Ionizing Argon Shocks with Precursor Phenomena Characterized by Optical Emission Spectroscopy

In the present study, plasma properties of ionizing argon shocks with precursor phenomena are characterized using optical emission spectroscopy (OES) combined with a detailed collisional-radiative (CR) model. Spatially resolved emission spectra of ionizing argon shocks are measured using an imaging spectroscopy system, and corresponding numerical spectra are synthesized from excited-state population densities calculated with the CR model. Key plasma properties, including electron temperature, electron number density, heavy-particle translational temperature, and argon atom number density, are obtained by fitting the numerical spectra to the measured spectra. The resulting spatial distributions are compared with computational fluid dynamics (CFD) predictions incorporating precursor photoionization effects, showing reasonable agreement except for the electron temperature profile. The electron temperature inferred from the CR model is elevated near the shock front and agrees well with the CFD results, whereas in the postshock nonequilibrium region it is significantly lower than that predicted by two-temperature modeling. This discrepancy is attributed to an overpopulation of excited argon atoms caused by enhanced electron-impact excitation processes associated with precursor phenomena, which consume electron energy. These results demonstrate the capability of CR-model-assisted OES to quantitatively characterize ionizing argon shocks with precursor phenomena and highlight the limitations of conventional two-temperature thermochemical models under strongly nonequilibrium entry-relevant conditions.