Journal of Directed Energy
Volume 1, Number 2
The papers listed below constitute Volume 1, Number 2 of the Journal of Directed Energy.
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Superheating and Material Ablation of Metals by Multiple Ultrashort Laser Pulses
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
Q and In Situ Measurements of Temperatures on the Surfaces of Samples Irradiated by a Sequence of Picosecond Laser Pulses
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
Experiments on the Propagation and Filamentation of Ultrashort, Intense Laser Pulses in Air
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
KEYWORDS: Intense short laser pulses, Laser atmospheric propagation, Laser filamentation
Stimulated Raman Scattering of Intense Free-Electron Laser Radiation in the Atmosphere
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
Investigation into Laser Beam Correction Through Unsteady Flowfields
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
Conceptual Design of a High-Power, High-Gain Free-Electron Laser Amplifier
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
Journal of Directed Energy, Volume 1, Number 2