Expansion of a Two-Electron-Population Plasma into Vacuum

The expansion of a two-electron-population plasma into vacuum is investigated in a controlled laboratory experiment. As the plasma expands, the colder electron population lags behind the energetic tail population, and a potential double layer, called a rarefaction shock, develops where the two separate. Upstream of this double layer, both electron populations exist; but downstream, only the tail electrons do. During the expansion, ions are accelerated to energies well above the tail electron energy.