Non-linearity Observed in Laboratory LIBS Acoustic Experiments.

Planetary acoustics is an emerging field of study, with the first microphones to be deployed on Mars onboard the NASA Perseverance rover. The goal of this work is to characterize acoustic waveform generated by laser-induced breakdown spectroscopy (LIBS) to help interpret data coming from Mars. LIBS works by focusing laser energy at the surface of a sample, creating a plasma whose optical spectrum reveals the elemental composition of the sample. During the first µs of the plasma expansion, when performed under an atmosphere, a shock wave is created and subsequentially relaxes into an acoustic wave. Recordings this laser-induced acoustic signal is complementary to LIBS analysis as the shock wave generation depends on material physical properties such as hardness, optical absorption, and composition.

To better understand the LIBS acoustic waveform, we performed a series of LIBS combined with acoustic experiments in a large (~2m long by 1 m diameter cylinder) thermal-vacuum (TVAC) chamber at the Los Alamos National Laboratory (LANL). Several rock samples were placed at one end of the TVAC chamber. By firing through the chamber window, a BigSky/Quantel class 4 laser, operating at 20 Hz, with energies ranging from 11 to 59 mJ/pulse, produced a series of LIBS sparks (250 to 350 shots per location) on the samples. Two Brüel Kjær (B&K) microphones, operating in the 20Hz to 20kHz frequency range, were placed at 28 and 67cm from the sample plate. For each shot, digitization at 1MHz of the acoustic signals was triggered by the laser pulse, enabling measuring the acoustic wave arrival time with high-precision.

We analyze in particular, a series of LIBS shots performed on a sandstone sample. Our measurements reveal that the acoustic arrival time is dependent on the sound peak pressure showing time delays of about 0.02ms. Shot-to-shot changes in peak pressure are related to material heterogeneity and the formation of a crater with each subsequent LIBS shot. This behavior was measured at different temperature and pressure conditions and is characteristic of non-linear processes inherited from the shockwave generation. We are also developing a theoretical model to further characterize the non-linearity of acoustic-LIBs signal and assess if some part of the variance observed by SuperCam onboard Perseverance can be related to the non-linearity.