Experimental study and modelling of a low-pressure N 2 -O 2 time afterglow

Eur. Phys. J. Appl. Phys.

2004

215

251

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.

Section: Electron Kineticsmentioning

confidence: 82%

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

Guerra¹,

Sá

Eur. Phys. J. Appl. Phys.

2004

215

251

“…Since we have electrons in the post-discharge, the question arises on whether or not they can be involved in electron-impact excitation processes. The existence of electron processes influencing the heavy-particle kinetics was first pointed out in [36] and subsequently in [13,37]. The theoretical analysis presented in [18] has demonstrated that although the high energy tail of the EEDF is rapidly depleted, the rate coefficients for some low-energy threshold processes, such as the stepwise excitation of N 2 (B 3 g ) and N 2 (C 3 u ) states from N 2 (A 3 + u ), are much slower modified.…”

Section: Resultsmentioning

confidence: 99%

Electron and metastable kinetics in the nitrogen afterglow

Guerra¹,

Sá

Plasma Sources Sci. Technol.

2003

This work presents a theoretical study of the time-relaxation of both the electron energy distribution function and the populations of the different species of interest in the nitrogen afterglow of a ω/2π = 433 MHz discharge at p = 3.3 Torr, in a tube with radius R = 1.9 cm. It is shown that collisions of highly excited N 2 (X 1 + g , v 35) molecules with N(4 S) atoms may be in the origin of the observed pronounced maxima for the concentrations of various species, including electrons, occurring downstream from the discharge after a dark zone. Slow electrons remain for very long times in the post-discharge (at least up to t ∼ 10 −3-10 −2 s), and the strong coupling between the electron and metastable kinetics is clearly pointed out.

“…This is a consequence of the large characteristic times for ambipolar diffusion, as calculated in [23] and measured experimentally in the breakdown time delay experiment reported in [22] and in a pulsed RF discharge in [9], so that the frequency for electron losses by diffusion is relatively small. The presence of a significant number of slow electrons for a long time makes it possible that electron stepwise excitation processes, typically with energies thresholds up to 2-3 eV, are effective under postdischarge conditions, as it was first suggested in [46] and [47] and calculated theoretically in [23]. However, these stepwise collisions cannot produce additional ionization in the postdischarge.…”

Section: B Afterglow Times Smentioning

confidence: 99%

Time-dependence of the electron energy distribution function in the nitrogen afterglow

Guerra¹,

Dias²,

IEEE Trans. Plasma Sci.

et al. 2003

In this paper, we present an investigation of the timerelaxation of the electron energy distribution function (EEDF) in the nitrogen afterglow of an 2 = 433 MHz flowing discharge at = 3 3 torr, in a tube with inner radius = 1 9 cm. We solve the time-dependent Boltzmann equation, including the term for creation of new electrons in associative/Penning reactions, coupled to a system of rate balance equations for the heavy-particles. The EEDFs are also obtained experimentally, from second derivatives of digitized probe characteristics measured using a triple probe technique, and compared with the calculations. It is shown that an equilibrium between the vibrational distribution function of ground-state molecules N 2 (1 6 +) and low-energy electrons is rapidly established, in times 10 7 s. In these early instants of the postdischarge, a dip is formed in the EEDF around 4 eV. The EEDF finally reaches a quasi-stationary state for 10 6 s, although the electron density still continues to decrease beyond this instant. Collisions of highly excited N 2 (1 6 + 35) molecules with N(4) atoms are in the origin of a maximum in the electron density occurring downstream from the discharge at 2 10 2 s. These reactions create locally the metastable states N 2 (3 6 +) and N 2 (1 6), which in turn ionize the gas in associative/Penning processes. Slow electrons remain for very long times in the postdischarge and can be involved in electron stepwise processes with energy thresholds smaller than 2-3 eV.