Listening to laser sparks: a link between Laser-Induced Breakdown Spectroscopy, acoustic measurements and crater morphology

Abstract

In preparation for the SuperCam/Mars Microphone scientific investigation, the acoustic signal associated with the plasma formation during Laser-Induced Breakdown Spectroscopy (LIBS) experiment is studied with regard to the shot-to-shot evolution of the laser induced crater morphology and plasma emission lines. A set of geological targets are depth profiled using a specifically designed LIBS setup coupled with acoustic test bench under ambient terrestrial atmosphere. Experiments confirm that the decrease of the acoustic energy as a function of the number of shots is well correlated with the target hardness/density and also demonstrate that the acoustic energy can be used as a remote tracer of the ablated volume of the target. Listening to LIBS sparks provides a new information relative to the ablation process that is independent from the LIBS spectrum.

Pulsed laser-induced heating of mineral phases: Implications for laser-induced breakdown spectroscopy combined with Raman spectroscopy

“…This unit consists mainly of the laser and the telescope, and is combined with the ChemCam Body Unit Engineering model including the spectrometers and detectors. A Nd : KGW laser beam delivering an energy of 24 mJ (11 mJ at the sample surface) at 1067 nm, with a pulse duration of 6 ns at full width at half maximum (FWHM) and a 3 Hz repetition rate, is focused onto a target in a laser spot of around 400±100 micrometers in diameter [15,19,20]. Distribution of the laser energy is not perfectly gaussian yielding LIBS crater with triangular shape visible on some targets.…”

“…Moreover, in order to investigate a possible influence of the atmospheric conditions on the LIBS effect on mineral structure, various environmental conditions have been tested by carrying out the LIBS analyses under (1) Earth atmosphere, (2) vacuum (pressure below 10 -3 mbar) and (3) Martian atmosphere (6 mbar of 96% CO2, 2.6% N2 and 1.4% Ar). The number of laser shots was also tested by performing series of 5,10,20,30,100 or 150 shots depending on the session and/or target. Thirty shots is the nominal number of laser shots for ChemCam analyses on Mars and this will likely be the same for SuperCam.…”

“…Some grains observed inside the crater were likely mechanically removed by the laser-target interaction or shockwave and then fell back into the crater.The crater depths are different for powdered materials vs. hard samples (crystals, rocks). The depth of the LIBS craters for pellet samples (apatite FOH, apatite OH and gypsum 1) can reach several hundreds of micrometers up to a millimeter, while for the hard type samples, the depth extends to a few micrometers only[15,19,20]. In terms of microtexture, the crater in apatite OH has a molten appearance including bubble-shaped structures, while the crater of gypsum 1 does not exhibit any microtextural feature indicative of possible melting(Figure 1e).…”

Shock Waves in Laser-Induced Plasmas

“…After more than 30 years the idea of using the acoustic wave intensity as a measure of the ablated mass was revived by Murdoch et al [41] and Chide et al [42], with the aim of improving the performances of the SuperCam LIBS instrument which is supposed to be delivered to Mars in 2020 [43].…”

“…The propagation of acoustic waves on Mars may be, on the other hand, quite problematic (the tests reported in [42] were performed in terrestrial conditions, at normal atmospheric pressure), and the entity of the interference of the Martian wind has yet to be quantified.…”