Electric field strengths, ion energy distributions, and ion density decay for low-pressure, moderate-current nitrogen discharges

Steady-state and time-dependent electric fields, ion densities, and ion velocity distributions are measured in the positive column of pulsed discharges in N₂ for conditions of moderate energy input per molecule, i.e., 0.1 eV/molecule. A nonintrusive laser absorption technique is used to determine axial velocity distributions of N₂⁺ ions and the time dependence of the N₂⁺ density from the shape and magnitude of absorption profiles of lines of the A ²Πᵤ{←}X ²Σg⁺ Meinel band of N₂⁺. The discharges were operated at pressures from 0.3 to 1 Torr, electric field to gas density ratios E/n from 130 to 75 Td, pulse lengths of 10 to 20 {μ}s, peak currents of about 1.5 A, and ion densities of 5×{}10¹⁷ ions/m³. Here 1 Td=10^-21V m² and 1 Torr=133 Pa. The line profiles were found to be consistent with the convolution of a theoretical, high field velocity distribution appropriate to charge transfer collisions and an above-thermal Maxwellian distribution. The electric field strength to gas density ratios E/n determined from the ion drift velocities are significantly lower than the predictions of the theory for a cold gas and suggest high ionization rates resulting from vibrationally excited N₂. The N₂⁺ density decay following the pulsed discharge is consistent with ion loss by electron-ion recombination and ambipolar diffusion for electrons heated by the vibrationally excited N₂ at a calculated vibrational temperature of about 1900 K. The discharge conditions are briefly discussed so as to indicate the dominant processes and data needed for detailed models.