Temporal electron collisions and adjunct ionization mechanisms in the positive column of a flowing N2 DC discharge

Abstract

We studied a flowing nitrogen DC discharge by Langmuir double probes and optical emission spectroscopy (OES). We measured the reduced electric field (E/N) and electron density (ne) with the double probes. The gas (Tg) and N2(X1Σ+g) vibrational temperature (θ) were measured by OES from the nitrogen first and second positive systems transitions. We constructed a kinetic numerical model accounting for N2(X1Σ+g, 0 ≤ v ≤ 45), N2(A3Σ+u), N2(B3Πg), N2(a1Πg), N2(a′1Σ−u), N2(w1Δu), N2(a″1Σ+g), N2(W3Δu), N2(E3Σ+g), N2(C3Πu), N2+(X2Σ+g), N2+(B2Σ+u), N+, N3+, N4+, N(4S), N(2D), and N(2P) states. The species densities are calculated as functions of the discharge gas residence time in pure nitrogen for our experimental conditions. The kinetic model assumes as input parameters the experimental discharge gas pressure (p), tube radius (R), electron density (ne), gas temperature (Tg), reduced electric field (E/N). In this Letter, the kinetic model is applied to calculations of the fractional contribution of each ionization mechanism to the total ionization generated in the discharge. We compute 24 different ionization mechanisms to the five ions species. We have observed that ionization is generated by different physical-chemical reactions depending on the discharge gas residence time. It changes from dominant electron impact ionization process in the earlier times of the discharge to adjunct ionization mechanisms depending on the pooling of N2(a′1Σ−u) and N2(A3Σ+u) metastable molecules and associative ionization of N2(a′1Σ−u), N(2D) and N(2P) metastable atoms and molecules. Moreover, we have observed the occurrence of approximate charge neutrality for our studied experimental condition: discharge current of 30 mA, gas flow rate of 1 Slm−1, and gas pressure of 230 Pa. Also, the kinetic model fits quite well the measured N2(X1Σ+g) vibrational temperature at the end of the positive column.