Modelling of the low-pressure argon positive column

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Abstract
The properties and operating characteristics of the positive column of a low-pressure positive column in argon have been investigated experimentally and theoretically. In the experiments the pressure was varied from 5×10−2 to 1 Torr, the discharge current from 0.2–20 mA, and the discharge tube was 2.6 cm inside diameter. Measurements of the axial electric field, electron density and population in 3P2 metastable states are reported for various discharge conditions. A test is made of the usefulness of a two-moment theory which is applicable in the intermediate pressure range of this work, i.e., in the transition from inertia-limited to mobility-limited flow of ions to the wall, for the prediction of the operating characteristics. For this purpose, the electron excitation and ionization rates, and transport parameters were calculated from the homogeneous Boltzmann equation. The predicted similarity laws for the positive column are in reasonable agreement with the measurements only in the lower pressure region, i.e., for a pressure-radius product less than about 0.1 Torr cm. The discrepancy between theory and experiment at the higher pressure is, presumably, the result of our failure to correct the calculated ionization rate and effective electron loss rate for the effects caused by ionization from metastables and by space-charge fields.