Sodium two-photon laser-induced fluorescence characterization with nanosecond and femtosecond excitation

摘要

Alkali metal dopants are common in particle combustion environments, biomass reforming for fuel, and have applications as dopants in combustion for diagnostic and plasma-based control approaches. Recently, alkali-doped solid propellant flames have been utilized for enhanced burning rates using microwave-based plasma enhancement. These systems exhibit significant particle scattering and high optical densities, leading to complications in single photon fluorescence detection. In this work, a two-photon excitation scheme is investigated to improve laser scattering rejection and fluorescence trapping issues exhibited for resonant single-photon excitation and detection. Both femtosecond and nanosecond excitation at 578 nm are examined with detection at 589 nm and 818 nm. An ion probe was used to detect ionization due to laser excitation as well as the flame. Significant resonance-enhanced multiphoton ionization is present for both nanosecond and femtosecond two-photon excitation schemes.