Excited oxygen kinetics at electronvolttemperatures via 5-MHz RF-diplexed laserabsorption spectroscopy br

A multi-MHz laser absorption sensor at 777.2 nm (12,863 cm-1) is developed for simultaneous sensing of(1) O(5S0)number density, (2) electron number density, and (3) translational temperature at conditions rel-evant to high-speed entry conditions and molecular dissociation. This sensor leverages a bias tee circuit with adistributed feedback diode laser and an optimization of the laser current modulation waveform to enable tempo-ral resolution of sub-microsecond kinetics at electronvolt temperatures. In shock-heated O2, the precision of thetemperature measurement is tested at 5 MHz and is found to be within +/- 5% from 6000 to 12,000 K at pressuresfrom 0.1 to 1 atm. The present sensor is also demonstrated in a CO:Ar mixture, in parallel with a diagnostic for COrovibrational temperature, providing an additional validation across 7500-9700 K during molecular dissociation.A demonstration of the electron number density measurement near 11,000 K is performed and compared to a sim-plified model of ionization. Finally, as an illustration of the utility of this high-speed diagnostic, the measurementof the heavy particle excitation rate of O(5S0)is extended beyond the temperatures available in the literature andis found to be well represented byk(3P -> 5S0)=2.7x10-14T0.5exp(-1.428x104/T)cm3middots-1from 5400 to12,200 K.(c) 2023 Optica Publishing Group