Two-photon laser-induced fluorescence in sodium-doped composite propellant flame

Abstract

<p>Alkali metal dopants are common in coal combustion environments, biomass reforming for fuel, and have applications as dopants in combustion for diagnostics and plasma-based control approaches. Recently, alkali-doped solid propellant flames have been demonstrated for enhancing burning rate using continuous microwave-based plasma enhancement, where sodium has been utilized as an electron donor. The performance of microwave coupled burning propellant flames may be optimized through knowledge of the ground-state sodium distribution. However, these systems exhibit significant particle scattering, gray body emission backgrounds, and high optical density, complicating measurements via laser diagnostics. In this work, to infer the sodium distribution in the propellant flame, a single-photon and two-photon absorption laser-induced fluorescence techniques are applied. Two-photon laser-induced fluorescence is shown to improve laser scattering rejection and fluorescence trapping issues exhibited in resonant single-photon laser-induced fluorescence. The 3s-3d excitation scheme is compared directly with 3s-4d excitation by examining fluorescence, amplified spontaneous emission, and ionization levels in sodium doped flame to optimize laser fluence levels. Significant resonance-enhanced multiphoton ionization and amplified spontaneous emission are presented in two-photon excitation schemes. Two-photon laser-induced fluorescence is applied to ground-state atomic sodium spatial distributions in microwave-coupled solid propellant flames.</p>