Underwater laser filamentation and guiding of electrical discharges

Summary form only given. A technique to laser trigger and guide electrical discharges in an underwater environment is currently being developed at the Naval Research Laboratory. This work has potential applications in advanced micromachining, similar to that currently performed using electric discharge machining and water-assisted femtosecond laser machining, and low-jitter pulsed power generator switches¹. In order to expand on this technique, we are studying the generation of underwater optical filaments. Optical filamentation is the process in which a sufficiently high power laser beam propagating within a medium will nonlinearly self-focus. Once the laser beam focuses down to an intensity high enough to ionize the medium, a plasma region forms resulting in plasma induced defocusing. These two processes reach equilibrium and guide the beam over a long distance forming a long "filament-like" plasma region. The residual plasma trail can also form a vapor channel after recombination, heating, and hydrodynamic expansion. Both the plasma filament and vapor channel can be useful structures for the guiding of electrical discharges. Previous work by our group has shown electrical guiding in air using high power femtosecond laser pulses to produce atmospheric optical filaments to guide discharges over several meters. While the generation of underwater filaments has been shown using femtosecond laser systems, our group has recently observed that much longer nanosecond pulse lasers are also capable of producing filaments. In particular, several filaments were observed using a 60 mJ Q-switched frequency doubled Nd:YAG laser. These filaments had characteristic diameters of-100 μm and persisted for over 10 Rayleigh lengths. The advantages of using such longer laser pulses are that they contain more energy, resulting in higher conductivity plasma and/or a larger subsequent vapor channel. Also, the corresponding laser systems are less expensive, more robust, - nd tend to exhibit better beam quality.Experimental results of underwater filamentation using both short and long pulse lasers will be presented. Additionally, plans and preliminary results for the guiding of electrical discharges will be discussed.