Research

The processes controlling the performance of thermal protection systems (TPS) during hypersonic flight primarily take place in the boundary layer region surrounding an air- or spacecraft. Recombination, accommodation, oxidation, pyrolysis, and radiative heat transfer are all processes that occur during the interaction of the high enthalpy flow with the heat shield material.

Developing models for these processes and the associated chemistry, that cover a range of pressures, temperatures and gas compositions, is critical for the reliable predictive design of TPS for high-speed transport and atmospheric reentry at earth and other planetary bodies in the solar system. Insights into the material response to such extreme flight conditions are also interesting for the development of plasma-facing components in fusion reactors and plasma-based material processing. 

A 3kW microwave plasma torch mounted in a vacuum vessel is situated in the CoRe Flow Lab, to simulate the high-enthalpy flow region behind shocks during hypersonic flight and to match the resulting surface heat flux. We are also in the process of commissioning a 10 kW inductively coupled plasma torch facility for higher heat fluxes and supersonic approach flows. Emission and laser spectroscopy diagnostics are available for non-intrusive measurements.

Of current scientific interest are processes controlling the ablation of non-reusable heat shields for high-speed interplanetary reentry in different gas compositions, the behavior of ultra-high temperature ceramics for reusable heat shields, and the impact of spacecraft demise on the earth’s atmosphere. To study the gas-material interactions we develop spatially resolved laser spectroscopic diagnostics and advanced statistical inference techniques.
 

Raman scattering is a powerful technique to probe rotational and vibrational state population distributions of molecular and even some atomic species. We are in the process of optimizing a one-dimensional Raman scattering setup for measurements at sub-atmospheric pressures. This will allow us to quantify species number densities, nonequilibrium state distributions, and equilibrium rotational and vibrational temperatures through the boundary layer of models immersed in plasmas and hypersonic flows. The setup utilizes volume Bragg gratings for a spectrally extremely narrow rejection of the excitation laser wavelength and an emICCD detector. Pulse stretchers will be added in the future to allow for higher Raman signals without causing Raman pumping or dielectric breakdown in the probe volume. 

The setup also serves as a basis for the extension to a coherent anti-Stokes Raman system, which provides a highly-directional measurement signal, species selectivity, and better single shot measurement potential.

Other projects in this area are the custom modification of an existing OH laser induced fluorescence measurement system to target atomic oxygen and nitrogen in a two-photon absorption laser induced fluorescence. And the development of a reference gas discharge cell.
 

While the application of Emission Spectroscopy (ES) is limited to radiating environments, it has a great appeal due to its perceived simplicity and low barrier-to-entry. Thus, ES remains a technique of great experimental interest. However, the quantitative interpretation of ES data is challenging due to the complex underlying chemical and radiative mechanisms and the many sources of uncertainty which are often not rigorously quantified. Emission spectroscopy (ES) detects radiation from excited states and requires models to relate this to ground state properties. The measurements are line-of-sight integrated and lack spatial resolution unless specific techniques are used. Spectra often include baseline radiation and depend on database parameters with unquantified uncertainties. Long integration times, used to enhance signal, can introduce bias in non-uniform environments due to the non-linear relationship between radiation and gas properties. 

To address these shortcomings and leverage the strengths of ES, we are developing an advanced framework for ES measurements that will take advantage of recent advances in algebraic reconstruction techniques, statistical inference, fast synthetization of spectra, and new hyperspectral measurement technologies. The framework attempts to address the inversion of the full ES measurement model, accounting for the actual light-collection volume and spatial reconstruction, spectroscopy models, kinetic-radiative models, Bayesian inversion for uncertainty quantification, and sparse spectral sampling for hyperspectral imaging. 

We are actively developing this framework, applying it to measurements of fundamental physical properties in inductive and capacitive plasmas, the evaluation of spectroscopic measurements from the UK KRUPS-capsules during hypersonic reentry, and spectroscopic data from fusion devices.