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DIRECTED
ENERGY
PROFESSIONAL
SOCIETY
Journal of Directed Energy
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Volume 4, Number 2 |
Winter 2010 |
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The papers listed below constitute Volume 4, Number 2 of the Journal of Directed Energy.
Print copies of this, and other issues of the Journal of Directed Energy are available through the
DEPS online store.
Access complete technical paper(s) through links in the paper titles.
Microwave Shielding Technology
(950 KB)
Lynn L. Hatfield and Bryan Schilder, Center for Pulsed Power and Power Electronics, Texas Tech University
A simple system for measuring the attenuation of microwaves in the frequency range of 700 MHz-13 GHz has been
used to compare attenuation by a large number of commercially available shielding materials. The
standard system for such measurements would require IEEE STD 299. Implementation of this standard
requires a number of different sources and receivers, making the measurements time consuming and
expensive. The simple system described here uses two microwave horns and a network analyzer to compare
the difference in attenuation due to a clear path from the transmitting horn to the receiving horn and a
path with a shielding material inserted. This ratio, expressed in decibels, was obtained quickly and
easily for a number of commercially available materials. The horns are A. H. Systems SAS-571 with a
usable range of 700 MHz-18 GHz. The network analyzer is a hp 1397C with a high-frequency limit of 13
GHz. The materials tested include conducting paints on cloth such as denim, conducting woven fabrics, and
metal meshes. The conducting paints and conducting fabrics mostly show large attenuation over the quoted
frequency range, although almost never as high as stated by the manufacturer, which may be due to
different methods used for the tests. Attenuation in decibels is given for 27 materials considered to be
generally useful in protecting sensitive electronic equipment under difficult circumstances.
KEYWORDS: Conducting fabrics, Metal mesh, Microwave shielding, RF shielding
PAGES 119-135
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Nonintrusive Field Characterization in Interior Cavities with Slab-Coupled Optical Sensor
(600 KB)
Bryson Shreeve, Richard Gibson, Daniel Perry, Richard Selfridge, and Stephen Schultz, Department of Electrical and Computer Engineering, Brigham Young University; Richard Forber and Wen Wang, IPITEK; and Jingdong Luo, University of Washington
This paper presents the advances made in electric field sensing using a slab-coupled optical sensor (SCaS). We
continue to enhance the use of optical fiber interrogation with electro-optic materials as a method of
field sensing. The fabrication materials are all insulators and therefore allow for detection offields
without altering them. The sensors are also much smaller than current metallic field sensors, allowing
them to be used in locations in which bulkier sensors cannot be placed. This work uses D-shaped fiber to
achieve resonant coupling with electro-optic crystals and polymer. This study reports how a scas sensor
can perform accurate, low-loss, X-band field detection. We also show how complex fields are analyzed by
creating two-dimensional sensors. Each of these advances proves that scas devices could be viable
solutions for electric field sensing challenges in the area of directed energy weapons.
KEYWORDS: Electric field, Optical fiber, Sensor
PAGES 136-146
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A System Engineering Approach for Active Track Jitter Performance Evaluation
(1,250 KB)
James Negro and David Dean, Boeing-SVS; Richard (Dick) Brunson, U.S. Air Force (retired); Joshua Kann, Boeing-UI'S; and Edward Duff, Air Force Research Laboratory/RDTP
This paper describes an engineering error budget approach for examining the design and jitter performance of
active track systems used for imaging and high-energy laser beam control applications. This
root-sum-square approach aggregates numerous design parameters into key performance variables that
capture in a simplified way the engagement, environment, and essential design characteristics of an
active track system. The study emphasizes the line-of-sight jitter performance of the active track
system operating in turbulent media. Major error components are described for tracker measurement noise,
residual atmospheric tilt turbulence, residual local optical system tilt jitter, jitter coupling error,
and active signature errors (including speckle and scintillation). Algebraic models for each of these
terms are derived from analytic models, simulations, or empirical experience. These models are combined
into an overall system engineering model error budget. The model is exercisedfor a generic
ground-to-space imaging application to illustrate the methodology. This active track model segregates
error terms unique to active track and shows the jitter performance penalty of active track systems in
comparison to comparable passive systems.
KEYWORDS: Active track, Jitter, Track performance
PAGES 147-167
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Characterization of High-Power Lasers
(1,300 KB)
Jack Slater, Schafer Corporation
A general methodology for characterization of high-average-power lasers with respect to power and beam
quality is presented. The techniques discussed largely capture the experience from the 100 kW Joint High
Power Solid State Laser (JHPSSL) program and plans for the Robust Electric Laser Initiative (RELl)
program.
KEYWORDS: Beam quality, High-power laser, Laser, Laser characterization
PAGES 168-188
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Fluence and Wavelength Dependence of a Painted Surface Absorptance during Short-Pulse Laser Illumination
(800 KB)
Michael E. Thomas, Daniel V. Hahn, and Kevin C. Baldwin, Applied Physics Laboratory, Johns Hopkins University; and Caroline McEnnis and James B. Spicer, Department of Material Science and Engineering, Johns Hopkins University
The absorptance of an opaque surface is completely characterized by the surface bidirectional reflectance
distribution function (BRDF). In particular for this study, a laser-ablated, painted aluminum substrate
is characterized in terms of its BRDF The sample is exposed to a raster-scanned high-intensity
Ti.sapphire laser operating up to a 1-kHz pulse repetition frequency with pulse duration of around
150 fs and pulse energies up to 650 j.1J at a wavelength of 800 nm. The ablated surface is then
characterized in terms of a measured BRDF at 633, 1,064, and 3,390 nm. In this way the specular and
diffuse nature of the paint can be determined. A novel physics-based semiempirical model is used to
represent the data as a function of laser fiuence and wavelength. How the model can handle such time
(fiuence)-dependent phenomena is discussed. Such a capability is essential in representing the
light-matter interaction between the laser beam and target.
KEYWORDS: Absorptance, BRDF, Diffuse reflectance, Fluence, High-energy laser, Specular reflectance
PAGES 189-204
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Optimum Beam Wavelength for Laser-Based Directed Energy Systems and Electrostatic Mitigation Approach for Uninterrupted Telemetry during Testing
(950 KB)
Madhusudhan Kundrapu and Michael Keidart, Department of Mechanical and Aerospace Engineering, The George Washington University; and Charles Jones, Air Force Flight Test Center
Laser beams of directed energy systems lose a significant amount of energy before they reach the target
surface, due to atmospheric attenuation, plasma shielding, and target surface reflection. The energy
losses depend on the beam wavelength. A detailed numerical analysis is carried out to optimize the beam
wavelength, in order to achieve identical rates of destruction with low fluences, for three different
target surfaces made of AI, Cu, and Ti. Plasma formed due to laser target interaction attenuates
telemetry during testing of directed energy systems. An electrostatic approach for the mitigation of
communication attenuation is analyzed to obtain the fluency limits up to which the approach can be
implemented. The effect of background pressure on the bias voltage requirement to create a sheath is
analyzed. A self-consistent numerical model that couples laser-target interaction with plasma formation
and plume expansion is employed to obtain the evaporation rates and plasma parameters. Transient sheath
calculations are performed to characterize the sheath. Ablation analysis shows that the optimum
wavelength for Al is 850 nm: It is found from sheath calculations that uninterrupted telemetry can be
achieved through Al plasma for
fluences below 4 Jlcm2 at a background pressure of 1 atm, using a maximum bias voltage of 10 kV.
KEYWORDS: Directed energy, Electrostatic sheath, Laser ablation, Optimum wavelength
PAGES 205-222
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Volume 4, Number 2, Journal of Directed Energy
Last updated: 8 September 2017
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