Self-consistent kinetic model of low-pressure N2-H2 flowing discharges:: I.: Volume processes

This work is the first of two companion papers devoted to the kinetic modelling of low-pressure DC flowing discharges in N-2-H-2 mixtures. While the present paper is mainly concerned with bulk discharge processes, the second one investigates surface processes involving dissociated N and H atoms, which are essential to understand the discharge properties. The global model combining bulk and surface processes as described in these two papers is self-contained in the sense that the sole input parameters it requires are those that can externally be chosen in experiments, namely: total gas pressure, radius and length of the discharge tube, discharge current, gas flow rate and initial gas temperature and composition (e.g., the relative hydrogen concentration X in the binary mixture (1 - X)N-2 + XH2 at the discharge inlet). For a given set of input parameters, this model enables one to calculate the following bulk plasma properties as a function of the axial coordinate z: concentration of N-2, H-2, NH, NH2, NH3 molecules and N, H atoms in the ground electronic state; population in the electronically excited states N-2(A (3)Sigma(u)(+), B (3)pi(g), a'Sigma(u)(-), a (1)pi(g), C (3)pi(u), a "Sigma(g)(+)), H-2(R) (an effective high Rydberg state) and N(D-2,P-2); concentration of the ions N-2(+), N-2(+)(B), N-4(+), H-2(+), H-3(+), HN2+ and H-; vibrational level populations of N-2(X(1)Sigma(g)(+)) and H-2(X(1)Sigma(g)(+)) molecules; electron density N-e, mean kinetic energy 3/2 kT(e), characteristic energy u(k) and drift velocity v(d); discharge sustaining electric field E; average gas temperature across the tube T and wall temperature T-w. The calculations are compared with data from different experiments in pure N-2 and H-2 discharges (measurements of electric field as a function of current and pressure) and in N-2-H-2 discharges (measurements of relative changes in the electric field and the N-2(C), N-2(+)(B) concentrations as a function of the H-2 percentage). From the comparison to experiment, rate coefficients for associative ionization upon collisions between two excited N2 molecules and deactivation of N-2(a') and N-2(X, v) by H atoms have been estimated from the model.