Multiplexed Polarization Spectroscopy for Single-Shot Multi-Species Diagnostics in High-Enthalpy Flows

Abstract This project will develop and demonstrate an innovative multiplexed two-photon polarization spectroscopy (MU-TIPS) diagnostic system for simultaneous measurement of O, O2, and NO species concentrations and temperatures in high-enthalpy flows. With the help of two full-time PhD students, the proposed research will create a non-intrusive diagnostic capable of providing nanosecond temporal resolution, millisecond repetition, and sub-millimeter spatial accuracy for near-surface measurements and spatial coverage on the order of 1 cm for single-shot one- dimensional temperature and concentration profiles. Polarization spectroscopy (PS) is a nonlinear optical technique capable of interrogating the electronic ground states of both atomic and molecular species and generating high signal-to-noise ratios with a diagnostic setup simpler than, e.g., Coherent Anti-Stokes Raman Scattering or Degenerate Four-Wave Mixing. Thus, the proposed research addresses critical measurement limitations in NASA’s arcjet facilities. Using a custom nanosecond Nd:YAGpumped broadband UV laser source, the system will enable single-shot multiplexed measurements of electronic ground-state populations and translational/rotational/vibrational temperatures in the challenging environments typical of atmospheric entry testing. The proposed diagnostic directly supports validation of ablative material response models by overcoming the spatial averaging limitations of passive and absorptive techniques and providing the high-fidelity datasets required for computational model development. The targeted species are of fundamental importance in Earth, Mars and Venus atmospheric entry research. Furthermore, PS can also target nitrogen, carbon, and hydrogen species, e.g. for missions to the gas-giants, by using different ultra-violet and mid-infrared diagnostic wavelengths. The project will advance the technology from TRL 1-2 to TRL 3 through systematic validation in laboratory flames and high-enthalpy plasma environments, establishing measurement protocols and hardware designs suitable for implementation in NASA’s ground testing infrastructure. For relevant benchmarking of the diagnostic, measurements will be made in the University of Kentucky microwave plasma High-Enthalpy Low-Cost Multi-User Test (HELMUT) Facility. The measurements will be conducted in both the high-enthalpy freestream and in the boundary layer of material samples.