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Journal of Directed Energy
Volume 4, Number 3 Spring 2011

The papers listed below constitute Volume 4, Number 3 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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Finding Potentially Susceptible Targets of High-Power Microwave Attack Using Time-Frequency Analysis
Walter Wall, Sun Hong, and Tim Andreadis, U.S. Naval Research Laboratory, Tactical Electronics Warfare Division

Potentially susceptible targets of HPM attack are recognized from the time-frequency representation of their transient high-power microwave (HPM) backscatter. The presence of high-Q resonances in an unknown electronic device should, in general, indicate an increase in the probability of affecting sensitive elements within the circuit. Each such resonance presents a potential entry point for HPM attack and thus identifying these phenomena is of particular interest for HPM control and hardening. In the timefrequency representation these resonances appear as persistent perturbations which distinguish them from other scattering phenomena. Therefore the surface area of the time-frequency domain is dependent on the number and Q of resonances in a target's backscatter and provides a natural measure of the potential susceptibility of unknown targets. The validity of this technique is evaluated in a case study of common electronic and nonelectronic devices. By identifying potential susceptibilities, this technique could provide valuable information for hardening electronics to HPM attack as well as operating HPM systems.
KEYWORDS: Vulnerability assessment, Time-frequency analysis, High-power microwave (HPM) hardening, Singularity expansion method (SEM), Short-time Fourier transform
PAGES 119-129

How to Pack Fiber Laser Arrays More Efficiently Than Hexagonal Close Pack
Chun-Ching Shih, Northrop Grumman Aerospace Systems

It is demonstrated that close pack may not be the most efficient way of packing a fiber laser array within a circular area. By rearranging individual elements, it is possible to increase the fill factor of the array and thus enhance the farfield performance. Specific examples are given for arrays packed with 19, 37, and 61 elements, which are the most probable number of fibers for a fiber laser system up to 100 kW.
KEYWORDS: Fiber laser, Laser array, Hexagonal array, Close pack, Fill factor
PAGES 130-135

Gaussian Beams from a Beam Control Perspective
Charles J. Albers, Jr., BAE Systems

New expressions are presented for describing Gaussian beams in terms natural to beam control rather than, as usual, in terms natural to resonators. These new expressions are much more useful for beam control analyses. For example, the formulations make it obvious that to maximize the irradiance at a given range requires that the (geometrical) focus be at that range and not the Fresnel maximum. Expressions relating the location of the Fresnel maximum and its irradiance to the focus and its irradiance are presented. It is also shown that the location of the Fresnel maximum is limited to a maximum distance from the aperture and that the location is not a single-valued function of the focus range. Relationships for beam size and on-axis irradiance at all ranges are also given. All results are in terms of the dimensionless parameter Xi = 2F lambda/At, where F is the geometrical optics focus range, lambda is the wavelength, and At is the equivalent aperture area. Examples are given to illustrate features of the theory. Finally, it is shown that the results presented here are consistent with the resonator-based results.
KEYWORDS: Gaussian beams, Beam control, Fresnel maximum, Beam spread parameter
PAGES 136-149

Solid-State Active Denial Weapons
K. Brown, R. Lowell, A. Brown, K.C. Hwang, and R. Scott, Raytheon and Joint Non-Lethal Weapons Directorate

To date active denial weapons have been implemented with average source powers ranging from 400 watts to 100 kW, using vacuum tube technology. At the required 95-GHz frequency, solid-state sources have been traditionally inadequate for use in active denial weapon systems. In fact, the highest published (2010) solid-state power generated to date at W-band is only 5 watts. In order to generate weapon system–level powers (i.e., kilowatts) using solid state, significant technological advances have been necessary. These technology advancements include development of a high-efficiency, W-band gallium nitride (GaN) semiconductor process, high-power millimeter-wave monolithic integrated circuit (MMIC) amplifier designs, efficient power-combining technologies, and unique weapon integration methodologies. These advances have made it possible to develop solid-state, active denial sources that generate several hundred watts of continuous wave (CW) power. These compact multi–hundred-watt sources have been implemented into a variety of active denial weapon systems including an active electronically steerable array (AESA) weapon demonstrator for the Joint Non-Lethal Weapons Directorate (JNLWD). This paper explores the technological advances that have made these revolutionary active denial weapons possible, the performance of the aforementioned weapon system, and the future of solid-state active denial.
KEYWORDS: RF directed energy, Non-lethal weapons, Active denial, Millimeter wave, Solid state, Galium Nitride
PAGES 150-157

Nonlinear Effects on the Cylindrical Cloaking Solution from Transformation Optics and Negative Index Metamaterials on Electromagnetic Shielding
Jeremy S. Hughes, Physics Department, Naval Postgraduate School

The effects of a nonlinear constitutive relationship on a cloaking solution described by transformation optics are presented. Failure modes of such a device used for electromagnetic shielding are defined and compared to numerical results obtained from a nonlinear model of a scattering problem in COMSOL Multiphysics software. Results show that distortion and breakdown effects dominate for intense fields. Analysis of negative index metamaterials is discussed.
KEYWORDS: Transformation optics, Nonlinear optics, Metamaterials, Numerical simulation, Electromagnetic shielding
PAGES 158-167

Determination of Rubidium Number Density under Optically Thick Conditions, Pressure Broadened by Helium or Methane
Matthew Rotondaro, C. Wisniewski, M. Post, C. Porter, and E. Smith, Department of Aeronautical Engineering, U.S. Air Force Academy; Gordon Hager, Air Force Institute of Technology

In this work we describe a direct spectroscopic method to determine the rubidium number density under optically thick conditions similar to those found in diodepumped, alkali metal vapor lasers. The rubidium number density was determined by examining the wings of the pressure-broadened (2S1/2 ? 2P1/2) transition. The wings were observed by using a narrow-band (300 KHz), tunable diode laser with a tuning range of 75 GHz. This technique can measure the absolute concentration of rubidium under conditions where the absorption cross-section, path length, and number density product yield conditions where the central region of the line is opaque. The measurement was accomplished by tuning the laser to a region sufficiently far into the short wavelength wing where transmission through the cell was possible. The laser was then scanned through the central opaque region of the line to the adjacent long-wavelength wing. The wavelength of the scan was calibrated by using a 1.5-GHz etalon and a frequency reference containing naturally occurring rubidium. The rubidium absorption spectra were measured at temperatures ranging from 40 to 100 şC while being broadened using helium and methane at pressures between 500 and 1900 Torr. These absorption spectra were subsequently fit to a pressure-broadened line profile, thereby allowing determination of the rubidium number density.
KEYWORDS: Rubidium, DPAL, Number density
PAGES 168-183

Lethality Experiment Using a Diode Array to Simulate a Weapon-Class Laser
W.H. Lowdermilk, A.M. Rubenchik, and H.K. Springer, Lawrence Livermore National Laboratory

This paper describes a lethality experiment with energetic material conducted in a contained firing facility using a semiconductor diode laser array (DLA) as a surrogate for a weapon-class, high-power laser. A large volume of propellant in a stainless steel canister was heated to ignition and the ignition temperature measured directly. A two stage "explosion" of the propellant was observed, and a qualitative explanation is suggested. This experiment demonstrates the utility of compact, lightweight, easily transportable, and relatively low-cost DLAs for laboratory lethality experiments.
KEYWORDS: Diode-laser array, Laser lethality, Energetic materials
PAGES 184-196

Beam Control System Trades for Near-Term Navy Tactical Laser Concepts
John R. Albertine, Independent Consultant

A paper entitled "Near-Term Tactical Laser Weapon Concepts and Trades" by Dr. J. Thomas Schriempf, Dr. Paul Berger, and the author was presented at the 2007 DEPS Systems Conference. It illustrated the potential utility of poor beam quality (BQ) fiber lasers for short-range tactical applications. Since that time, efforts (such as the Navy Laser Weapon System, or LaWS) have begun that embodied these ideas and demonstrated interesting results. This paper carries this earlier analysis past the initial concept studies and looks at a tradeoff space that encompasses a variety of existing or potential 100-kW–class, fiber and slab solid-state laser constructs. It leads to conclusions about performance optimization among power, beam quality, jitter, and aperture configurations to be used against tactical surface and flying targets of interest to the Navy. The analysis also demonstrates, using a simple laser system, a process for identifying those subsystem improvements that would provide the greatest benefit at the system level.
KEYWORDS: HEL system design, Beam control optimization, Solid-state lasers
PAGES 197-209

Volume 4, Number 3, Journal of Directed Energy

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