Abnormal glow discharges in Ar: experiments and models

We review measured and calculated voltages and cathode-fall thicknesses aga inst current and electrode separation for parallel-plane Ar glow discharges in the above-normal, or abnormal, current range. We consider dc and pulsed discharges, but not radio-frequency discharges. Reasons for the very high degree of variability among experiments and models are reviewed and suggest ions are made for improving the reliability of both. Variations of a factor of two in experimental cathode-fall voltages at a fixed normalized current density are attributed to differences in the cathode emission properties, varying electrode separation, and effects of discharge tube walls. We explo re a wide range of normalized current densities, models of the ionization c oefficient against E / n, and the effective electron yields per positive io n reaching the cathode using simple scaling laws. Ionization source terms f or local-field and non-local-field models are compared and used as the basi s for a simplified non-local model. We discuss the present author's recomme ndation that, because of current large uncertainties in the electron yield per ion for practical cathodes, the yield used in cathode-fall models shoul d be treated as an unknown. Recent models successfully predict the behaviou r of transient abnormal discharges on the microsecond time scale.