^{1,a)}, Christian Sarra-Bournet

^{1,b)}, Nicolas Naudé

^{1}, Françoise Massines

^{2}and Nicolas Gherardi

^{1,c)}

### Abstract

In this paper, two-photon absorption laser induced fluorescence spectroscopy is used to follow the nitrogen atom density in flowing dielectric barrier discharges fed with pure nitrogen and operating at atmospheric pressure. Two different dielectric barrier discharge regimes are investigated: the Townsend regime, which is homogeneous although operating at atmospheric pressure, and the more common filamentary regime. In both regimes, densities as high as are detected. However, the N atoms kinetic formation depends on the discharge regime. The saturation level is reached more rapidly with a filamentary discharge. For a given discharge regime, the N atom density depends strongly on the energy dissipated in the plasma between the gas inlet and the measurement position, whether the energy is varied by varying the position of the measurements, the gas flow, or the dissipated power. Experiments performed in the postdischarge show that the N atom decay cannot be simply attributed to three-body recombination of atomic nitrogen with nitrogen molecules, meaning that other mechanisms such as surface recombination or gas impurities play a role.

This work was partially supported by the French Agence Nationale de la Recherche in the frame of the IPER project (Grant No. ANR-05-BLAN-0090). The authors thank A. Boulanger for precious help in the technical set-up. C.S.-B. acknowledges a Canada graduate scholarship from NSERC.

I. INTRODUCTION

II. TALIF SPECTROSCOPY

A. Principle and experimental apparatus

B. Calibration method: Two-photon excitation of Kr

III. DISCHARGE APPARATUS

IV. RESULTS AND DISCUSSION

A. Nitrogen ground state atom density in TDBD

B. Nitrogen ground state atom density in TDBD post-discharge

C. Effect of discharge regime

V. CONCLUSION

### Key Topics

- Townsend discharges
- 21.0
- Gas discharges
- 20.0
- Electric measurements
- 12.0
- Atmospheric pressure
- 11.0
- Ground states
- 11.0

## Figures

Simplified energy level diagram of atomic nitrogen and krypton indicating the excitation scheme and the observed fluorescence wavelengths.

Simplified energy level diagram of atomic nitrogen and krypton indicating the excitation scheme and the observed fluorescence wavelengths.

Schematic diagram of the experimental apparatus used in the TALIF diagnostics.

Schematic diagram of the experimental apparatus used in the TALIF diagnostics.

Log-log plot of the N atom fluorescence signal at resonance as a function of the 206.65 nm laser beam energy. The slope of the line is 2.0 ± 0.1.

Log-log plot of the N atom fluorescence signal at resonance as a function of the 206.65 nm laser beam energy. The slope of the line is 2.0 ± 0.1.

Schematic of the DBD setup.

Schematic of the DBD setup.

Oscillogram of measured current , discharge current , applied voltage , gas voltage and laser pulse, (a) in the case of a TDBD and (b) in the case of a FDBD .

Oscillogram of measured current , discharge current , applied voltage , gas voltage and laser pulse, (a) in the case of a TDBD and (b) in the case of a FDBD .

Absolute density of ground state nitrogen atoms in a Townsend discharge as a function of the position from the discharge entrance, measured for three different gas flows of .

Absolute density of ground state nitrogen atoms in a Townsend discharge as a function of the position from the discharge entrance, measured for three different gas flows of .

Absolute density of ground state nitrogen atoms in a Townsend discharge at a given position and as a function of the gas flow (, ).

Absolute density of ground state nitrogen atoms in a Townsend discharge at a given position and as a function of the gas flow (, ).

Absolute density of ground state nitrogen atoms in a Townsend discharge as a function of the mean equivalent residence time or the mean energy dissipated in the gas for the given measurement position (. The gas flow and/or the position is varied).

Absolute density of ground state nitrogen atoms in a Townsend discharge as a function of the mean equivalent residence time or the mean energy dissipated in the gas for the given measurement position (. The gas flow and/or the position is varied).

Effect of ○ the mean equivalent residence time with constant power and ● the varied power for a constant mean equivalent residence time (62 ms, is varied from ) on the absolute density of ground state nitrogen atoms in a Townsend discharge.

Effect of ○ the mean equivalent residence time with constant power and ● the varied power for a constant mean equivalent residence time (62 ms, is varied from ) on the absolute density of ground state nitrogen atoms in a Townsend discharge.

Absolute density of ground state nitrogen atoms as a function of the residence time in the postdischarge area of a Townsend discharge (, gas flow and/or the position is varied). The symbols are experimental values; the curve corresponds to the calculated values from Eq. (8).

Absolute density of ground state nitrogen atoms as a function of the residence time in the postdischarge area of a Townsend discharge (, gas flow and/or the position is varied). The symbols are experimental values; the curve corresponds to the calculated values from Eq. (8).

Absolute density of ground state nitrogen atoms in a filamentary discharge as a function of the mean energy dissipated in the gas for the given measurement position (. The gas flow and/or the position is varied).

Absolute density of ground state nitrogen atoms in a filamentary discharge as a function of the mean energy dissipated in the gas for the given measurement position (. The gas flow and/or the position is varied).

## Tables

Values of the parameters involved in the N density calibration via krypton (Ref. 23). is the mean laser pulse energy (μJ) for which measurements have been done, ν is the photon frequency, is the transition probability of the observed fluorescence spectral channel, is the total transition probability of the excited state (e.g., inverse of the radiative lifetime), and are the respective densities of and Kr, and are the quenching coefficient of the atom analyzed (Kr or N) with respectively and Kr, is the total transmission of the collection optics at the wavelength corresponding to the analyzed atom (Kr or N), and η is the detector quantum efficiency at this wavelength.

Values of the parameters involved in the N density calibration via krypton (Ref. 23). is the mean laser pulse energy (μJ) for which measurements have been done, ν is the photon frequency, is the transition probability of the observed fluorescence spectral channel, is the total transition probability of the excited state (e.g., inverse of the radiative lifetime), and are the respective densities of and Kr, and are the quenching coefficient of the atom analyzed (Kr or N) with respectively and Kr, is the total transmission of the collection optics at the wavelength corresponding to the analyzed atom (Kr or N), and η is the detector quantum efficiency at this wavelength.

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