Time-resolved spectroscopy and modeling of underwater laser ionization and filamentation for electrical discharge guiding

Summary form only given. Laser guiding of underwater electrical discharges is being investigated at the Naval Research Laboratory. Guided underwater discharges would be useful for advanced micromachining and pulsed power switching¹. Possible mechanisms for guiding underwater discharges include either the ionized optical filament column or a vapor column generated by optically heating a filament. Our group has demonstrated the generation of underwater optical filaments over 55 cm in length (corresponding to more than 35 Rayleigh lengths), using 532 nm, 6 ns pulses with up to 50 mJ². Such meter-scale filaments could enable techniques for guiding meter-scale underwater discharges, useful for low-frequency, remote undersea acoustic generation. Spectroscopy revealed that a significant portion of laser energy during underwater filament generation and propagation is scattered in the laser focal volume by molecular Raman interaction, but not from the filament itself. Recent simulations using the HELCAP 4D nonlinear laser propagation code accurately predicted measured filament fluence profiles, however they also indicate complex time-dependent and axially non-uniform plasma behavior. Ongoing HELCAP modeling will include improved spatial and temporal resolution, and molecular stimulated Raman scattering effects. Femtosecond time-resolution perpendicular pump-probe shadowgraph images reveal gas microbubble generation throughout the pump beam path, using ns pump pulses. Such microbubbles affect laser propagation and photoionization. Planned experiments aim to elucidate microbubble generation mechanisms, and will involve deionized water, ionic water solutions, and other liquids. In addition, time-resolved spectroscopy of underwater laser-generated plasmas reveal blackbody, molecular, and atomic emission lasting up to 100 ns after the pump pulse, far exceeding typical fs collision times in liquid water. Ongoing time-resolved spectroscopy will aim to further- characterize intense underwater laser propagation, Raman scattering, and underwater plasma dynamics. Recent and ongoing measurements and simulations of underwater optical filament generation and laser ionization will be presented.