The ring-shaped spatial distribution of the argon excimer, \(\hbox {Ar}_2^{*}\), in the effluent of the kINPen-sci

Spatially resolved measurements of the argon excimer, \(\hbox {Ar}_2^{*}\) $\text{Ar}_2^{\ast }$ , were performed in the effluent of a cold atmospheric plasma jet (CAPJ), the so called kINPen-sci, using cavity ringdown spectroscopy. The spatial distribution of the density of \(\hbox {Ar}_{2}^{*}\) $\text{Ar}_2^{\ast }$ could be distinguished into two distinct populations: a Gaussian and a toroidally shaped distribution. The production mechanisms of these populations seem to differ. On the one hand, a strong correlation was found between the Gaussian \(\hbox {Ar}_{2}^{*}\) $\text{Ar}_2^{\ast }$ population and the spatial distribution of the filaments produced in the effluent of the kINPen-sci. On the other hand, measurements performed while varying the experimental conditions under which the kINPen-sci was operated indicate that the gas flow velocity must play a major role in the formation of the toroidal \(\hbox {Ar}_2^{*}\) $\text{Ar}_2^{\ast }$ population. However, the mechanism of formation of the toroid \(\hbox {Ar}_{2}^{*}\) $\text{Ar}_2^{\ast }$ population remains unclear.