Electron density measurements behind a hypersonic shock wave in argon

The purpose of the research is to characterize the ionization process behind a shock wave with precursor photoionization in argon. In this study, the H-β line is observed by spectroscopic measurements using a hypersonic shock tube and the Stark broadening of the H- β line is evaluated to obtain the electron density behind a shock wave. As a result, the electron density is on the order of 1021 m-3 and tends to decrease with increasing the distance from the shock front. The decrease of the measured electron density is caused by the radiative energy loss from the test gas because the radiative energy loss corresponds to a decrease of the temperature or the density of the test gas, both of which can decrease the electron density. Previous study showed that photoionization occurred ahead of the shock wave, making use of the radiation energy emitted from the region behind the shock wave. The fact might be related to the radiative energy loss obtained in the present study. From the present study, it is found that the radiative energy loss is dominant behind the shock wave under the condition that precursor photoionization occurs ahead of the shock wave. In future, we should investigate the radiative transfer phenomena around the shock wave for further clarification.