Excited oxygen kinetics at electronvolt temperatures via 5-MHz RF-diplexed laser absorption spectroscopy

Abstract

A multi-MHz laser absorption sensor at 777.2 nm ( 12 , 863 c m − 1 ) is developed for simultaneous sensing of (1)  O ( 5 S 0 ) number density, (2) electron number density, and (3) translational temperature at conditions relevant to high-speed entry conditions and molecular dissociation. This sensor leverages a bias tee circuit with a distributed feedback diode laser and an optimization of the laser current modulation waveform to enable temporal resolution of sub-microsecond kinetics at electronvolt temperatures. In shock-heated O 2 , the precision of the temperature measurement is tested at 5 MHz and is found to be within ± 5 % from 6000 to 12,000 K at pressures from 0.1 to 1 atm. The present sensor is also demonstrated in a CO:Ar mixture, in parallel with a diagnostic for CO rovibrational 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 simplified model of ionization. Finally, as an illustration of the utility of this high-speed diagnostic, the measurement of the heavy particle excitation rate of O ( 5 S 0 ) is extended beyond the temperatures available in the literature and is found to be well represented by k ( 3 P → 5 S 0 ) = 2.7 × 10 − 14 T 0.5 exp ⁡ ( − 1.428 × 10 4 / T ) c m 3 ⋅ s − 1 from 5400 to 12,200 K.