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Journal of Directed Energy
Volume 3, Number 2 Spring 2009

The papers listed below constitute Volume 3, 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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Leaky Coaxial Cable as a Transmitting Antenna for HEMP Shielding Effectiveness Testing
Thomas F. Podlesak and Canh Ly, U.S. Army Research Laboratory

MIL-STD-188-125 requires high-altitude electromagnetic pulse (HEMP)-shielded facilities to be tested at minimum 7-year intervals, or whenever a shield is modified in any way. This requires extensive measurements at many test points on all sides of a facility. The standard procedure is to position a transmitter and transmitting antenna at various points along the shield perimeter. This is labor intensive and repetitious. Further, in some cases it is extremely difficult to position transmitting antennas. Given the likelihood that these critical installations will have many decades of service life, these measurements will need to be repeated many times over the facility life. The Army Research Laboratory, which has the mission to make such measurements at a variety of Department of Defense installations, is currently evaluating such a facility in which such measurements are unusually difficult. The military facility is underground and consists of several buildings placed in rock-walled tunnels. There is very little space between the tunnel walls and the building walls, floor and roof to maneuver transmitting antennas. A solution is being investigated in which the normal transmitting antennas, which are loops and bilogical arrays, may be replaced with leaky coaxial cables. There are coaxial cables with gaps in their shielding, which allows the center conductor to radiate. The cables therefore function as antennas, not transmission lines. They are broadband and normally used in wireless networking systems. Another common use is for communication systems in mining operations, an application not unlike the task in question. The cables are strung out longitudinally along the walls and floor of the facility, and testing is done by moving conventional receiving antennas inside the facility to the various test points along the cable, per the requirements of Appendix A, MIL-STD-188-125. Upon completion of testing, the cable antennas will be left in place for the next occasion of testing. They therefore have to be placed only once. We present the findings of the investigation of this new method for HEMP testing, as well as comparisons with more conventional testing also being done.
KEYWORDS: HEMP, Leaky Coaxial cable, testing
PAGES 97-106

Two-Dimensional Re-Imaging Assisted Phased Array Architecture for Coherent Beam Combination
Andrew Bratcher and Radoslaw Uberna, Lockheed Martin Coherent Technologies

Many applications, most notably directed energy applications, call for high-power lasers with good beam quality. Current roadblocks of solid-state and fiber laser technology limit the amount of power that can be achieved in a single diffraction-limited output. One technique to increase the total output power of a laser system is to combine multiple lower power lasers into a single, high-power beam. A Re-imaging Assisted Phased Array Architecture (REAPAR) based on self-imaging waveguides was recently developed at Lockheed Martin Coherent Technologies (LMCT) and implemented to coherently combine multiple laser beams in waveguides. Beam combination in a waveguide has the ability to produce a single diffraction-limited spot without the sidelobes typical of a free-space phased array, theoretically enabling lossless combination. Prior work at LMCT investigated the suitability and combining efficiency of one-dimensional waveguides, and recent thrusts of the REAPAR concept have extended to beam combination in two-dimensional (2-D) waveguides. We present the recent results of the combination of four fiber-launched beams in a 2-D hollow glass waveguide with the opportunities for improved efficiency.
KEYWORDS: Beam combination, Phased array, Self-imaging, Waveguide
PAGES 107-117

Variation of the Dynamic Bidirectional Reflectance Distribution Function Across High-Energy Laser Spots
Larry McKee, Science Applications International Corporation

The capability to accurately measure the high-energy laser (HEL) irradiance incident on an airborne target is a key shortfall for HEL test and evaluation. The hope has been that analyzing remote imagery of the HEL spot on the target could provide the required measurement to an acceptable accuracy. The key feasibility issue for the technique is being able to identify and use an appropriate bidirectional reflectance distribution function (BRDF) for the target surface as the surface is being heated and damaged by the HEL interaction. However, experimental results and laboratory measurements show that the BRDF of a surface heated by a HEL is highly variable, both temporally and spatially. Further, no basis could be identified for predicting or determining the dynamic BRDF values across the laser spot without knowledge of the incident irradiance profile. Because the measurement objective is to determine the incident irradiance, the use of passive laser spot imagery alone is judged to be an impractical solution approach for range applications.
KEYWORDS: Bedirectional reflection distribution function, Irradiance measurement, Remote imagery
PAGES 118-133

HAL: Humidity and Aerosol LIDAR for Profiling Atmospheric Water Vapor and Aerosols
J.P. Sherman, Appalachian State University and P.D. Dao, Air Force Research Laboratory

The design and initial field testing of a prototype water vapor differential absorption LIDAR (DIAL) developed at the Air Force Research Laboratory are discussed in this paper. The operational version of a steerable LIDAR, named HAL (humidity and aerosol LIDAR), will profile line-of-sight aerosols and water vapor with high spatial and temporal resolution along downward-pointing, long-slant paths during clear-sky and moderately turbid conditions as a support tool for tactical high-energy laser experiments. A high pulse-energy alexandrite transmitter produces more than 1 W of narrow-line average power at the 729.23-nm H2O vapor line, permitting high resolution using a modest receiver aperture. Stable transmitter operation for continuous periods exceeding 30 h has been demonstrated. The prototype LIDAR was pointed upward for direct and indirect comparisons of LIDAR-derived absolute humidity profiles with colocated and regionally launched radiosondes and with a colocated microwave radiometer. Qualitative (and often quantitative) agreement was observed for upward-pointing water vapor retrievals up to 4.5-km (3.5 km) altitudes during nighttime (daytime) hours, in addition to qualitative aerosol layer/cloud profiles up to 14 km. The H2O vapor range will be expanded by a modified receiver currently being developed. Potential applications include characterizing the effects of water vapor and aerosols on high-energy electromagnetic wave propagation and cloud prediction.
KEYWORDS: Aerosols, Beam propagation, DIAL, Thermal blooming, Water vapor
PAGES 134-148

Recent Advances in Explosive Pulsed Power
L.L. Altgilbers, U.S.Army Space and Missile Defense Command; A.H. Stults, U.S. Army Aviation and Missile Research, Development, and Engineering Center; M. Kristiansen, A. Neuber, J. Dickens, A. Young, T. Hold, and M. Elsayed, Texas Tech University; R. Curry and K. O'Connor, University of Missouri; J. Baird and S. Shkuratov, Loki, Inc.; B. Freeman, KTECH Corp; D. Hemmert, HEM Technologies; F. Rose, Z. Shotts, and Z. Roberts, Radiance Technologies; W. Hackenberget and E. Alberta, TRS Technologies; and M. Rader and A. Dougherty, Naval Research Laboratory

Explosive pulsed power (EPP) has been investigated since the early 1950s. However, in the late 1980s interest in it began to decline. Interest was revived in 1998, when the Air Force Office of Scientific Research established Multidisciplinary University Research Initiative and New World Vista programs at Texas Tech University (TTU) to study explosive pulsed power devices, with an emphasis on compact or small systems. In 2004, a series of Small Business Innovative Research programs were initiated to further develop compact explosive pulsed power. On the basis of these efforts, we now have a better understanding of the basic physics of small EPP systems and of their weaknesses and strengths. As a result, we can now build reliable generators that provide consistent results and that can be utilized in practical applications. In this paper, a brief introduction to these generators will be given, along with some of the most recent advances in our understanding of them. This paper will report only on advances made by Army and Navy researchers and those of their contractors. A description of an explosive-driven high-power microwave test bed that was built at TTU will be presented. Finally, the results of recent experiments in which EPP was used to drive an antenna will be presented.
KEYWORDS: Explosive pulsed power, HPM test bed, Power conditioning, Seed sources
PAGES 149-191

Volume 3, Number 2, Journal of Directed Energy

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