Ion kinetics and symmetric charge-transfer collisions in low-current, diffuse (Townsend) discharges in argon and nitrogen

Translational kinetic-energy distributions of mass-selected ions have been measured in diffuse, low-current Townsend-type discharges at high electric field-to-gas density ratios (E/N) in the range of 1×${10}^{\mathrm{-}18}$–2×${10}^{\mathrm{-}17}$ V ${\mathrm{m}}^2$ (1–20 kTd). The discharges were generated in Ar and ${\mathrm{N}}_2$ under uniform-field conditions and ion energies were measured using a cylindrical-mirror energy analyzer coupled to a quadrupole mass spectrometer. The mean ion energies determined from measured energy distributions of ${\mathrm{Ar}}^{+}$ in Ar and ${\mathrm{N}}_2^{+}$ in ${\mathrm{N}}_2$ are compared with the mean energies predicted from solutions of the Boltzmann transport equation based on the assumption that symmetric resonant charge transfer is the predominant ion-neutral interaction. The results for ${\mathrm{Ar}}^{+}$ and ${\mathrm{N}}_2^{+}$ are consistent with predictions made using a constant (energy independent) cross section for which an effective ion temperature can be defined. However, for both ions, the measured mean energies tend to fall increasingly below the predicted values as E/N increases. The possible causes and significance of the differences between the measured and calculated mean ion energies are examined by considering collisions other than charge transfer that can affect ion energies as well as uncertainties in the charge-transfer cross sections used in the calculations. Measurements were also made of the relative contributions from ${\mathrm{N}}^{+}$ and ${\mathrm{Ar}}^{++}$ to the ion flux. Over the E/N range of interest, ${\mathrm{N}}^{+}$ accounts for less than 15% of the ion flux in nitrogen and ${\mathrm{Ar}}^{++}$ accounts for less than 5% of the ion flux in argon.