Hybrid fs/ps coherent anti-Stokes Raman scattering for multiparameter measurements of combustion and nonequilibrium
Hybrid femtosecond/picosecond coherent anti-Stokes Raman scattering (fs/ps CARS) is developed for measuring internal energy distributions, species concentration, and pressure for highly dynamic gas-phase environments. Systems of interest include next-generation combustors, plasma-based manufacturing and plasma-assisted combustion, and high-speed aerodynamic flow. These challenging environments include spatial variations and fast dynamics that require the spatial and temporal resolution offered by hybrid fs/ps CARS.
A novel dual-pump fs/ps CARS approach is developed to simultaneously excite pure-rotational and rovibrational Raman coherences for dynamic thermometry (300--2400 K) and detection of major combustion species. This approach was also used to measure single-shot vibrational and rotational energy distributions of the nonequilibrium environment of a dielectric barrier discharge plasma. Detailed spatial distributions and shot-to-shot fluctuations of rotational and vibrational temperatures spanning 325--450 K and 1200--5000 K were recorded across the plasma and surrounding flow, and are compared to plasma emission spectroscopy measurements. Dual-pump hybrid fs/ps CARS allows for concise, kHz-rate measurements of vibrational and rotational energy distributions or temperatures at equilibrium and nonequilibrium without nonresonant wave-mixing or molecular collisional interference.
Additionally, a highly transient ns laser spark is explored using CARS to measure temperature and pressure behind the shock wave and temperature of the expanding plasma kernel. Vibrational energy distributions at the exit of a microscale gaseous detonation tube are presented. Theory required to model fs/ps CARS response, including nonthermal energy distributions, is presented. The impact of nonequilibrium on measurement accuracy is explored, and a coherent line-mixing model is validated with high-pressure measurements. Temperature and pressure sensitivity are investigated for multiple measurement configurations, and accuracy and precision is quantified as a function of signal-to-noise for the fs/ps CARS system.