Measurements of Absolute Atomic Nitrogen Density by Two-Photon Absorption Laser-Induced Fluorescence Spectroscopy in Hot Air Plasma Generated by Microwave Resonant Cavity

Abstract

For the first time, absolute densities of atomic nitrogen in its ground state (N 4 S) have been measured in hot dry and humid air plasma columns under post-discharge regime. The determination of space-resolved absolute densities leads to obtain the dissociation degrees of molecular nitrogen in the plasma. The hot plasmas are generated inside an upstream gas-conditioning cell at 600 mbar when the air gas flow is directly injected at 10 slm in a microwave resonant cavity (2.45 GHz, 1 kW) placed in the downstream side. Density measurements based on laser induced fluorescence spectroscopy with two-photon excitation (TALIF), are more particularly performed along the radial and axial positions of the plasma column. Calibration of TALIF signals is performed in situ (i.e. in the same gas-conditioning cell but without plasma) using an air gas mixture containing krypton. Optical emission spectroscopy is considered to estimate the rotational gas temperature by adding a small amount of H 2 in dry air to better detect OH (A-X) spectra. The rotational temperatures in humid air plasma column (50% of humidity) are larger than those of dry air plasma column by practically 30% near the nozzle of resonant cavity on the axis of the plasma column. This is partly due to attachment heating processes initiated by water vapor. A maximum of the measured absolute nitrogen density is also observed near the nozzle which is also larger for humid air plasma column. The obtained dissociation degrees of molecular nitrogen in both dry and humid air plasma along the air plasma column are lower than the cases where only thermodynamic equilibrium is assumed.

Investigation of N(4S) kinetics during the transients of a strongly emissive pulsed ECR plasma using ns-TALIF

“…In fact, Krypton is a rare gas that provides a TALIF system, i.e., [(4p 6 ) 1 S0, 5p'[3/2]2, 5s[3/2]1], with wavelength values for laser-two-photon-absorption (2x204.13 nm) and fluorescence (587.09 nm) that are similar to those of N-atom. This method was successfully used for space-and time-resolved measurements of the absolute N-atom density in a variety of moderately-emissive plasmas and plasma post-discharges [14][15][16][17][18][19][20][21][22][23][24][25]. Surprisingly, despite this large success, achieving N-atom absolute density measurements in highly emissive nitrogen plasmas remains a challenging task from an experimental point of view [16,17,26].…”

“…[(4p 6 ) 1 S 0 , 5p [3/2] 2 , 5s[3/2] 1 ], with wavelength values for laser two-photon absorption (2 × 204.13 nm) and fluorescence (587.09 nm) that are similar to those of the N-atom. This method was successfully used for space-and time-resolved measurements of the absolute N-atom density in a variety of moderately-emissive plasmas and plasmas post-discharge [14][15][16][17][18][19][20][21][22][23][24][25].…”