Populations in the metastable and the resonance levels of argon and stepwise ionization effects in a low-pressure argon positive column

Abstract

The contribution of the ionization from the two metastable and the two resonance levels of argon to the total ionization rate in a low-pressure argon positive column is investigated theoretically and experimentally. A simple yet self-consistent discharge model is developed by coupling the balance between the electron total production and loss rates to the steady-state rate balance equations for the excited states. The predictions of this model, i.e., the maintenance field for the positive column and the populations in the individual excited states as a function of the gas pressure and the discharge current are compared with the experiment (0.05<p<1 Torr; 0.2<I<20 mA; φ=2.6-cm-i.d. discharge tube). It is shown that stepwise ionization becomes predominant for pressures larger than a few tenths of a Torr even for the lowest currents of this experiment. The predicted values of the maintenance field are considerably lower than those that would be required to sustain the discharge by ground-state ionization and compare favorably with the measurements. The calculated populations in the metastable states also compare reasonably well with the experiment. The discrepancies are, however, quite large in the case of the resonance levels owing, presumably, to the lack of accuracy of the theoretical escape factors for the resonance radiation.