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Journal of Directed Energy
Volume 2, Number 1 Summer 2006

The papers listed below constitute Volume 2, Number 1 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.

Guest Editorial: From Technology Trenches (150 KB)
Donald Lamberson, DEPS

There comes a time when technology for technology's sake is not enough. There comes a time when getting more power, more efficiency, or more anything else from the latest laser is not enough. There comes a time when implementing the latest algorithm in beam control or gaining a few more decibels from microwave transmitters is not enough. In mathematical terms, while all of this is necessary, it is not sufficient to justify our continued existence. We must find a way to serve the Warfighter. We must transition our technology, some of which is very mature, to acquire the weapons Warfighters need. We must get out of the "technology trenches" and onto the battlefield. I think that time has come.
KEYWORDS: Directed energy weapons

Modeling of Laser Charring and Material Removal in Fiberglass Materials (650 KB)
Vladimir V. Semak and Timothy F. Miller, Pennsylvania State University

A numerical model of the physical processes governing laser charring of fiberglass materials has been developed. The model is three-dimensional and transient and incorporates submodels for volumetric and surface laser energy absorption, pyrolysis chemistry kinetics, pyrolitic gas outfluxing, char removal via chemical reaction, and radar attenuation resulting from char formation. This model differs from previous char modeling work because it deals with volumetric, in addition to surface, energy absorption. Modeling results are compared directly with measurements of temperature, char thickness, and optical and radar transmission made on fiberglass material test slabs in a wind tunnel. The comparisons show very good agreement and illustrate subtle behaviors associated with changes in heat transfer and char thickness. Because of uncertainties in important physical properties, some empirical input of thermophysical properties to the model is necessary. However, the amount of this empirical input is manageable and enables close prediction over a large range of laser flux
KEYWORDS: Charring, Fiberglass, High-energy laser, Modeling
PAGES 5-21

Appropriate Measures and Consistent Standard for High-Energy Laser Beam Quality (1,550 KB)
T. Sean Ross and William P. Latham, Air Force Research Laboratory

Along with power output of the laser system, laser optical quality or beam quality provides a suitable measure of performance. Power and beam quality are standards for the comparison of laser systems with each other and against mission requirements. An understanding of the meaning of beam quality is necessary to completely define laser performance capability. The current state of our community includes a multitue of different and not well understood beam quality measures: M2, Strehl ratio, brightness, power in the bucket, "times diffraction limited," and mode content determined by a variety of beam radius measures: half-widths, second-moment radius, widths at 1/e or 1/e2 points, width of primary lobe, etc. Another complication is that different elements of the community use different measures to evaluate optical quality characteristics. We examine the assumptions behind common measures of beam quality and compare the various measures as they relate to beams from lasers employing stable resonant optical cavities. We show how the mode composition of a beam depends on prior determination of beam radius and how the term "times diffraction limited" can mean different things depending on the method used to measure beam radius. We show the ambiguities that arise between certain classes of beams and measures of beam quality and advocate for a laser beam quality standard that relates directly to mission requirements.
KEYWORDS: High-power lasers, Laser beam quality, Laser standards, M squared
PAGES 22-58

Analysis of Thermo-Mechanical Failure Initiation in Tensioned Aluminum Strips Under Irradiation from an IR Heat Source (400 KB)
Jesse McClure and Michael C. Larson, Tulane University

When a thin, tensioned specimen is irradiated by an IR beam, the temperature of the irradiated zone may increase rapidly and induce large thermal gradients between the heat-affected zone and the surrounding material. The material within the heated zone loses strength, lowering the threshold for rupture. However, the material in the heated zone also softens and expands, thereby relieving a portion of the applied tensile stress, which is taken up by the surrounding material. This redistribution of the load increases the likelihood of cracks forming in the high-thermal-gradient areas surrounding the heated zone. Experimental results show that the magnitude of the applied tension plays a key role in determining the tradeoff between these potential failure site locations. As the tension is increased, the crack initiation region shifts laterally away from the center of the heated zone toward the edges of the zone where the temperature is lower but thermal gradients are highest. Dynamic crack growth ensues after a period of relatively slow fracture development, resutling in failure across the width of the specimen. Results from a finite element model are compared to those of the experiments and are used to propose a quantitative predictor of the actual site location of crack initiation for particular load cases
KEYWORDS: Aluminum, Crack growth, Failure, Fracture, Irradiation
PAGES 59-70

Optimum Wavelength and Power for Efficient Laser Propagation in Various Atmospheric Environments (2,100 KB)
Phillip Sprangle, Joseph Penano, and Bahman Hafizi; Naval Research Laboratory

This paper addresses the key physical processes that affect the propagation of high-energy lasers in the atmosphere. The main objective is to discuss the optimum laser wavelength and power for efficient propgation in maritime, desert, rural, and urban atmospheric environments. The theoretical/numerical model used in this study includes the effects of aerosol and molecular scattering, aerosol heating and vaporization, thermal blooming due to aerosol and molecular absorption, atmospheric turbulence, and beam quality. These processes are modeled in a fully three-dimensional and time-dependent manner. It is found that aerosol particles, which consist of water, sea salt, organic matter, dust, soot, biomass smoke, urban pollutants, etc., are particularly important because they result in laser scattering and absorption and enhanced thermal blooming. In the water vapor transmission windows, the total absorption coefficient driving thermal blooming can be caused mainly by aerosols and not water vapor. In certain maritime environments the deleterious effects of aerosols can be reduced by vaporization. Aerosol particles that cannot be vaporized, such as those consisting of dust, soot, etc., can significantly increase thermal blooming. We show that moderate values of the laser beam quality parameter have little effect on the propagation efficiency. The laser power, averaged over dwell time, delivered to a distant target as a function of transmitted power is obtained for a number of wavelengths and atmospheric environments. The optimum wavelength and power are found for each atmospheric environment. KEYWORDS: Aerosols, High-energy laser propagation, Thermal blooming
PAGES 71-95

Volume 2, Number 1, Journal of Directed Energy

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