Effect of plasma boundary and electrode asymmetry in planar DC discharge system

Abstract

This paper present presents a detailed characterization and analysis of plasma formation using different anode sizes in two contrasting configurations in a planar DC discharge system. One configuration has a conducting boundary (CB) formed by the conducting wall of the vacuum chamber that acts as an extended cathode. The second configuration, the Small Volume Insulated Boundary (SVIB) with a volume 22.5 times smaller than the CB system, is realized by confining the plasma completely within a fully insulating boundary. Anode sizes may be equal to the cathode size (symmetric electrodes) or smaller (asymmetric electrodes). In general, CB discharges require much lower applied voltages, showing very little variation with the pressure. Although the symmetric CB discharges have only single electron population, the asymmetric electrode discharges exhibit two electron populations, a high-density bulk population (Te ∼ 2–3 eV) and a very low-density warm population (Tw ∼ 40 eV) that serves to enhance ionization and compensate for reduced anode size. In contrast, the SVIB discharges require high voltages, show considerable variation in discharge voltage both with pressure and anode size, and have higher densities. In addition, one finds two electron populations for all anode sizes. From estimates of the anode sheath drop, it is possible to show that all CB discharges have an electron-rich anode sheath for all anode sizes. In contrast, the SVIB discharges exhibit ion-rich anode sheaths for all anode sizes, although for small-sized anodes and high pressures the sheaths transform to an electron-rich sheath.