Time-resolved emission spectroscopy of impact plasma

One phenomenon observed at hypervelocity impacts (HVI) is the generation of plasma with a very short lifetime of a few microseconds. Due to this short lifetime, characteristic plasma parameters such as the electron density and the electron temperature of the expanding plasma were not investigated thoroughly in the past. This paper will present a method to measure these parameters with a time-resolution of 500 ns for the full period of impact plasma expansion and discuss results gained in impact experiments. At the Fraunhofer EMI, impact experiments on solar panels were performed using a two-stage light-gas gun to accelerate aluminum spheres with a diameter of a few millimeters up to a speed of 8 km/s. A measurement system consisting of a spectrograph and a streak camera was applied for time-resolved spectroscopy of the impact plasma. To derive plasma properties, the recorded streak image was evaluated using different methods for different expansion states of the plasma cloud. The spectra show strong self-absorption lines in the first microseconds of expansion. In the present work, these features are explained by the electron density and temperature gradient in the plasma. For the determination of electron temperature and density, a one-dimensional radiative transfer model was adapted to the measured spectra. After 2 ms of expansion, the plasma is optically thin and emission lines can be observed. For this expansion state, the electron temperature was determined by the ratio of line to continuum radiation, whereas the electron density was determined through the line broadening due to the Stark effect. Using these methods, it was found that the electron temperature decreases in the first 3 ms of propagation from 45,000 K to 2,000 K in the experiments performed. The electron density decreases from 1019 cm-3 to 1017 cm-3.