Walten Owens

Laser Spectroscopic Investigation of Surface-Catalyzed Reactions for Mars Exploration Vehicles

Discrepancies exist as to whether appreciable amounts of CO2 recombine at the surface of Martian entry vehicles. This is a significant concern as overly conservative design measures need to be used to accommodate this uncertainty owing to the the variation in heat-fluxes that arise for different recombination models used. The University of Vermont (UVM) has recently developed an 30kW Inductively Coupled Plasma (ICP) Torch Facility. This facility is designed for investigation of material testing and gas-surface interaction chemistry. Two-photon absorption laser induced fluorescence (TALIF) and diode laser absorption spectroscopy (DLAS) strategies to characterize O, CO and CO2 concentrations near sample material surfaces are presented here.

Investigation of CN Production from Carbon Based Materials in Nitrogen Plasmas

The numerical approaches used to calculate ablative heat shield thickness have come into question recently, with particular attention given to uncertainties in chemical reaction rates. One such example is the carbon nitridation reaction, C(s) + N → CN where atomic nitrogen extracts solid carbon from a heat shield surface. Few measurements have been performed for this reaction at conditions relevant to re-entry, and large differences exist in reported values. Experiments aimed at measuring this particular reaction rate have been conducted recently in the 30 kW Inductively Coupled Plasma Torch Facility at the University of Vermont, which simulates re-entry conditions on scaled material samples. Two-photon laser-induced fluorescence is used to measure nitrogen atom density and translational temperature in the reacting boundary layer above the graphite surface. These measurements are used to calculate the nitrogen atom flux reaching the surface. Combining this result with measurements of sample mass loss allows an estimation of the nitridation rate for the test conditions. Initial tests were done with modest surface temperatures in the range of 1620 to 1800 K. Preliminary results from this investigation are reported.

Near Surface CO2 Detection in an Inductively Coupled Plasma Facility Using Diode Laser Absorption

Uncertainty exists as to whether appreciable amounts of CO2 are formed by surface recombination for Mars entry vehicles. It is well understood that highly catalytic thermal protection system (TPS) material can lead to nearly two times the heat flux to the vehicle surface as a non-catalytic material. Conservative design practice assumes a supercatalytic surface to accommodate this uncertainty. Risk avoidance leads to oversized TPS geometries that add mass to entry vehicle designs that could be otherwise employed. Determining whether significant near surface production of CO2 occurs at trajectory heating conditions would add significant insight. The University of Vermont (UVM) has developed a 30kW Inductively Coupled Plasma (ICP) Facility to provide a pure high temperature plasma environment for aerospace material testing and gas-surface interaction chemistry investigations. The goal of the present work is to develop and test a diode laser absorption spectroscopy (DLAS) sensor to target near sample surface CO2 concentrations. A DFB diode laser at 2744 nm with a tuning range of around 3 nm is chosen as the light source. Current investigations show no appreciable amount of CO2 near a highly catalytic surface (water cooled copper) within the sensors’ detectivity limits. Work is ongoing to further improve these sensitivity limitations.

Surface Catalyzed Reaction Efficiencies in Nitrogen and Oxygen Plasmas from Laser Induced Fluorescence Measurements

Recent efforts to advance laser diagnostics in plasma facilities have resulted in the capability to measure gradients of temperature and species in the reacting zone above a high-temperature material. These measurements allow a direct qualitative estimate of the surface-catalyzed reaction efficiency when the sub-millimeter spatial distribution of the reactive species is compared with measurements over reference materials. Measurements of nitrogen atom and oxygen atom distributions are presented for reference materials and silicon carbide for both nitrogen and oxygen plasmas to elucidate the relative efficiencies for homogeneous surface catalyzed recombination.

Surface Catalyzed Reaction Efficiencies in Air Plasmas Using Laser Induced Fluorescence Measurements

Recent efforts to advance laser diagnostics in plasma facilities have resulted in the capability to measure gradients of temperature and species in the reacting zone above a hightemperature material. These measurements allow a direct quantitative estimate of the surface-catalyzed reaction efficiency when the spatial distribution of the reactive species is compared with measurements over reference materials. Measurements of temperature, relative nitrogen atom density, relative oxygen atom density, and relative NO molecule density in a chemically reacting boundary layer were performed in air plasmas to elucidate the behavior of catalytic efficiencies.

Flexible TPS Surface Catalysis Testing in a 30 kW ICP Torch Facility

Ground-facility testing of a silicon carbide (SiC) fabric for flexible based thermal protection systems is underway using the 30 kW Inductively Coupled Plasma (ICP) Torch located at the University of Vermont Plasma Test and Diagnostics Laboratory (PTDL). The fabric is exposed to high enthalpy nitrogen and air plasma flows to quantify surfacecatalyzed recombination reactions relevant to earth atmosphere re-entry. The catalytic behaviour is characterized using multiple surface and gas-phase measurements including infrared pyrometry, cold-wall heat flux, and laser induced florescence (LIF). The flexible material catalytic recombination efficiency is assessed by comparing the atomic species gradients for the flexible material with those of a solid surface with the same elemental composition and with reference materials of varying catalytic efficiency. This paper presents an overview of the motivation for the investigation, a description of the experimental configureation, and comparative results on nitrogen recombination for rigid SiC, the flexible material, and the bare coupon holder.