Journal of Directed Energy
Volume 1, Number 2 Spring 2004

The papers listed below constitute Volume 1, Number 2 of the Journal of Directed Energy. Print copies of issues of the Journal of Directed Energy are available through the online store.

Superheating and Material Ablation of Metals by Multiple Ultrashort Laser Pulses (500 KB)
J.K. Chen and J.E. Beraun, Air Force Research Laboratory

For metals and metal-like materials subjected to high-power ultrashort-pulse laser heating, phase explosion is believed to be a dominating mechanism that sputters the material away. The two-step heating equations along with an isothermal solid-liquid phase transformation are proposed to evaluate the amount of ablated material by ultrafast lasers. Numerical analysis is performed for gold films in vacuum irradiated by an ultrashort UV laser. When the superheated liquid temperature at a grid point reaches 0.9Ttc (Ttc denoting the thermodynamic equilibrium critical temperature), phase explosion is assumed and the grid point is thus removed. The calculated ablation depth matches well with experimental data. It is found that two consecutive pulses split from a high-power, ultrashort-pulse laser beam could ablate more material than the single beam if the first pulse fluence is greater than the ablation threshold.
KEYWORDS: Laser ablation, Phase explosion, Superheating, Ultra-short pulsed laser
PAGES 93-109

Q and In Situ Measurements of Temperatures on the Surfaces of Samples Irradiated by a Sequence of Picosecond Laser Pulses (650 KB)
J. Grun and others, Naval Research Laboratory and other affiliations

One-inch-diameter samples of various materials, such as stainless steel, painted and unpainted aluminum, fused silica, and fiberglass composites, were irradiated for several seconds, at pulse repetition rates of tens of megahertz, by a sequence of picosecond-duration pulses from a free-electron laser operating at a wavelength of 3.1 µm. In a typical experiment the laser irradiated a few-millimeter-diameter spot with a number of kilojoules, resulting in kilowatt-per-square-centimeter average irradiances and substantially higher peak irradiances. These experiments were the first to examine the lethality, measured as Q* (laser energy in kilojoules required for removing 1g of material), of a long sequence of picosecond-duration laser pulses in an irradiance regime relevant to high energy laser lethality, i.e., utilizing laser intensities that have a chance of propagating a significant distance in the atmosphere without much loss of energy. In situ diagnostics were used to measure space- and time-resolved temperature profiles on the laser-irradiated and backsides of the samples. Heating and cooling of the samples, melting, boiling, hole burning, flow of liquid matter, and breakup of composites were measured. Results of these experiments and a comparison to postshot analysis of the samples are presented.
KEYWORDS: Free electron, Laser, Lethality, Picosecond, Q*, Temperature
PAGES 111-129

Experiments on the Propagation and Filamentation of Ultrashort, Intense Laser Pulses in Air (500 KB)
A. Ting and others, Naval Research Laboratory and other affiliations

Ultrashort (femtosecond), high-power laser pulses can exceed the threshold for nonlinear self-focusing in air resulting in extended propagation distances due to the dynamical balance between the plasma formation and the nonlinear focusing. Experiments were performed using the chirped-pulse-amplification (CPA) lasers at the Naval Research Laboratory to study the physics of self-guiding of the laser beams for extended distances and formation of multiple laser and plasma filaments. Filament dimensions and spectral content of the filaments were measured. An empirical value of the nonlinear index of refraction of air was determined by comparing the experimental profiles of the filaments with the simulation results from a fully self-consistent three-dimensional laser propagation model. The nonlinear index is found to be substantially smaller then the values reported for long (nanosecond) pulses and to agree with recent reported values for femtosecond laser pulses. The measured optical spectra of the “white” light generated in the laser propagation revealed the presence of molecular plasmas useful for chem/bio agent identification.
KEYWORDS: Intense short laser pulses, Laser atmospheric propagation, Laser filamentation
PAGES 131-141

Stimulated Raman Scattering of Intense Free-Electron Laser Radiation in the Atmosphere (1,150 KB)
J.R. Penano, P. Sprangle, P. Serafim, B. Hafizi, and A. Ting; Naval Research Laboratory and other affiliations

Stimulated rotational Raman scattering is known to be one of the main factors limiting the propagation of high-power laser beams in the atmosphere. A set of three-dimensional, fully time-dependent propagation equations describing the stimulated Raman interaction and propagation of short, ~picosecond laser pulses and pulse trains is presented and discussed. The laser pulses considered in this study are indicative of those generated by a megawatt-class free-electron laser (FEL) based on a radio frequency linac. In addition to the Raman interaction, the equations include other effects such as the optical Kerr nonlinearity due to bound electrons and group velocity dispersion, both of which are important for FEL pulses. The effective time-dependent nonlinear refractive index containing both Kerr and Raman processes is derived. Numerical simulations based on solving the propagation equations in three dimensions show the detailed evolution of the Raman scattering instability for various pulse formats. Stabilization of the Raman instability for pulses with durations much shorter than the rotational period is demonstrated. The interaction of FEL pulses in a train through the Raman polarization field is also illustrated.
KEYWORDS: Atmospheric propagation, Free-electron laser, Stimulated Raman scattering
PAGES 143-159

Investigation into Laser Beam Correction Through Unsteady Flowfields (1,300 KB)
R.S. St.John, D.J. Link, B. Foucault, M.I. Jones, and E.E. Bender; Science Applications International Corporation and other affiliations

Time-varying unsteady aircraft flowfields can seriously distort propagating high-energy laser (HEL) beams. Computational fluid dynamic (CFD) codes have matured sufficiently to accurately calculate time-resolved unsteady flow properties for airborne HEL system installations. Aero-optical postprocessing software that calculates wavefront errors from unsteady CFD predictions has also been developed and interferometrically validated. These codes may be utilized to derive stroke, bandwidth, and modality requirements for adaptive optic (AO) systems needed to correct time-varying flowfield aberrations and maximize HEL target irradiance. Aero-optic wavefront error maps from unsteady flowfields surrounding an HEL turret at realistic flight conditions were generated by Lockheed Martin Aeronautics Company. Science Appplications International Corporation, using the aforementioned wavefront error maps, utilized an idealistic AO system to examine AO requirements for airborne HEL beam correction. A Hartmann wavefront sensor (WFS) and deformable mirror (DM) combination was used for higher order wavefront error correction and a tracker and fast-steering mirror combination for tilt correction. Temporal outputs of original and residual wavefront errors, Zernike coefficients, DM commands, WFS outputs, and Strehl analyses, as well as derived AO system requirements, are given for various WFS subaperture field of view and for various DM and fast-steering mirror control bandwidths.
KEYWORDS: Adaptive optics, Aero-optics, Unsteady flowfield aero-optics wavefront errors
PAGES 161-170

Conceptual Design of a High-Power, High-Gain Free-Electron Laser Amplifier (650 KB)
D.C. Nguyen, S.S. Kurennoy, L.M. Young, and H.P. Freund; Los Alamos National Laboratory and other affiliations

High-gain free-electron laser (FEL) amplifiers offer unique advantages such as robust operation without a high-Q optical cavity and potentially high extraction efficiencies with the use of tapered wigglers. Although a high average power, continuous-wave FEL amplifier has not been demonstrated, many key physics issues such as electron beam brightness requirements, single-pass gain, saturation, etc., have been resolved. We study the feasibility of a high-power FEL based on the high-gain, combination-wiggler amplifier. We show that with suitable electron beam parameters, peak output power of 1 GW can be directly achieved. We also outline a possible configuration of a high-power, high-gain FEL amplifier with energy recovery.
KEYWORDS: Energy recovery, High-gain FEL amplifier, High-power FEL
PAGES 171-181

Journal of Directed Energy, Volume 1, Number 2

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