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Journal of Directed Energy
Volume 4, Number 2 Winter 2010

The papers listed below constitute Volume 4, 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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Microwave Shielding Technology
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

Nonintrusive Field Characterization in Interior Cavities with Slab-Coupled Optical Sensor
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

A System Engineering Approach for Active Track Jitter Performance Evaluation
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

Characterization of High-Power Lasers
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

Fluence and Wavelength Dependence of a Painted Surface Absorptance during Short-Pulse Laser Illumination
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

Optimum Beam Wavelength for Laser-Based Directed Energy Systems and Electrostatic Mitigation Approach for Uninterrupted Telemetry during Testing
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

Volume 4, Number 2, Journal of Directed Energy

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