Generation and evolution of laser-induced shock waves under martian atmospheric conditions

Abstract

SuperCam on NASA's Mars 2020 mission is the first laser-induced breakdown spectroscopy (LIBS) instrument on Mars that also employs a microphone to help with the evaluation of the LIBS spectra. When a LIBS measurement is performed in an atmosphere, an acoustic signal is emitted that can be recorded and used, for example, as a normalization standard to help in the interpretation of spectral data. The investigation of this pressure wave in Martian atmospheric conditions is important to better understand the relation between the acoustic signals and the LIBS spectra that are measured by SuperCam. However, the mechanisms involved in the generation of the pressure wave in Martian atmospheric conditions have not been studied in detail yet. To improve our understanding of how the acoustic signal is generated and how it can be used to aid in the analysis of the spectroscopic data, we developed an experimental setup combining different experimental methods to study the pressure waves generated during LIBS measurements in a simulated Martian atmosphere. It incorporates a schlieren imaging system for shock wave analysis, a plasma imaging setup for spatially and temporally resolved plasma analysis and a microphone to study the acoustic signal generated by the shock wave. The schlieren system is used to investigate the shock wave's density structure and expansion dynamics. The system was designed to record schlieren images in low-pressure Mars-analogue atmosphere with a high temporal resolution. We present first measurements including schlieren images of the laser-induced shock wave expanding from an iron target in a simulated Martian atmosphere. The measurements confirm numerical simulations of the density structure of laser-induced shock waves for the first time. Furthermore, we find that the Taylor-Sedov model gives a good estimate of the shock wave dynamics up until sonic speeds, when a linear model is appropriate. By comparing the dynamics of the shock wave to the expansion of the plasma, we find that the shock wave decouples from the luminous plasma between 400 ns and 500 ns after plasma ignition. We demonstrate the capabilities of this new setup by giving a detailed description of how the acoustic signal is generated from the expanding plasma. In future studies, the presented setup will be used to investigate the influence of material properties and experimental parameters on the generation of the shock wave and the acoustic signal to support the analysis of SuperCam measurements on Mars.