Influence of humidity on the three-dimensional OH distribution in the effluent of the COST-Jet

Abstract Atmospheric plasma jets enable the precise generation and transport of reactive oxygen and nitrogen species to a designated target, making them particularly valuable for applications in plasma medicine. In these contexts, it is crucial to understand their generation mechanisms in the plasma and interactions with the surrounding atmosphere during transport. This is studied for the hydroxyl (OH) radical in the COST reference microplasma jet (COST-Jet) operated with helium (He) feed gas. Using laser-induced fluorescence spectroscopy, the radical’s absolute density in the jet’s effluent is resolved in all three dimensions. By separately controlling the humidity levels in the feed gas and ambient air atmosphere, the plasma and post-plasma generation of OH is explored. When water vapor is added to the feed gas, comparably high and homogeneously distributed OH densities ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:mrow> <mml:mo>∼</mml:mo> </mml:mrow> <mml:mn>1</mml:mn> <mml:mo>×</mml:mo> <mml:msup> <mml:mn>10</mml:mn> <mml:mrow> <mml:mn>14</mml:mn> </mml:mrow> </mml:msup> </mml:mrow> </mml:math> cm −3 ) occur at the jet’s nozzle showing a rapid radial and axial decay down the effluent. In contrast, OH production from ambient moisture alone is observed only at the interface between the He effluent and the surrounding air, resulting in densities roughly one order of magnitude lower. Higher relative humidities in the surrounding air cause the OH density peak to shift closer to the nozzle in every direction, as moisture penetrates deeper into the effluent.