Spatial variations in the charge density of argon discharges in the Gaseous Electronics Conference reference reactor

The spatially dependent ion and electron concentrations of argon discharges in the gaseous electronics conference reference reactor reach a saddle point maximum in the center of the glow. The concentrations were measured using a Langmuir probe. The charge density decreases in the axial direction from the glow center to the electrode sheath edges as expected. In contrast, the charge density increases in the radial direction from the glow center to the radial electrode edge. The maximum occurs approximately 1 cm inside of the actual electrode edge. The most plausible explanation for this increase is enhanced ionization at the electrode edges due to field enhancement there. The electric field is enhanced by the close proximity of the ground shield and driven electrode at the radial edge of the electrode. We estimate that the ionization rate in the plasma near the electrode edge must be almost double of that in the glow center.