Laser Absorption Measurements of CN(X) Population and Temperature Evolution in the Low Density Electric Arc Shock Tube

Accurate prediction of radiation from the cyano radical (CN) during Titan entry is a priority for NASA’s Dragonfly mission. In order to validate predictions, previous tests conducted at NASA Ames’ Electric Arc Shock Tube (EAST) facility explored low altitude/high density trajectory conditions. For this work, conditions relevant to higher altitudes of Titan entry trajectory were investigated with the new Low Density Shock Tube (LDST) at EAST. Experiments were conducted with test gas 2.2% by mole methane in nitrogen, fill pressures ranging from 0.03 to 0.1 Torr, and shock velocities from 3.8 to 7.6 km/s. A multi-pass diagnostic utilizing tunable diode laser absorption spectroscopy (TDLAS) measured multiple absorbance features of the CN red system around 927 nm and 1128 nm, from which the gas temperature and ground state CN number density were inferred. Measurements suggest close temperature agreement with simulations by the Data Parallel Line Relaxation code (DPLR), and close agreement with CN number density at high incident shock velocities, but exhibit appreciable deviations at low shock velocities, the lowest pressures, and in time periods of thermal non-equilibrium. Additionally, these ground state measurements provide an important benchmark for analyzing non-Boltzmann behavior in the excited electronic states of CN, and aid in the gasdynamic characterization of the LDST.