Energy relaxation and heating in the afterglow of high electric field ns-discharges in ambient air using spontaneous Raman scattering

Abstract

The spatio-temporal rovibrational excitation and relaxation mechanisms of N2(X) in the post-discarge of a 10 ns high-voltage diffuse discharge are studied by Spontaneous Raman Scattering. It is shown that the vibrational excitation of nitrogen molecules remains high despite the strong electric fields applied during the discharge itself and the relaxation processes are similar to lower voltage ns discharges. The main differences with the lower field discharges are rather visible at the beginning of the discharge with a specific spatial volume distribution and a significant vibrational non-equilibrium between v=0,1 and v>1. The spatial distribution of the rovibrational excitation of the diffuse discharge is very wide radially, consistent with the sustainability of fields greater than 100 Td over nearly 8 mm during propagation. The initial rovibrational excitation is inhomogeneous along the axis. The gas temperature reaches up to about 1200 K close to the pin (85 kV, ambient air) while it remains below 500 K in the rest of the volume. It is possible to control the heating of the discharge without greatly modifying the energy transfer mechanisms by adjusting the duration of the voltage pulse. In terms of reactivity, high atomic oxygen densities seem to be very localized in the vicinity of the pin (10 24 m -3 at 1.5 mm from the pin, corresponding to about 20 % dissociation). This inhomogeneity reflects the distribution of energy in the volume of the discharge. The main effects of humidity are also studied. It amplifies the fast heating and accelerates the decay of atomic oxygen in the post-discharge. No significant acceleration of the V-T relaxation of nitrogen due to the addition of water vapour was observed for the studied conditions. A shock wave was identified which is triggered at around 500 ns.

Electron density and temperature in a diffuse nanosecond pulse discharge in air at atmospheric pressure

“…As reported in [55], the O ground states density can be as high as 20% of the initial O 2 density at 1.5 mm from the pin in the early post-discharge. Due to the high electric fields measured up to 6 ns in the pin region [7], the O 2 dissociation rate can be expected to be much higher closer to the pin.…”

“…With R2 equal to 19.43, the final expression of ∆λ VdW is given in table A1 as a function of pressure and gas temperature. In the first hundred of nanoseconds of the discharge dynamics, the increase of the temperature which is observed near the pin is so fast that it can be considered as isochore [29,55]. T g reaches 450 K at 8 ns within the first 500 µm from the pin, which corresponds to a pressure of 1.54 bar.…”

Experimental study of the effect of water vapor on dynamics of a high electric field non-equilibrium diffuse discharge in air

“…Those results show that the water vapor addition can enhance the heating processes in a few nanoseconds in the high field region of the pin but not in the areas where the field is lower. The gas temperature during the early postdischarge has been also obtained by spontaneous Raman scattering [55]. Those measurements showed that the temperature spatial mapping in the first 300 ns of the post-discharge suggests that 1 ± 0.4 mJ is spent in fast heating at the pin in dry air and 1.5 ± 0.4 mJ in humid air.…”

N2 vibrational excitation in atmospheric pressure ns pulse and RF plasma jets

“…At these conditions, efficient vibrational excitation is produced by electron impact, over a wide range of reduced electric fields (E/N). Strong vibrational excitation of N 2 has also been detected in ns pulse discharges in nitrogen and air [17][18][19][20][21][22].…”