Joseph Penano

Incoherent Combining and Atmospheric Propagation of High-Power Fiber Lasers for Directed-Energy Applications

High-power fiber lasers can be incoherently combined to form the basis of a directed high-energy laser system which is highly efficient, compact, robust, low-maintenance and has a long operating lifetime. This approach has a number of advantages over other beam combining methods. We present results of the first field demonstration of incoherent beam combining using kilowatt-class, single-mode fiber lasers. The experiment combined four fiber lasers using a beam director consisting of individually controlled steering mirrors. Propagation efficiencies of ~90%, at a range of 1.2 km, with transmitted continious-wave power levels of 3 kW were demonstrated in moderate atmospheric turbulence. We analyze the propagation of combined single-mode and multimode beams in atmospheric turbulence and find good agreement between theory, simulations and experiments.

Propagation of ultra-short, intense laser pulses in air

Recent theoretical, computational, and experimental work carried out at the Naval Research Laboratory on the propagation of ultra-short laser pulses in air is presented. Fully time-dependent, three-dimensional, nonlinear equations describing the propagation of laser pulses in air under the influence of diffraction, group velocity dispersion,Kerr nonlinearity, stimulated Raman scattering,ionization, and plasma wakefield excitation are presented and analyzed. The propagation code, HELCAP [P. Sprangle, J. R. Penano, and B. Hafizi, Phys. Rev. E 66, 046418 (2002)], is used to simulate the propagation of laser pulses in air under the influence of the physical processes mentioned above. Simulations of laser filamentation together with experimental measurements are used to confirm that the filamentation process is dependent on pulse duration. An equilibrium configuration for optical and plasma filaments in air is derived and the dynamic guiding and spectral broadening of a laser pulse is modeled. The effect of atmospheric turbulence on nonlinear self-focusing is demonstrated. Simulations of a recent electromagnetic pulse (EMP) generation experiment are also presented and the efficiency of EMP generation is determined and found to be extremely small.

Relativistic effects on intense laser beam propagation in plasma channels

Propagation characteristics of a radiation beam in a preformed, tapered plasma channel are analyzed by means of an envelope equation for the beam spot size. The model allows for relativistic focusing and ponderomotive channeling, radial and axial density gradients, and is valid for arbitrary intensity. The characteristics of laser beam propagation are shown to be governed by two parameters, the ratio of laser power to the critical power for relativistic focusing, and a dimensionless focusing strength parameter that includes contributions from both relativistic and channel focusing. The envelope equation provides a unified approach for exploring diverse applications such as designing a tapered laser wakefield accelerator or a plasma lens. The model is employed in interpretation of pump–probe laser propagation experiments and an x-ray source experiment. Full-scale simulations of a plasma channel lens are presented and shown to be in excellent agreement with the analytical results.

Generation of ELF electromagnetic waves in the ionosphere by localized transverse dc electric fields: Subcyclotron frequency regime

It is commonly believed that Alfvenic waves observed in the ionosphere originated at trans ionospheric altitudes. However, recent observations of waves localized over skin depth scales, which are prone to Landau damping, and upward going Poynting flux suggest an ionospheric source. This theoretical study establishes the generation of electromagnetic waves at subcyclotron frequencies by localized static electric fields of the type commonly observed in the auroral ionosphere. The problem is formulated in terms of an eigenvalue system of equations which can describe all cold plasma normal modes, ion acoustic waves, and the various mode couplings induced by nonuniform electric fields. It is found that the velocity shear associated with the background electric field can significantly affect the observable properties of Alfven waves. In particular, Alfven waves can be destabilized when the magnitude of the velocity shear frequency exceeds the ion cyclotron frequency. Velocity shear can also significantly modify the ratio of E/B for these waves. The analysis further reveals electromagnetic Kelvin-Helmholtz instabilities, which are non-Alfvenic, with lower velocity shear thresholds. We model conditions encountered by several rockets and satellites, for example, FAST, Freja, AMICIST, AT2, to analyze wave properties and find that for typical ionospheric parameters the Kelvin-Helmholtz instabilities can generate electromagnetic waves with broadbanded spectra and other physical characteristics similar to observations. It is also shown that these waves are capable of resonant interaction with electrons over localized regions and hence may be important to understanding the generation of suprathermal electron bursts.

Remote lasing in air by recombination and electron impact excitation of molecular nitrogen

We analyze and simulate the physical mechanisms for a remote atmospheric lasing configuration which utilizes a combination of an ultrashort pulse laser to form a plasma filament of seed electrons, and a heater beam to heat the seed electrons. Nitrogen molecules are excited by electron impact and recombination processes to induce lasing in the ultraviolet. Recombination excitation, thermal excitation, gain, and saturation are analyzed and simulated. The lasing gain is sufficiently high to reach saturation within the length of the plasma filament. A remotely generated ultraviolet source may have applications for standoff detection of biological and chemical agents.

GeV acceleration in tapered plasma channels

To achieve multi GeV electron energies in the laser wakefield accelerator (LWFA) it is necessary to propagate an intense laser pulse long distances in a plasma without disruption. A three-dimensional envelope equation for the laser field is derived that includes nonparaxial effects, wakefields, and relativistic nonlinearities. In the broad beam, short pulse limit the nonlinear terms in the wave equation that lead to Raman and modulation instabilities cancel. Long pulses (several plasma wavelengths) experience substantial modification due to these instabilities. The short pulse LWFA, although having smaller accelerating fields, can provide acceleration for longer distances in a plasma channel. By allowing the plasma density to increase along the propagation path electron dephasing can be deferred, increasing the energy gain. A simulation example of a GeV channel guided LWFA accelerator is presented. Simulations also show that multi-GeV energies can be achieved by optimally tapering the plasma channel.

Remote atmospheric breakdown for standoff detection by using an intense short laser pulse

A remote atmospheric breakdown is a very rich source of UV and broadband visible light that could provide an early warning of the presence of chemical-biological warfare agents at extended standoff distances. A negatively chirped laser pulse propagating in air compresses in time and focuses transversely, which results in a rapid laser intensity increase and ionization near the focal region that can be located kilometers away from the laser system. Proof-of-principle laboratory experiments are performed on the generation of remote atmospheric breakdown and the spectroscopic detection of mock biological warfare agents. We have generated third harmonics at 267 nm and UV broadband radiation in air from the compression and focusing of femtosecond laser pulses. Fluorescence emission from albumin aerosols as they were illuminated by the femtosecond laser pulse has been observed.

Longitudinal compression of short laser pulses in air

We have performed laboratory experiments to study long distance propagation of large bandwidth ultrashort laser pulses in air. Initial pulse length, frequency chirping, and laser pulse energy were varied where the maximum propagation distance was up to 105 m. We have demonstrated the compression of initially negatively chirped low intensity laser pulses due to the linear group velocity dispersion of air. The characteristics of the compressed pulse such as pulse duration and spectral chirping were found to be significantly affected by the laser pulse intensity, with higher intensities corresponding to longer minimum compressed pulse duration.

Velocity shear-driven instabilities in a rotating plasma layer

The linear stability of a radially localized layer rotating about the cylindrical axis in a magnetized plasma is investigated using an eigenvalue analysis. The eigenvalue equation is solved numerically in a parameter regime characteristic of the Space Physics Simulation Chamber (SPSC) experiments [Amatucci et al., Phys. Rev. Lett. 77, 1978 (1996)] at the Naval Research Laboratory (NRL). Four types of instabilities are predicted. They are type-A and type-B Kelvin-Helmholtz instabilities, a transverse current-driven instability, and the inhomogeneous energy density driven instability (IEDDI). A quantitative comparison between theory and experiment indicates that an experimentally observed fluctuation in a rotating plasma layer is an IEDDI.

Microwave diagnostics of femtosecond laser-generated plasma filaments

We present a simple non-intrusive experimental method allowing a complete single shot temporal measurement of laser produced plasma filament conductivity. The method is based on filament interaction with low intensity microwave radiation in a rectangular waveguide. The suggested diagnostics allow a complete single shot temporal analysis of filament plasma decay with resolution better than 0.3 ns and high spatial resolution along the filament. The experimental results are compared to numerical simulations, and an initial electron density of 7 × 1016 cm−3 and decay time of 3 ns are obtained.