Approved for public release; distribution is unlimited. LUULUJUIJ U0"t REPORT DOCUMENTATION PAGEForm Public reporting burden for this collection of information is estimated to average 1 hour per response, including the time for reviewing instructions, searching existing data sources, gathering and maintaining the data needed, and completing and reviewing the collection of information. ABSTRACT {Maximum 200 words)The propagation of short, intense laser pulses in the atmosphere may have applications in the areas of active and passive remote sensing, electronic countermeasures, and induced electric discharges. For example, localized ultraviolet radiation generated at a remote distance can provide a source for active fluorescence spectroscopy of biological and chemical agents in the atmosphere. The generated directed pulses of intense white light may find applications in the areas of hyperspectral imaging and differential absorption spectroscopy. The propagation of short, intense laser pulses through the atmosphere is investigated. A 3D, nonlinear propagation equation is derived which includes the effects of dispersion, nonlinear self-focusing due bound electrons, stimulated molecular Raman scattering, multiphoton and tunneling ionization, pulse energy depletion due to ionization, relativistic focusing and pondcromotively excited plasma wakefields. A method for generating a remote spark in the atmosphere is proposed. Examples involving beam focusing, compression, ionization, and white light generation are studied by numerically solving the full set of 3D, nonlinear propagation equations. Coupled equations for the spot size, plasma density and power, allowing for pulse energy depletion due to ionization are derived, demonstrating the absence of extended self-guided propagation.

Intense, ultrashort laser pulses propagating in the atmosphere have been observed to emit sub-THz electromagnetic pulses (EMPS). The purpose of this paper is to analyze EMP generation from the interaction of ultrashort laser pulses with air and with dielectric surfaces and to determine the efficiency of conversion of laser energy to EMP energy. In our self-consistent model the laser pulse partially ionizes the medium, forms a plasma filament, and through the ponderomotive forces associated with the laser pulse, drives plasma currents which are the source of the EMP. The propagating laser pulse evolves under the influence of diffraction, Kerr focusing, plasma defocusing, and energy depletion due to electron collisions and ionization. Collective effects and recombination processes are also included in the model. The duration of the EMP in air, at a fixed point, is found to be a few hundred femtoseconds, i.e., on the order of the laser pulse duration plus the electron collision time. For steady state laser pulse propagation the flux of EMP energy is nonradiative and axially directed. Radiative EMP energy is present only for nonsteady state or transient laser pulse propagation. The analysis also considers the generation of EMP on the surface of a dielectric on which an ultrashort laser pulse is incident. For typical laser parameters, the power and energy conversion efficiency from laser radiation to EMP radiation in both air and from dielectric surfaces is found to be extremely small, < 10(-8). Results of full-scale, self-consistent, numerical simulations of atmospheric and dielectric surface EMP generation are presented. A recent experiment on atmospheric EMP generation is also simulated.

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. One of the purposes of this paper is to evaluate the stability properties of intense laser pulses propagating extended distances (many tens of Rayleigh ranges) in plasma channels. A three-dimensional envelope equation for the laser field is derived that includes nonparaxial effects such as group velocity dispersion, as well as wakefield and relativistic nonlinearities. It is shown that in the broad beam, short pulse limit the nonlinear terms in the wave equation that lead to Raman and modulation instabilities cancel. This cancellation can result in pulse propagation over extended distances, limited only by dispersion. Since relativistic focusing is not effective for short pulses, the plasma channel provides the guiding necessary for long distance propagation. Long pulses (greater than several plasma wavelengths), on the other hand, experience substantial modification due to Raman and modulation instabilities. For both short and long pulses the seed for instability growth is inherently determined by the pulse shape and not by background noise. These results would indicate that the self-modulated LWFA is not the optimal configuration for achieving high energies. The standard LWFA, although having smaller accelerating fields, can provide acceleration for longer distances. It is shown that by increasing the plasma density as a function of distance, the phase velocity of the accelerating field behind the laser pulse can be made equal to the speed of light. Thus electron dephasing in the accelerating wakefield can be avoided and energy gain increased by spatially tapering the plasma channel. Depending on the tapering gradient, this luminous wakefield phase velocity is obtained several plasma wavelengths behind the laser pulse. Simulations of laser pulses propagating in a tapered plasma channel are presented. Experimental techniques for generating a tapered density in a capillary discharge are described and an example of a GeV channel guided standard LWFA is presented.

Public reporting burden for this collection of information is estimated to average 1 hour per response, including the time for reviewing instructions, searching existing data sources, gathering and maintaining the data needed, and completing and reviewing this collection of information. Send comments regarding this burden estimate or any other aspect of this collection of information, including suggestions for reducing this burden to Department of Defense, Washington Headquarters Services, Directorate for Information Operations and Reports (0704-0188), 1215 Jefferson Davis Highway, Suite 1204, Arlington, VA 22202-4302. Respondents should be aware that notwithstanding any other provision of law, no person shall be subject to any penalty for failing to comply with a collection of information if it does not display a currently valid OMB control number. PLEASE DO NOT RETURN YOUR FORM TO THE ABOVE ADDRESS. REPORT DATE (DD-MM-YYYY)2 The interaction of intense, femtosecond (fsec) laser pulses with a dielectric medium is examined using a numerical simulation. The simulation uses the 1-D electromagnetic wave equation to model laser pulse propagation. In addition, it includes multiphoton ionization, electron attachment, ohmic heating of free electrons, and temperature dependent collisional ionization. Laser pulses considered in this study are characterized by peak intensities -1012 to 1014 W/cm and pulse durations -10 to 100 fsec. These laser pulses, interacting with fused silica, are shown to produce abovecritical plasma densities and electron energy densities sufficient to attain experimentally measured damage thresholds. Significant transmission of laser energy is observed even in cases where the peak plasma density is above the critical density for reflection. A damage fluence based on absorbed laser energy is calculated for various pulse durations. The calculated damage fluence is found to be consistent with recent experimental results. SPONSORING I MONITORING AGENCY NAME(S) AND ADDRESS(ES) 10. SPONSOR / MONITOR'S ACRONYM(S) ONR SUBJECT TERMSUltra-short laser pulse; Laser-dielectric interaction; Ionization; Laser damage 16.

A relativistically intense femtosecond laser pulse propagating in a plasma channel undergoes dramatic photon deceleration while propagating a distance on the order of a dephasing length. The deceleration of photons is localized to the back of the pulse and is accompanied by compression and explosive growth of the ponderomotive potential. Fully explicit particle-in-cell simulations are applied to the problem and are compared with ponderomotive guiding center simulations. A numerical Wigner transform is used to examine local frequency shifts within the pulse and to suggest an experimental diagnostic of plasma waves inside a capillary.