Shock Layer Optical Attenuation and Emission Spectroscopy Measurements During Arc Jet Testing with Ablating Models

Abstract

Measurements of optical attenuation and emission spectra for the radiating bow shock region upstream of several ablating arc jet test models are reported. Attenuation measurements were intended to assess light transmission parallel to the model face at k633 nm by the shock layer formed over the ablating surface; the attenuation is attributed to the presence of particles. As the models ablated, the surface receded, effectively translating the detection line of sight upstream from the model surface. Substantial loss of transmission correlated with the macroscopic ablation and surface heating rates, and persisted upstream of the shock front. Simultaneously, optical emission spectra were measured in the same plane as the attenuation laser. These measurements were intended to determine the extent to which ablation products (particles or vapor) influence the bow shock radiation. The spectra were measured rapidly using miniature fixed-grating spectrometers with fiber optic input. With a field of view of 2 mm and acquisition time less than 100 ms, spatial resolution was retained and time dependent intensity trends were observed. Several components of dissociated air can be identified based on their spectral signatures; the radiative contribution by ablation products appears minor, however.