to be produced as a transient state. Preliminary experiments for ion mixture left after pulsed discharges in liquid helium II are d emonstrated.