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

Document Type

Article

Publication Date

1-1-1994

Abstract

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 Πu2X g+2 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 μs, 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. © 1994 The American Physical Society.

Publication Title

Physical Review E

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