When laser energy is deposited in a target material, numerous complex processes take place at length and time scales that are too small to visually observe. To study and ultimately fine-tune such processes, researchers look to computer modeling. However, these simulations rely on accurate equation of state (EOS) models to describe the thermodynamic properties - such as pressure, density and temperature - of a target material under the extreme conditions generated by the intense heat of a laser pulse.
One process that is insufficiently addressed in current EOS models is ablation, where the irradiation from the laser beam removes solid material from the target either by means of vaporization or plasma formation (the fourth state of matter). It is this mechanism which launches a shock into the material, ultimately resulting in the high densities required for high pressure experiments such as inertial confinement fusion (ICF).
To better understand laser-matter interactions with regard to ablation, researchers from Lawrence Livermore National Laboratory (LLNL), the University of California, San Diego (UCSD), SLAC National Accelerator Laboratory and other collaborating institutions conducted a study that represents the first example of using X-ray diffraction to make direct time-resolved measurements of an aluminum sample's ablation depth. The research appears in Applied Physics Letters.
Controlling material ablation depths is crucial for various scientific and industrial processes, including laser fusion and astrophysical research, among other areas. However, measuring ablation depth in the picosecond time regime (one trillionth of a second) is a longstanding issue of laser-induced shock experiments. This is because previous approaches have generally relied on post-irradiation analysis of the target material, which makes it difficult or impossible to track the evolution of material response and can result in discrepancies due to effects such as spall (a material deformation effect in response to stress waves) that occur over the duration of the experiment.
The study, led by Sophie Parsons, a UCSD graduate student participating in LLNL's Academic Cooperation Program, utilized X-ray diffraction data that was previously collected by LLNL scientists Mike Armstrong, Harry Radousky and Jon Belof during laser experiments in 2016. Parsons, along with her LLNL mentors Radousky and Armstrong, analyzed this data to extract new information from the solid phases of aluminum, whereas previous analyses had only accounted for the point at which the laser shot melted the sample.
The team mathematically compared the unshocked thickness of aluminum to the amount of ablated aluminum over time to obtain in situ measurements as shock waves propagated through the aluminum layer of the target. Their in-situ method can zero in on the laser-matter interactions taking place on the picosecond (one trillionth of a second) time scale, enabling them to directly measure and isolate the effects that occur during the initial laser-surface interaction.
Within the first 10 picoseconds of the laser's interaction with the aluminum surface, researchers observed a rapid decrease in the volume of solid material.
"This is likely due to the rapid formation of an approximately 500-nanometer thick plasma layer at the laser-illuminated surface, which is what we are referring to as the 'ablation depth,'" said Armstrong, co-author of the paper. Thereafter, the loss of ambient material is steady over time as the shock wave moves through the remaining unablated aluminum.
This research is part of an ongoing series of studies under the Materials Science in Extreme Environments University Research Alliance (MSEE). Future work could include the generation of a comprehensive database of ablation depths, said Radousky, LLNL's MSEE principal investigator. As more data is collected, researchers can use this technique to further understand the physics of laser ablation.
-Shelby Conn
