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DIRECTED
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Journal of Directed Energy

Volume 2, Number 2 
Fall 2006 


The papers listed below constitute Volume 2, Number 2 of the Journal of Directed Energy. Print copies of issues of the Journal of Directed Energy are available through the online store.. DEPS members enjoy access the complete technical papers through links in the paper titles. Members should sign in to their online account and return to this page to access this additional content.  Join DEPS 
Modeling of Antimortar Lethality by a SolidState HeatCapacity Laser
C.D. Boley and A.M. Rubenchik, Lawrence Livermore National Laboratory
We have studied the use of a solidstate heatcapacity laser (SSHCL) in mortar defense. This type of laser, as built at
Lawrence Livermore National Laboratory, produces highenergy pulses with a wavelength of about 1 µm and a pulse
repetition rate of 200 Hz. Currently, the average power is about 26 kW. Our model of target interactions includes
optical absorption, twodimensional heat transport in the metal casing and explosive, melting, wind effects (cooling
and melt removal), highexplosive (HE) reactions, and mortar rotation. The simulations continue until HE initiation is
reached. We first calculate the initiation time for a range of powers on target and spot sizes. Then we consider an
engagement geometry in which a mortar is fired at an asset defended by a 100kW SSHCL. Propagation effects such as
diffraction, turbulent broadening, scattering, and absorption are calculated along the trajectory, by means of a
validated model. We obtain kill times and fluences, as functions of the rotation rate. These appear quite feasible.
KEYWORDS: Engagement scenario, High Explosive, Lethality, Mortar, Solidstate laser
PAGES 97106

Propagation of HighEnergy Laser Beams Through Atmospheric Stagnation Zones
Joseph Penano, Phillip Sprangle, and Bahman Hafizi , Pennsylvania State University
Stagnation zones, i.e., regions in which the effective transverse wind velocity is zero, can greatly enhance the thermal
blooming of highenergy laser beams (HELs) in the atmosphere. An approximate expression for the Strehl ratio of a
focused HEL beam propagating through a stagnant zone is derived. The propagation of a HEL beam in a maritime
atmosphere is modeled in a fully threedimensional and timedependent manner using the HELCAP propagation code. In the
simulations, the beam is focused onto a remote target, and a stagnation zone is created by slewing the laser in the
direction of the wind. The laser power delivered to the target is calculated as a function of the slew rate. For the
parameters considered, it is found that a stagnation zone near the laser source has little effect on the propagation
efficiency while a stagnation zone near the target can significantly reduce the power on the target.
KEYWORDS: Atmospheric propagation, Highenergy laser, Stagnation zone
PAGES 107117

Compact, Optically Guided, Megawatt FreeElectron Laser Amplifier for Maritime Propagation
Phillip Sprangle, Joseph Penano, and Bahman Hafizi, Naval Research Laboratory
We present a conceptual design and parameters for a compact, megawattclass, optically guided, freeelectron laser (FEL)
amplifier. Fullscale laser propagation simulations, including the effects of aerosols, turbulence, and thermal
blooming, indicate that operating the FEL in water vapor transmission windows (i.e., 1.045, 1.625, and 2.141 µm) can
result in high propagation efficiencies in a maritime environment (>50% over a 5km range). In the present design we
choose the FEL wavelength to be 2.141 µm because of propagation and generation advantages as well as for eye safety
reasons. The average output power of this highgain FEL amplifier is ~1.5 MW, and the input is provided by a
lowaveragepower (<50W) FEL oscillator. Diffractive spreading of the FEL output beam is sufficiently large ot allow
the first grazing angle relay mirror to be close to the exit of the wiggler without exceeding the mirror damage
intensity threshold level, which is taken to be 50 kW/cm2. In this design the wiggler length is <2 m and, depending on
the mirror grazing angle and the optical beam quality, the distance from the wiggler exit to the relay mirror can be
<3 m.
KEYWORDS: Freeelectron laser amplifier, Maritime propagation, Optical guiding
PAGES 119132

Most Compact Pulse Power Supply for Narrowband HighPower Microband Systems
W.C. Nunnally, R. O'Connell, and D. Giorgi; University of MissouriColumbia and other affiliations
The most compact pulse power system is one in which the functions of energy storage, voltage scaling, and pulse shaping
are conducted in the same unit. A stacked Blumlein line (SBL) system addresses these functions in the same volume such
that the energy density approaches the density of the energy storage dielectric. The critical elements in an SBL are
the stage switches, which must close with precise timing, have inherently low inductance and rapid resistance
transition time, and handle the required voltage and current. The newly developed OptiSwitch units are capable of
enabling a microsecond 500kV1MV pulse power unit to be constructed with pulse rates of up to several kilohertz.
Since volume and mass are important parameters, the energy storage transmission lines are the major portion of the
system volume when optical fiber coupled, OptiSwitch devices are employed. Higher permittivity ceramic capacitors are
used to reduce the volume of the system and the length of the transmission lines required. A point design of an
OptiSwitched SBL system, including system volume and system performance parameters of SBL using highenergydensity
capacitors with associated trigger and charging systems, is described.
KEYWORDS: Blumlein line, Compact, Low inductance, Pulse power, SiC switches
PAGES 133143

Highly Efficient Cesium Vapor Laser
R.J. Knize, T. Ehrenreich, and B. Zhdanov; U.S. Air Force Academy
The results of our work on optically pumped cesium vapor laser development are presented. We demonstrated efficient cesium
laser operation with diode laser pumping. The measured optical efficiency was more than 32% with an overall
electricaltooptical efficiency of 15%. With an improved pump source we have demonstrated a Cs laser with slope
efficiency of 81% and overall optical efficiency of 63%.
KEYWORDS: Alkali lasers, Diode pumped lasers, Optically pumped lasers
PAGES 145150

Search for LowAbsorption Regions in the 1.6 and 2.1um Atmospheric Windows
W.E. Bicknell, S. DiCecca, M.K. Griffin, S.D. Swartz, and A. Flusberg, MIT Lincoln Laboratory and other affiliations
Atmospheric absorption, often dominated by water vapor absorption in a naval environment, causes thermal blooming that will
limit operational capability of a highenergy laser (HEL) weapon defense system. To support selection of an optimum
wavelength for naval operation of an HEL and for scenario modeling, water vapor absorption in the 1.6 and 2.1µm
windows has been measured. This paper reviews the instrumentation and describes results and the impact of the
measurements. It is an update of results reported at the 7th Directed Energy Symposium at Rockville, Maryland, in
October 2004.
KEYWORDS: Laser beam propagation, Water vapor absorption
PAGES 151161

Improved Simulations of Photoinjectors for HighAverageCurrent FreeElectron Lasers
C.L. Bohn, D. Mihalcea, I.V. Pogorelov, I.V. Sideris, and B. Terzic; Northern Illinois University and other affiliations
We report a sucessful implementation of a threedimensional waveletbased solver for Poisson's equation with Dirichlet
boundary conditions, optimized for use in particleincell beam dynamics simulations. We explain how the new algorithm
works and the advantages it brings to accelerator simulations. The solver is integrated into a full
photoinjectorsimulation code (IMPACTT), and the code is then benchmarked by comparing its output against that of
other codes (verificiation) and against laboratory measurements (validation). To enable detailed verification, we
developed and applied a new technique that involves quantifying chaos in particle orbits. We also simulated the
AES/JLab photoinjector using a suite of codes. This activity revealed certain performance limitations and their causes.
Finally, with an eye on eventually doing endtoend simulations of driver accelerators for highaveragepower
freeelectron lasers, we sketch a path for future code improvements and applications for which this code is
particularly and possibly even uniquely, useful.
KEYWORDS: Freeelectron laser, Photoinjector, Poisson solver, Space charge, Wavelets
PAGES 163188

Volume 2, Number 2, Journal of Directed Energy
