T.G. Jones

Formation and propagation of meter-scale laser filaments in water

We report the demonstration, characterization, and modeling of meter-scale underwater optical filaments using a nanosecond pulsed laser. We observed single filament formation for P/PCRIT = 1–5, where PCRIT ∼ 1 MW in water. We employed a variable distance water tube to characterize laser pulse evolution and filament formation. Filaments with uniform radius 50 ± 10 μm persisted for 55 cm (>35 Rayleigh lengths). Significant forward Stimulated Raman Scattering (up to 60%) was observed and characterized. Simulation results for propagation distances and radii were in agreement with experiment, and predict a structured plasma with peak density of 1.5 × 1018 cm−3.

Trapping and acceleration of nonideal injected electron bunches in laser wakefield accelerators

Most conceptual designs for future laser wakefield accelerators (LWFA) require external injection of precisely-phased, monoenergetic, ultrashort bunches of MeV electrons. This paper reports simulation and Hamiltonian models of several nonideal injection schemes that demonstrate strong phase bunching and good accelerated beam quality in a channel-guided LWFA. For the case of monoenergetic, unphased (long bunch) injection, there is an optimum range of injection energies for which the LWFA can trap a significant fraction of the injected pulse while producing an ultrashort, high-quality accelerated pulse. These favorable results are due to a combination of pruning of particles at unfavorable phases, rapid acceleration, and strong phase bunching. Also, the plasma channel introduces a favorable shift in the region of accelerating phase where electrons are focused, which can significantly reduce the required injection energy. Simulation results agree well with the predictions of the Hamiltonian model. Simulations of phased injection with a broad injected energy spread also exhibit final accelerated bunches with small energy spread. These results suggest that relatively poor quality injection pulses may still be useful in LWFA demonstration experiments.

Stimulated Raman scattering and nonlinear focusing of high-power laser beams propagating in water

The physical processes associated with propagation of a high-power (power > critical power for self-focusing) laser beam in water include nonlinear focusing, stimulated Raman scattering (SRS), optical breakdown, and plasma formation. The interplay between nonlinear focusing and SRS is analyzed for cases where a significant portion of the pump power is channeled into the Stokes wave. Propagation simulations and an analytical model demonstrate that the Stokes wave can re-focus the pump wave after the power in the latter falls below the critical power. It is shown that this novel focusing mechanism is distinct from cross-phase focusing. The phenomenon of gain-focusing discussed here for propagation in water is expected to be of general occurrence applicable to any medium supporting nonlinear focusing and stimulated Raman scattering.

Simulation of accelerated electron spectra in laser wakefield accelerators

The generation of high quality electron beams in a laser wakefield accelerator (LWFA) is generally thought to require phased injection of an ultrashort electron bunch. However, simulations have shown that longer bunch, unphased injection may also produce high quality, short electron bunches if the injection energy is property chosen. The process involves pruning of electrons that move into defocusing portions of the wakefield, along with strong phase bunching and rapid acceleration. Simulation results are consistent with a simple Hamiltonian model that numerically integrates particle orbits in an idealized wake potential. Simulations are also presented for a channel-guided LWFA system using optical injection.

Stimulated Raman and Brillouin scattering, nonlinear focusing, thermal blooming, and optical breakdown of a laser beam propagating in water

The physical processes associated with propagation of a high-power laser beam in a dielectric include self-focusing, stimulated Raman scattering, stimulated Brillouin scattering, thermal blooming, and multiphoton and collisional ionization. The interplay between these processes is analyzed using a reduced model consisting of a few differential equations that can be readily solved, enabling rapid variation of parameters and the development of theoretical results for guiding new experiments. The presentation in this paper is limited to propagation of the pump, the Stokes Raman, and the Brillouin pulses, ignoring the anti-Stokes Raman. Consistent with experimental results in the literature, it is found that self-focusing has a dramatic effect on the propagation of high-power laser beams in water. A significant portion of the pump laser energy is transferred to Stokes Raman forward scatter along with a smaller portion to Brillouin backscatter.

Particle-in-cell simulations of optical injectors for plasma accelerators

Two possible methods of optically injecting electrons into a plasma accelerator are the self-modulated laser wakefield accelerator (SMLWFA) and laser ionization and ponderomotive acceleration (LIPA). A magnetic selection scheme is proposed to select a narrow band of energies from the intrinsically broad beam produced by the SML- WFA. The scheme is analyzed using 2D particle-in-cell (PIC) simulations and 3D ray tracing. The effects of space charge on the ideal LIPA distribution are examined using full 3D PIC simulations.

Nonlinear Propagation of 100 ps, UV Laser Pulses in Water with Strong Stimulated Raman Stokes Coupling

Underwater UV laser pulse propagation experiments were performed at intensities spanning the linear and nonlinear regimes. Measurements and simulations show strong coupling to molecular Raman modes and suggest strong ionization-induced refraction near the beam focus.

Intense underwater laser propagation, ionization and heating for remote shaped plasma generation

Summary form only given. Intense underwater laser propagation, filamentation, and ionization are beingIntense underwater laser propagation, filamentation, and ionization are being investigated at NRL for many Navy applications, including remote undersea laser acoustic generation for low-frequency long-range sonar, as well as advanced micromachining using underwater laser-guided discharges. The key to low-frequency laser acoustic generation is shaping of the heated underwater volume, with more elongated volumes producing longer-duration acoustic pulses with more energy at low-frequencies. A patented scheme for generating elongated, meter-scale, high energy density underwater plasmas1 is under study, in which a filamenting laser pulse could serve as a target for a second energetic “heater” laser pulse. Early experiments suggest improved ionization efficiency using a ps filament pulse at either 266 nm or 355 nm, together with a 532 nm ns heater pulse. Time-resolved absorption spectroscopy of intense underwater propagation and ionization enabled measurement of hydrated electron density of 5.4 × 1018 cm-3 and lifetime of 350 ps. High-resolution fluorescence imaging of ns underwater laser propagation, using twophoton absorbing dye, confirmed previous measurements of 100 μm diameter filament structures2. Intense underwater laser propagation can involve both strong forwardpropagating stimulated Raman scattering (SRS) and backward-propagating stimulated Brillouin scattering (SBS), motivating our construction of a new lab-frame nonlinear laser propagation code. 1D and 2D analytical calculations and numerical simulations are underway to predict beam envelope propagation, filamentation, and early photoionization and plasma heating behavior. Analytical modeling revealed that cross-phase modulation by SRS can dominate the pump beam envelope dynamics for some conditions3. Results from recent experiments and modeling will be presented.

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.

High-resolution femtosecond measurements of underwater laser ionization and filamentation for electrical discharge guiding

Summary form only given. Laser triggering and guiding of underwater electrical discharges is being investigated at the Naval Research Laboratory. Laser-guided underwater discharges have potential applications in advanced micromachining and pulsed power switching1. Key elements of this technology are underwater laser ionization and the generation of extended underwater optical filaments. We report several new measurements of underwater laser ionization, including high resolution imaging using a 2-laser pump-probe technique with femtosecond time resolution. We used 532 nm, 4 ns, 20 mJ lens-focused pump pulses to ionize a water sample, and independently-timed 400 nm, 50 fs, submillijoule perpendicular-propagating probe pulses to generate shadowgraphs and interferograms. Shadowgraph images appear to show gas bubbles approximately 5 μm in diameter throughout the pump beam path. The number and density of these bubbles was observed to increase with time during the pump pulse. A sufficiently dense trail of residual gas bubbles can merge to form a vapor channel, which has been shown to guide underwater discharges1. Time-resolved spectra of nanosecond laser-ionized water reveal black-body radiation lasting more than 10 ns after the ionizing pump pulse. Interferograms of underwater volumes ionized using 800 nm, 50 fs, 20 mJ laser pulses revealed plasma lifetimes of order 10 ps or less. Recently, using 532 nm, 4 ns pulses with up to 60 mJ, our group demonstrated the generation of underwater optical filaments over 55 cm in length, corresponding to more than 35 Rayleigh lengths. To our knowledge, this is a record length for an underwater optical filament, and could enable techniques for guiding underwater discharges over distances of order of a meter or longer2. Underwater beam profiles were imaged at 4 cm intervals, revealing a characteristic filament diameter of ~100 μm. Simulations using the HELCAP 4D nonlinear laser propagation code also predict reproducible optical filaments of the same diameter. Plans and recent measurements for underwater optical filament characterization, as well as laser guiding of electrical discharges, will be discussed.