Kinetic modeling of low-pressure nitrogen discharges and post-discharges

Abstract

The kinetic modeling of low-pressure (p ∼ 1−10 torr) stationary nitrogen discharges and the corresponding afterglows is reviewed. It is shown that a good description of the overall behavior of nitrogen plasmas requires a deep understanding of the coupling between different kinetics. The central role is played by ground-state vibrationally excited molecules, N2(X 1 Σ + g , v), which have a strong influence on the shape of the electron energy distribution function, on the creation and destruction of electronically excited states, on the gas heating, dissociation and on afterglow emissions. N2(X 1 Σ + g , v) molecules are actually the hinge ensuring a strong link between the various kinetics. The noticeable task done by electronically excited metastable molecules, in particular N2(A 3 Σ + u) and N2(a 1 Σ − u), is also pointed out. Besides contributing to the same phenomena as vibrationally excited molecules, these electronic metastable states play also a categorical role in ionization. Furthermore, vibrationally excited molecules in high v levels are in the origin of the peaks observed in the flowing afterglow for the concentrations of several species, such as N2(A 3 Σ + g), N2(B 3 Πg), N + 2 (B 2 Σ + u) and electrons, which occur downstream from the discharge after a dark zone as a consequence of the V-V up-pumping mechanism.

Modes of longitudinal combined discharge in low pressure nitrogen

“…Second, the flow of positive ions and high energy photons enters the 'cathode' sheath from the plasma region (which knock out secondary electrons hitting the electrode surface), as well as metastable atoms and molecules (which enhance the flow of secondary electrons under deactivation as well as via set ionization in the 'cathode' sheath). As known [25], metastable electron states N 2 (A 3 + u ) and N 2 (a 1 − u ) play an important role in sustaining the dc discharge; meanwhile ionization processes (including associative ionization) occur according to the reactions…”

Relaxation of heavy species and gas temperature in the afterglow of a N2 microwave discharge

“…N( 4 S) + N 2 , which has a rate coefficient given by 10 À13 exp(À510/T g ) cm 3 s À1 [1,11] in our calculations. As a matter of fact, it has been recently pointed out in the study of nanosecond discharges in nitrogen [17] that the recent theoretical calculations dedicated to the estimation of the rate coefficient for this reaction [18,19] propose a value given by 4.52 Â 10 À14 T 0:678 g exp(À1437.7/T g ) cm 3 s À1 , which leads to larger values for this rate coefficient.…”

“…For this time range, the gas heating mechanisms come no longer from the energy stored on the vibrational mode, but to a small (and therefore not shown in Fig. 2) contribution from wall processes N 2 (X, v) + wall and N( 4 S) + wall (see [2,10,11] for more details concerning the description of these mechanisms as gas heating channels) with heating rates of about 900 and 80 K/s, respectively, for an afterglow time of 50 ms, decreasing afterwards till the end of the afterglow. It is interesting to note that if all gas heating terms are removed from the gas thermal balance equation in the afterglow, remaining only the heat conduction term to the wall, the value of the gas temperature is about 500 K for an afterglow time of 1 ms and almost 300 K for afterglow times longer than 10 ms.…”

“…This equation determines the radially averaged gas temperature T g assuming a parabolic radial profile for the gas temperature and considering further that heat conduction is the main cooling mechanism. Furthermore, this equation must account for all available energy to gas heating (translational mode) from volume and wall processes, described with detail in [1,2,[9][10][11][12].…”

“…To shorten this paper to an appropriate length, the reader should refer to references [4][5][6][7]9,11] for more details on collisional data, modelling aspects concerning the electron Boltzmann equation, microwave discharge and post-discharge conditions, to references [1,2,10,11] for additional information on gas heating mechanisms, gas thermal balance equation and energy data, as well as to [1,[13][14][15] for the values of the molar heat capacity and thermal conductivity under non-equilibrium conditions.…”