Determination of OH Radicals in an Atmospheric Pressure Helium Microwave Plasma Jet

A recent study has reported the observation of the OH radicals in the far downstream of an atmospheric argon microwave plasma jet. The far downstream is referred to as the location where the ratio of the distance from the jet orifice to the length of the jet column is greater than three. In this paper, we report that this phenomenon also exists in a similar plasma jet that is 2.5 mm long, operated in helium gas. A detailed characterization of the helium microwave plasma jet was carried out by using UV-pulsed cavity ringdown spectroscopy and optical emission spectroscopy. The absolute number densities of the OH radicals were measured along the plasma jet column from the jet orifice to the far downstream, and the OH concentrations vary from 9.64 × 10¹⁵ near the jet orifice to 7.32 × 10¹² molecules/cm³ in the far downstream (16 mm away from the jet orifice, corresponding to a distance ratio of 6.4). The dependence of the OH concentration on plasma power and gas flow rate at different locations along the jet axis is characterized, and the result suggests that the OH concentration increases with an increase in plasma power and gas flow rate in the downstream.