Active species evolution in the presence of different targets impacted by a helium plasma jet at atmospheric pressure

Abstract

A low-temperature plasma jet is generated by a dielectric barrier discharge poweredby a pulsed high voltage in helium flow (3 L/min) at atmospheric pressure inpresence of different targets (glass slide or ultra-pure water or a grounded metal plate)positioned perpendicular to the plasma propagation axis. Experimental electricalcharacterizations as discharge current, voltage and powerand optical ones as intensifiedcamera ICCD, Schlieren imaging and emission spectroscopy to follow specific excitedspecies have been achieved. The transition from laminar to turbulent regime wereobserved during the discharge ignition with a larger spreading of the plasma on thesurface target with lower dielectric permittivity and the generation of two dischargesduring each voltage pulse is highlighted during the propagation of the ionization wavethat has shown a variable speed along the plasma axis not depending on the target kind.The evolution of some active species (as OH, O and excited nitrogen and helium) areinvestigated using time resolved mapping of the emissions of radiative excited speciespropagating in ambient air between the plasma jet output andthe target. For a lowrelative permittivity target (glass), the volume ionization wave at its arrival on thetarget spreads on its surface thus behaving as a surface ionization wave. For thehighest relative permittivity (metal), a conductive channel appears between the targetsurface and the plasma jet during the first discharge, followed by a diffuse plasma plumefrom the target surface towards the plasma jet after the impact of the ionization waveon the target. A hybrid behavior is highlighted for the ultra-pure water which leadsto a short spreading of the ionization wave on the target surface, the formation of aconductive channel in ambient air between tube output and target and the formationof a plasma plume on the target surface.

Laser-induced fluorescence measurements of spatially resolved OH densities in a helium plasma jet interacting with three different targets in the open air

“…As in their results, the OH-LIF intensity was found to be much higher for the metal and the water surface compared to the glass plate. Previously, we reported the evolution of radiative excited species (OH * , N * 2 ) during a voltage pulse, while a He-APPJ impacted various targets [31]. It was found that the regions of the detected excited OH * radicals were the same as the regions where we observed, in the present work, ground state OH radicals.…”

“…It is also partly the case for the ultra-pure surface but with a less important surface spreading than on glass surface because a part of the plasma discharge remains in the volume as a small conductive channel, while for the metal, there is mainly the formation of a stronger volumic conductive channel between the surface and the tube outlet. These effects linked to the nature of the surfaces are already detailed in our previous paper [31] and can therefore explain the differences observed between pLIF signal intensities in the case of the lower permittivity surfaces (glass and water) and the highest one (metal).…”

“…The helium APPJ device used in this work has already been described elsewhere [31,32], so that only the key features will be reminded. Two 20 mm wide aluminum electrodes were wrapped around a quartz tube (4 mm inner diameter and 6 mm outer diameter) and separated by a distance of 10 mm.…”

“…Since OH radicals are mainly distributed within regions, where the helium flow comes in contact with humidity from the open air, it can be concluded that the production of OH radicals is mainly due to the dissociation of water in these regions. In fact, this helium flow involves directed streamers or ionization waves propagating first inside the glass tube near the walls with a known ring shape [31] and then develop outside the tube a conical shape when helium begins to be mixed with the ambient open air. Downstream in the helium flux, the OH density decreases as the helium-air ratio decreases and the ionization waves are more and more attenuated when moving away from the tube exit.…”

“…So, it is important to consider the influence of the target surface during the treatment with an APPJ [30]. In a previous work [31], we studied the changes in the plasma jet of some physical, electrical and hydrodynamics properties and also the qualitative evolution of some active species (including OH) versus three different targets.…”