Investigation of Gas-surface Interactions of Ablative Materials in Low Power Plasma Facilities

In the proposed work, a novel methodology for investigating gas surface interactions of a plasma with an ablative heat shield material will be demonstrated. The data gained in such experiments will be provided as validation data for high fidelity material response codes currently under development. Such codes are needed to predict heat shield performance during atmospheric entries to enable a reliable and optimized thermal protection system (TPS) design. The general idea is to gather data in a highly controlled environment while separating as many contributing processes as possible. The environment itself does not necessarily have to reproduce re-entry conditions, as long as it creates a situation relevant to the chemistry under investigation and can be reproduced by numerical simulation. A systematic variation of plasma conditions and surface temperatures will step by step build a database on gas-surface interaction of ablative materials in both equilibrium and non-equilibrium, providing information on selected reaction paths through the use of different working gases. A suitable hierarchy would be given through experiments with argon, argon/oxygen, argon/nitrogen, and argon/air, yielding increasing complexity of the possible chemical interactions between plasma and ablation products. A characterization of the actual interaction processes is foreseen through monitoring the spectral emission of different interaction products such as OH, NH, CN, but also known ablation and trace elements from the ablator such as C, K, Ca, and Na. The emission of these products has already been observed in during flight observation and has been shown to be characteristic for boundary layer characterization in ground testing. In future research, additional diagnostic methods such as laser diode absorption and laser induced fluorescence might be applied. The actual validation process requires a spectral simulation based on the numerical results of the code to be validated with codes as NEQAIR or similar radiation codes which will be compared to the data to be measured in this work. Similar methodologies have already been applied to arc-jet experiments at NASA Ames and to data measured during airborne observations of capsule re-entries. Within this project, the methodology and the capability of the facilities shall be developed and demonstrated at the example of selected working conditions. A comprehensive characterization of the material response will exceed the scope of this work, though. The work compliments current activities at the University of Kentucky which is already known as a center for ablation research. It will extend activities in the frame of current NASA EPSCoR work on investigation of micro spallation of ablative TPS materials. The final goals are to present an experimental validation test case to the community.