Development of Langmuir and spatially resolved OES diagnostics for arc jet shock and boundary layer measurements relevant to atmospheric re-entry environments

The second-generation miniature Arc Jet Research Chamber at NASA Ames requires physical characterization of its supersonic plasma flow to verify numerical models and material sample testing. Two types of plasma diagnostics will be used: Langmuir probes and optical emission spectroscopy (OES). Several single and triple Langmuir probes have been constructed and will be inserted parallel to the flow via a sweep arm to measure post-shock electron temperature and number density along radial profiles at various axial distances. Due to high heat flux (200-3000 W/cm²), sweep durations are necessarily short (< 0.1 s). This will be an important metric for validating coupled CFD-material codes and providing insight into longstanding thermocouple anomalies attributed to charging of heatshield surfaces. An OES system has also been assembled to obtain spatially resolved density and temperature for various flow species. The system consists of a convex lens that forms an image of the plasma onto a 16-channel linear fiber that simultaneously captures the spectra from sixteen different locations. The light emission is then focused by a spherical mirror into a spectrometer and recorded with a camera. These measurements non-intrusively provide a new spatially resolved flow characterization capability, which will initially be used to resolve issues with shock radiation interference in pyrometer measurements of material surface temperature. The design of each diagnostic and any preliminary results will be discussed.