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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..
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Modeling of Antimortar Lethality by a Solid-State Heat-Capacity Laser
C.D. Boley and A.M. Rubenchik, Lawrence Livermore National Laboratory

We have studied the use of a solid-state heat-capacity laser (SSHCL) in mortar defense. This type of laser, as built at Lawrence Livermore National Laboratory, produces high-energy pulses with a wavelength of about 1 Ám and a pulse repetition rate of 200 Hz. Currently, the average power is about 26 kW. Our model of target interactions includes optical absorption, two-dimensional heat transport in the metal casing and explosive, melting, wind effects (cooling and melt removal), high-explosive (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 100-kW 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, Solid-state laser
PAGES 97-106

Propagation of High-Energy 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 high-energy 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 three-dimensional and time-dependent 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, High-energy laser, Stagnation zone
PAGES 107-117

Compact, Optically Guided, Megawatt Free-Electron 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, megawatt-class, optically guided, free-electron laser (FEL) amplifier. Full-scale 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 Ám) can result in high propagation efficiencies in a maritime environment (>50% over a 5-km range). In the present design we choose the FEL wavelength to be 2.141 Ám because of propagation and generation advantages as well as for eye safety reasons. The average output power of this high-gain FEL amplifier is ~1.5 MW, and the input is provided by a low-average-power (<50-W) 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: Free-electron laser amplifier, Maritime propagation, Optical guiding
PAGES 119-132

Most Compact Pulse Power Supply for Narrowband High-Power Microband Systems
W.C. Nunnally, R. O'Connell, and D. Giorgi; University of Missouri-Columbia 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 500-kV-1-MV 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 OptiSwitch-ed SBL system, including system volume and system performance parameters of SBL using high-energy-density capacitors with associated trigger and charging systems, is described.
KEYWORDS: Blumlein line, Compact, Low inductance, Pulse power, SiC switches
PAGES 133-143

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 electrical-to-optical 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 145-150

Search for Low-Absorption Regions in the 1.6- and 2.1-um 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 high-energy 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 2.1-Ám 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 151-161

Improved Simulations of Photoinjectors for High-Average-Current Free-Electron 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 three-dimensional wavelet-based solver for Poisson's equation with Dirichlet boundary conditions, optimized for use in particle-in-cell 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 photoinjector-simulation code (IMPACT-T), 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 end-to-end simulations of driver accelerators for high-average-power free-electron lasers, we sketch a path for future code improvements and applications for which this code is particularly and possibly even uniquely, useful.
KEYWORDS: Free-electron laser, Photoinjector, Poisson solver, Space charge, Wavelets
PAGES 163-188

Volume 2, Number 2, Journal of Directed Energy

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