In-situ diagnostics of the interaction region between a nitrogen – oxygen plasma jet and hot C/C – SiC ceramic materials
Ingo Altmann, Gerd Bauer, Kurt Hirsch, Herbert Jentschke, Stefan Klenge, Bernhard Roth, Detlef Schinköth, Uwe Schumacher
Several in-situ diagnostic methods were developed and applied to investigate heat shield materials of space vehicles for re-entry missions. The requirements for those heat protection materials are simulated experimentally in the plasma wind tunnel. A jet of a nitrogen – oxygen arc plasma is expanded by a nozzle into a vacuum vessel. Several diagnostic systems examine the plasma chemical reactions in front of the heat shield, measure the surface temperature, observe surface reactions on the heat protection materials, and determine erosion rates continuously during the experiment. From emission and absorption spectroscopic measurements supported by laser Thomson scattering the parameters of the plasma and the erosion products are determined. Typical parameters in the interaction region are: the electron temperature, Te approx. Tgas approx. 10 000 K, and the electron density, ne approx. 3 x 1018 m-3. Spectrograms (two-dimensional spectral intensity distributions) show the streaming behaviour of the plasma near the target and permit in-situ recording. Spectral emissions of the main plasma components and of the erosion products are strongly correlated. Thus fluctuations of line intensities observed at different positions parallel and perpendicular to the plasma jet direction explore localised streaming velocities. The erosion behaviour is investigated by three different methods. These are emission and absorption spectroscopy in the ultraviolet, in-situ imaging of the surface emissivity, and Fourier transform infrared (FTIR) spectroscopy. Time development studies of the different emissivities of the hot carbon fibres, carbon matrix (C/C), and SiC layers, respectively, determine the specific erosion rate of those layers. They are in good agreement with gravimetric measurements and the silicon flux evaluated from silicon densities and the streaming velocity.