The Physics of Confined Laser Ablation and Its Applications
Wesley Keller | 19-ERD-027
Tamper confinement is utilized in pulsed laser drive applications to increase shock pressure and momentum transfer to the target. This enhancement in drive can extend the range of testing for fundamental material studies or provide more capable laboratory testbeds for material shock conditioning and propulsion system work. Under ideal conditions, the tamper is transparent to the incident laser and only serves to confine the ablative blow-off from an absorbing workpiece, thereby concentrating energy near the ablation surface to enhance recoil pressure on the target. In theory, the effectiveness of tamper confinement is only limited by thresholds for free surface energy coupling, bulk absorption, and nonlinear self-focusing. Experimental data across a range of applications utilizing tamper confinement, however, have shown saturation of impulsive drive well below these thresholds. In this study, we investigate a potential contribution to impulse saturation where thermally induced dielectric breakdown in the tamper is triggered by conductive heating from the absorbing target, leading to the formation of a laser absorption front within the tamper. The resulting migration of energy away from the target surface can significantly degrade shock drive and can be problematic for pulsed laser drive conditions well below the thresholds of concern for tamper transmission effects. This study extends previous work in this area by using time-resolved intra-pulse plasma imaging within the tamper to visualize the evolution of laser absorption during the pulse, as well as photon Doppler velocimetry measurements of workpiece vibration to characterize impulsive drive. A first principles-based modeling approach capturing dielectric breakdown and absorption within the tamper was developed within Lawrence Livermore National Laboratory's three-dimensional radiation hydrodynamics code HYDRA.
This study developed and experimentally validated new modeling capability within Livermore's radiation hydrocode HYDRA for simulating tamper confined laser drive experiments. The new modeling capability will support pre-test planning activities, robust parameter studies for process optimization, and will extend current test diagnostic capability by providing additional insight into the underlying physical processes beyond what is readily accessible through experimental measurements. The modeling capability and knowledge base generated by the study advances the lasers and optical sciences core competency of Livermore and will support a wide range of experimental programs at the Laboratory, the NNSA's nuclear security enterprise, and greater DOE national laboratory complex.
Publications, Presentations, and Patents
W. Keller, S. Ly, J. Crowhurst, A. Rousso, J. Lee, C. Boley, A. Rubenchik. N. Shen, S. Sepke, G. Guss, D. Weisz, J. Koning, R. Negres, P. Pax, S. Moon, J. Zaug, M. Matthews, J. Stolken, C. Halvorson, B. Blue, LLNL-PRES-816718 (2021).