Plasma boundary induced electron-to-ion sheath transition in planar DC discharge

This paper reports parallel plate DC glow discharge plasma experiments showing significant changes in the nature of the anode sheath when the plasma boundary is changed from a large, grounded, conducting boundary (CB; the vacuum vessel) acting as an extended cathode to the one wherein the plasma is encapsulated completely by an insulating boundary (IB). The most distinctive difference observed between the two configurations is a conventional electron sheath formation at the anode for the CB case, whereas an “unconventional ion sheath” is formed for the IB case. These observations are deduced from the fact that for the former case, anode potential is “higher” than plasma potential, while for the latter case, the opposite is true. Coupled with this is the observation of two electron populations for the IB case: (i) a high density, low temperature (bulk) population and (ii) a very low density warm population (density ∼ 1% of the bulk density and temperature ∼ 45 eV). The role of the latter is to afford higher ionization levels to compensate for the limited cathode area available for maintaining the higher densities. In comparison, for the CB case, the cathode area is unrestricted and a single temperature population suffices. Initially, IB experiments were conducted in a glass tube confining the plasma between the anode and the cathode. However, to ensure that the observations are not simply a volumetric effect, another set of experiments was undertaken with the “entire chamber wall and other conducting parts” insulated with mica sheets/glass tubes, etc. The two IB cases yielded identical results.