Cathode fall characteristics in a dc atmospheric pressure glow discharge

Abstract

Atmospheric pressure glow discharges are attractive for a wide range of material-processing applications largely due to their operation flexibility afforded by removal of the vacuum system. These relatively new atmospheric plasmas are nonequilibrium plasmas with gas temperature around 100°C and electron temperature in the 1-10 eV range. Their appearance is characteristically diffuse and uniform, and their temporal features are repetitive and stable. Of the reported numerical studies of atmospheric glow discharges, most are based on the hydrodynamic approximation in which electrons are assumed to be in equilibrium with the local electric field. Spectroscopic and electrical measurements suggest however that the cathode fall region is fundamentally nonequilibrium. To this end we consider a hybrid model that treats the cathode fall region kinetically but retains a hydrodynamic description for the region between the thin cathode fall layer and the anode. Using this hybrid model, a helium discharge system excited at dc is studied numerically for a very wide current density range that spans from Townsend dark discharge, through normal glow discharge, to abnormal glow discharge. Numerical results confirm many distinct characteristics of glow discharges and compare well with that of low-pressure glow discharges. Generic relationships, such as that between the electric field and the current density, are also established and are in good agreement with experimental data. This hybrid model is simple and insightful as a theoretical tool for atmospheric pressure glow discharges.

Three modes in a radio frequency atmospheric pressure glow discharge

“…Similar to moderate-pressure glow discharges, the electron density of APGD is approximately proportional to the current density in a linear fashion 23 and Fig. 8 shows their relationship for helium according to the cathode fall theory of dc APGD.…”

“…8 shows their relationship for helium according to the cathode fall theory of dc APGD. 23 For n e ϭ2.06ϫ10 13 cm Ϫ3 , the corresponding current density is 2.3 A/cm 2 . From our earlier discussion in Sec.…”

“…It is known that electron density often peaks outside the electrode sheaths 21,23 and so n es is a fraction of n c . According to a recent theory for dc APGD, 23 typically f e ϭn es /n e ϭ0.25-1 depending on sheath thickness. As a rough illustration of the relevant order of magnitude, we assume f e ϭ0.5.…”

“…Given that the sheath breakdown is predominately electrostatic in nature, the cathode fall theory recently developed for dc APGD 23 can be used to relate the electron density to the current density. Similar to moderate-pressure glow discharges, the electron density of APGD is approximately proportional to the current density in a linear fashion 23 and Fig.…”

“…Although kinetic effects in electrode sheaths may be important, 21,23 we employ the hydrodynamic model as an approximation and assume that the first Townsend ionization coefficient can be treated as a function of the local field only. 5,7 For helium this is given by 24…”