Caltech Water-Ice Dusty Plasma Experiment Upgrade and Analysis of Creation of Hot Electron Tail and its Effect on Dust Charging

The Caltech water-ice dusty plasma experiment has water vapor injected into a capacitively coupled RF plasma with LN2-cooled electrodes. Water ice grains form spontaneously in the weakly-ionized plasma with extremely cold background gas. The experiment is undergoing a significant upgrade wherein the RF electrodes will be cooled by a liquid helium cryocooler to enable lower and more controlled temperatures. Status of this upgrade will be reported.

The electron energy distribution in a weakly-ionized dusty plasma depends on a competition between electron creation by ionization, electron cooling by collisions with neutrals, and electron loss via recombination.  Free electrons created by ionization have energy of a few eV but then cool down to a small fraction of an eV via collisions with the cold neutrals. There is thus a steady-state hot “tail” of energetic just-created free electrons coexisting with a much larger population of cold electrons. Detailed computations of the collision cascade show that the hot tail can be so large as to substantially increase the dust grain charge.