Quenching-Independent Two-Photon Absorption Laser-Induced Fluorescence of Atomic Nitrogen in High-Enthalpy Air–Carbon Gas–Surface Interaction

A technique for quenching-independent, two-photon absorption laser-induced fluorescence (TALIF) measurements of atomic nitrogen in high-enthalpy facilities is presented. The technique relies on high-laser intensity, which results in the photoionization rate dominating other depopulation channels for the induced excited state. The photoionization-dominated technique is applied here to study the distribution of atomic nitrogen in the vicinity of an ablating graphite sample in a plasma plume. Three different plasma conditions are investigated: a 17 MJ/kg air plasma delivering 145 W/cm² to the graphite surface, an 18 MJ/kg air plasma delivering 195 W/cm², and a 24 MJ/kg nitrogen plasma delivering 85 W/cm². The number density of atomic nitrogen is measured along the stagnation streamline of the flow on the graphite sample in each case. The highest number densities are observed in the nitrogen plume, as would be expected given the pure nitrogen composition and high enthalpy. The 195 W/cm² air condition has the second-highest atomic nitrogen distribution, followed by the 145 W/cm² air condition. This technique may provide a valuable method for studying atomic nitrogen, an important species to air/carbon gas-surface interaction at hypersonic-relevant conditions, in high-enthalpy facilities.