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The search for viable replacement fire extinguishing compounds for the presently used Halons 1301 and 1211 has teen fully underway for some time. A successful replacement compound has to satisfy a number of criteria, including superior fire extinguishment characteristics. Recent advances in combustion science have opened up the possibility of identifying the detailed physical and chemical properties that are responsible for fire inhibition and extinction. This work indicates that each inhibiting compound exhibits a unique behavior in both experimental and computational results with respect to flame structure and adiabatic flame speed characteristics. Overall, our results indicate that the detailed chemical kinetic models have the potential for not only predicting the relative fire extinguishment properties of new Halon replacement compounds, or their mixtures, but also to predict the possible formation of toxic compounds during their use in actual fires.

The potential application of the laser-induced fluorescence method for determining number densities and temperatures of gas species is investigated through computer modeling of the detailed relaxation of laser-excited molecular and atomic-molecular systems. The rate equation models for a sodium-diatomic molecular system and a single diatomic molecule system are explained and applied to the case of sodium-nitrogen and pure carbon monoxide.

The optical switching capabilities of Distributed Bragg Reflector (DBR) structures, including Vertical Cavity Surface Emitting Lasers (VCSELs) are examined. Reflectivity switching is demonstrated using both thermal and carrier generated effects to alter the DBR/VCSEL layers' refractive indices. Optical bistability is demonstrated at room temperature, under CW photopumped excitation. The optical bistability hysteresis is controllable by spectral location of the pump on the stop band edge. In the VCSEL, reflective bistability is also evidenced; additionally, this bistability is accompanied by a bistability in the VCSEL lasing output intensity, spot size, and wavelength. Modeling of the DBR/VCSEL thermally induced bistability was accomplished using an iterative, three dimensional Green's function solution to the laser induced heat equation. The model predicts thermally generated bistability, indicating a wavelength dependence, reflectivity jump, self focusing induced spot size change, and wavelength jump commensurate with the experimental data. Reflectivity switching due to carrier effects is also demonstrated, the DBR and VCSELs exhibiting subpicosecond switching speeds and contrast ratios of 3:1. Carrier heating, two photon absorption, and thermal components of the reflectivity switching capabilities are examined and found to be consistent with measured nonlinear response characteristics for the refractive index in AlGaAs.

It is demonstrated that the linear response scheme within the framework of density functional method can provide reliable atomic data for various atoms and ions of experimental interest. This model is particularly useful in those situations where conventional independent particle models fail to generate accurate data. The atomic structure code for multielectron ions of arbitrary Z is of central importance in these calculations. Proper relativistic treatment is included in our work for high Z ions. The mechanism of time-dependent polarization of the atom is seen to be important in describing the experimental observations. For most experimental situations, the present method is adequate and very useful. If the applied radiation field strength is very high, suitable generalization of the present model or developement of new techniques are necessary to treat those conditions.

The first part of this project involved continued development of theoretical models of diatomic molecular electronic level structure for application to the production of ultracold diatomic molecules via photoassociation. Recently, there has been considerable interest in producing molecules at the temperatures achieved for laser-cooled atoms, for applications to coherent chemistry, studies of molecule-atom and molecule-molecule interactions. Bose condensates of molecules and for quantum computing. It is advantageous to produce cold molecules from laser-cooled atoms. but this requires an accurate model of the energy level structure near the dissociation limit for excited atoms, and for other intermediate states. With this in mind. we have modeled experimental photoassociation data from Rb2. spectroscopic data on the lowest excited states of Na2, and hyperfine structure of the lowest triplet state of Cs2. The second part involved continued development of experimental techniques for intense atomic beam collimation using a bichromatic laser field. We solved the equations for evolution of atoms in such a laser field, expressed the results in the form of trajectories on the Bloch sphere for a two-level atoms, and then used these results to design and perform efficient atomic beam reflection experiments. At the same time, we are developing methods to use these highly collimated beams of metastable helium atoms for precision lithography, using sensitized surfaces.

Options for tracking dynamic underwater targets using optical methods is currently limited. This thesis examines optical reflectance intensities utilizing Lambert’s Reflection Model and based on a proposed underwater laser tracking system. Numerical analysis is performed through simulation to determine the detectable light intensities based on relationships between varying inputs such as angle of illumination and target position. Attenuation, noise, and laser beam spreading are included in the analysis. Simulation results suggest optical tracking exhibits complex relationships based on target location and illumination angle. Signal to Noise Ratios are a better indicator of system capabilities than received intensities. Signal reception does not necessarily confirm target capture in a multi-sensor network.

We investigate the corrective effect of adaptive optics (AO) on High Energy Laser (HEL) beam propagation through atmospheric turbulence. The effect of adaptive optics systems was first integrated into a laser propagation scaling code, ANCHOR, that was developed within the Direct Energy (DE) research group at the Naval Postgraduate School (NPS). Using a host of plausible input parameters, we then compare the estimated target irradiance increase accomplished by various adaptive optics systems. In various cases, adaptive optics is shown to substantially increase the effective range of HELs.

High resolution (approx. .05/cm) molecular line absorption measurements have been performed in the 3-5 micrometer spectral region. This data completes the measurements activities directed toward developing field verified predictive modeling of DF laser atmospheric molecular absorption. Some 181 HDO line profile measurement sets are presented for the path length and partial pressure combinations required to determine the air broadened HDO line parameters. Also, synthetic atmospheric molecular absorption spectra have been generated for the several molecular absorbers (N2O, CH4, HDO, CO2) known to be important in the DF laser region. During the course of the DF transmission modeling effort, it became apparent that the line of sight molecular content should depend on the location and environmental conditions. To monitor the HDO molecular content, a Gas Filter Correlation Spectrometer (GFSC) was designed, constructed and delivered to NRL. Since the variability of HDO in the real atmosphere was shown by NRL to be quite dramatic and important, a design study to expand the GFCS capability was initiated. This study is included in this report. (Author)

A computational plasma aerodynamics model is developed to study the performance of a laser propelled Lightcraft. The computational methodology is based on a time-accurate, three-dimensional, finite-difference, chemically reacting, unstructured grid, pressure-based formulation. The underlying physics are added and tested systematically using a building-block approach. The physics modeled include non-equilibrium thermodynamics, non-equilibrium air-plasma finite-rate kinetics, specular ray tracing, laser beam energy absorption and refraction by plasma, non-equilibrium plasma radiation, and plasma resonance. A series of transient computations are performed at several laser pulse energy levels and the simulated physics are discussed and compared with those of tests and literatures. The predicted coupling coefficients for the Lightcraft compared reasonably well with those of tests conducted on a pendulum apparatus.

In the present paper we propose to update the standard calculation of Stark line profiles for autoionizing atomic states in dense plasmas, i.e. warm dense matter and strongly coupled plasmas. This is motivated by the importance of taking into account the effect of the electric microfield generated by the plasma charge constituents in the calculation of the atomic populations of autoionizing states which leads to the modification of the calculation of Stark line profiles in the frame of the standard theory. This is due to the properties of the autoionizing atomic states that are far from Boltzmann distribution even in dense plasmas bringing serious doubts on the statistical approach used in the standard theory of Stark profile calculations. We discuss the importance of the present analysis in view of the radiation emission originating from dense plasmas created by the interaction of the X-ray Free Electron Lasers (XFEL's) with solid density matter.

The universe is permeated by magnetic fields, with strengths ranging from a femtogauss in the voids between the filaments of galaxy clusters to several teragauss in black holes and neutron stars. The standard model behind cosmological magnetic fields is the nonlinear amplification of seed fields via turbulent dynamo to the values observed. We have conceived experiments that aim to demonstrate and study the turbulent dynamo mechanism in the laboratory. Here we describe the design of these experiments through simulation campaigns using FLASH, a highly capable radiation magnetohydrodynamics code that we have developed, and large-scale three-dimensional simulations on the Mira supercomputer at Argonne National Laboratory. The simulation results indicate that the experimental platform may be capable of reaching a turbulent plasma state and study dynamo amplification. We validate and compare our numerical results with a small subset of experimental data using synthetic diagnostics.

This thesis explores the possibility of power beaming from terrestrial sites to orbiting spacecraft. Solar cell annealing to repair radiation damaged cells and power augmentation for satellites that are battery limited are two of the possible uses for power beaming. Through modeling and simulation a more complete understanding of the expected system response was obtained. A virtual wafer fabrication tool (VWFT) called Silvaco ATLAS was used to simulate a previously optimized InGaP/GaAs/Ge triple junction solar cell's response to a three laser system. The software was used to determine the optimum wavelength for each layer of the cell and then the intensity of the lasers was determined to achieve the same output of the cell as experienced under solar illumination, thereby replacing the sun. The original cell boasted 36.29% efficiency under solar illumination, but by applying the laser system the same cell operated at 51.15% efficiency. The approach and lessons learned can be applied to any solar cell design.

In this project we investigate the impact of the maritime environment on the propagation of laser beams. This study primarily uses data collected at the Naval Academy with the goal of quantifying the correlation between the statistics of the environmental parameters and the statistics of laser beam intensity at the target. The project has two parts to it: 1) we present a computational analysis of different probability density function approximation techniques; and 2) we introduce preliminary steps towards developing a stochastic model for the maritime laser beam propagation. In the first part of this work we apply three mathematical methods to construct the probability density function of the data: i) the Kernel Density Estimator (KDE) method, ii) the Barakat Method using lower-order moments, and iii) the Bayesian Mixture Model. We compare and contrast the features of the three approximation techniques, first in the context of a synthetic data whose true pdf is known, and next in the context of the laser data. In the second task, we analyze how a complex medium causes the photons of the laser light to behave differently than if they were acting in freespace, by focusing on the stochastic behavior that our data exhibits. We develop a stochastic paraxial wave equation in order to have a mathematical model capable of accepting statistical parameters from the atmosphere as input to allow us to investigate the statistical properties of light intensity at a specified target.

The use of a high-resolution, ground-based 3D laser scanner was recently evaluated for terrestrial site characterization of variable-surface minefield sites and generation of surface and terrain models. The instrument used to conduct this research was a Leica HDS3000 3D laser scanner. The high-speed, highly accurate ranging system has a 360 deg horizontal 270 deg vertical field of view that delivers positional, range, and angular (vertical and horizontal) single point accuracies (range 1 to 50 m) of 6 mm, 4 mm, and 60 micro-radians, respectively. The laser is a class 3R and is completely eye-safe with a wavelength of 523 nm and spot size of less than or equal to 6 mm at a distance of 50 m. The pulse rate is 1,000 points/ sec with an optimal effective range up to 100 m which is capable of producing a maximum point cloud spacing of 1.2 mm in the horizontal and vertical direction. Two study sites located in the midwestern United States were used for this analysis. A very dense vegetation site (Grass Site) and a bare soil site with intermittent rocks and sparse vegetation (Dirt Site) were selected for data collection to simulate both obscured and semi-obscured minefield sites. High-density scans (range 0.2 to 2.0 cm spacing) were utilized for Cyra target acquisition and were commensurate with size and distance to target from scanner location. Medium-density scans (range 2.0 to 5.0 cm spacing) were chosen for point cloud generation of the entire site(s) with approximately 10 percent edge overlap between field scans. In order to provide equivalent, unobstructed viewing perspectives from all scan locations at each site, the scanner was positioned on a trailer-mounted, chain-driven lift and raised to an approximate scan height of 7.6 m above the ground. Final registration to UTM projected coordinate system of the multiple scan locations for the Dirt Site and Grass Site produced mean absolute errors of 0.014 and 0.017 m, respectively.

We summarize activities that we carried out during the grant period in collaboration with scientists at the Naval Research Laboratory. Our joint research focused on two activities. The first was the study of propagation in chalcogenide fibers and materials and focused on super continuum generation, negative curvature fibers, and motheye structures. We found ways to achieve a broad bandwidth in chalcogenide fibers and to maximize the transmission into and out of these fibers. The second was the study of high-current photodetectors and focused on determining the sources of nonlinearity and methods to mitigate them.

Pulsed laser communication systems have been proposed which work through all types of clouds. Predicting the performance of such systems requires a knowledge of the statistics of the spatial, angular and temporal spreading of pulses by clouds. Current models are particularly weak in predicting pulse transmission/reflection when the transmitter is near the horizon with respect to the receiver. A database consisting of the probability of cloud pattern types has been developed to be used with a database of cloud pattern properties. Together these two databases are used to predict system performance statistics for operational scenarios. In the future, real-time operational planning for message delivery will require short term predictions of cloud fields and their effects on performance. Models of reflection of pulses from clouds, as well as transmission through them are needed to make accurate evaluations of the probability of interception and disclosure of receiver or transmitter position. Reprints.

The Earth-Moon-Sun system has traditionally provided the best laboratory for testing the strong equivalence principle. For a decade, the Apache Point Observatory Lunar Laser-ranging Operation (APOLLO) has been producing the world's best lunar laser ranging data. At present, a single observing session of about an hour yields a distance measurement with uncertainty of about 2~mm, an order of magnitude advance over the best pre-APOLLO lunar laser ranging data. However, these superb data have not yet yielded scientific results commensurate with their accuracy, number, and temporal distribution. There are two reasons for this. First, even in the relatively clean environment of the Earth-Moon system, a large number of effects modify the measured distance importantly and thus need to be included in the analysis model. The second reason is more complicated. The traditional problem with the analysis of solar-system metric data is that the physical model must be truncated to avoid extra parameters that would increase the condition number of the estimator. Even in a typical APOLLO analysis that does not include parameters of gravity physics, the condition number is very high: $8 \times 10^{10}$.

The Earth-Moon-Sun system has traditionally provided the best laboratory for testing the strong equivalence principle. For a decade, the Apache Point Observatory Lunar Laser-ranging Operation (APOLLO) has been producing the world's best lunar laser ranging data. At present, a single observing session of about an hour yields a distance measurement with uncertainty of about 2~mm, an order of magnitude advance over the best pre-APOLLO lunar laser ranging data. However, these superb data have not yet yielded scientific results commensurate with their accuracy, number, and temporal distribution. There are two reasons for this. First, even in the relatively clean environment of the Earth-Moon system, a large number of effects modify the measured distance importantly and thus need to be included in the analysis model. The second reason is more complicated. The traditional problem with the analysis of solar-system metric data is that the physical model must be truncated to avoid extra parameters that would increase the condition number of the estimator. Even in a typical APOLLO analysis that does not include parameters of gravity physics, the condition number is very high: $8 \times 10^{10}$.

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A computational approach to modeling direct metal laser sintering (DMLS) additive manufacturing process is presented. The primary application of the model is for determining the temperature history of parts fabricated using DMLS to evaluate residual stresses found in finished pieces and to assess manufacturing process strategies to reduce part slumping. The model utilizes MSC SINDA as a heat transfer solver with imbedded FORTRAN computer code to direct laser motion, apply laser heating as a boundary condition, and simulate the addition of metal powder layers during part fabrication. Model results are compared to available data collected during in situ DMLS part manufacture.

Journal of Research of the National Institute of Standards and Technology

Journal of Research of the National Institute of Standards and Technology

Solar cells have been used to convert sunlight to electrical energy for many years and also offer great potential for non-solar energy conversion applications. Their greatly improved performance under monochromatic light compared to sunlight, makes them suitable as photovoltaic (PV) receivers in laser power beaming applications. Laser beamed power to a PV array receiver could provide power to satellites, an orbital transfer vehicle, or a lunar base. Gallium arsenide (GaAs) and indium phosphide (InP) solar cells have calculated efficiencies of more than 50 percent under continuous illumination at the optimum wavelength. Currently high power free-electron lasers are being developed which operate in pulsed conditions. Understanding cell behavior under a laser pulse is important in the selection of the solar cell material and the laser. An experiment by NAsA lewis and JPL at the AVLIS laser facility in Livermore, CA presented experimental data on cell performance under pulsed laser illumination. Reference 5 contains an overview of technical issues concerning the use of solar cells for laser power conversion, written before the experiments were performed. As the experimental results showed, the actual effects of pulsed operation are more complicated. Reference 6 discusses simulations of the output of GaAs concentrator solar cells under pulsed laser illumination. The present paper continues this work, and compares the output of Si and GaAs solar cells.

Free space optical (FSO) communication technology is increasingly used for both commercial and military purposes. For military communication that requires fast, secure, and reliable transmission of information, the benefits of using FSO technology are important. However, there are some inherent limitations, especially due to environmental conditions such as turbulence, that can significantly affect the performance of FSO communication systems. Since turbulence plays a critical role in FSO systems, the purpose of this thesis is to compare different experimental and theoretical methods for estimating turbulence in a maritime environment. This analysis is achieved by measuring meteorological parameters using a local weather station and then using that data to calculate (using machine learning regression analysis) the refraction structure parameter that indicates the severity of turbulence. Also, the impact of turbulence on the signal-to-noise ratio (SNR) for a notional FSO system is estimated to show that even moderate amounts of turbulence can significantly reduce the SNR.

Free space optical (FSO) communication technology is increasingly used for both commercial and military purposes. For military communication that requires fast, secure, and reliable transmission of information, the benefits of using FSO technology are important. However, there are some inherent limitations, especially due to environmental conditions such as turbulence, that can significantly affect the performance of FSO communication systems. Since turbulence plays a critical role in FSO systems, the purpose of this thesis is to compare different experimental and theoretical methods for estimating turbulence in a maritime environment. This analysis is achieved by measuring meteorological parameters using a local weather station and then using that data to calculate (using machine learning regression analysis) the refraction structure parameter that indicates the severity of turbulence. Also, the impact of turbulence on the signal-to-noise ratio (SNR) for a notional FSO system is estimated to show that even moderate amounts of turbulence can significantly reduce the SNR.

Scattering calculations using a more detailed model of the multimode laser beam in the forward-scattering spectrometer probe (FSSP) were carried out by using a recently developed extension to Mie scattering theory. From this model, new calibration curves for the FSSP were calculated. The difference between the old calibration curves and the new ones is small for droplet diameters less than 10 micrometers, but the difference increases to approximately 10% at diameters of 50 micrometers. When using glass beads to calibrate the FSSP, calibration errors can be minimized, by using glass beads of many different diameters, over the entire range of the FSSP. If the FSSP is calibrated using one-diameter glass beads, then the new formalism is necessary to extrapolate the calibration over the entire range.

The Free Electron Laser (FEL) and Rail Gun are electric weapons which will require a significant amount of stored energy for operation. These types of weapons are ideal for use onboard an all-electric ship. An investigation is made of the effects these weapons will have on a proposed electrical system architecture using simulation modeling. Specifically, this thesis identifies possible design weaknesses and shows where further research and modeling is needed in order to ensure the proper integration of these electric weapons onboard an all-electric ship. The integration of these electric weapon systems with the power systems on electric ships will have an impact on naval operations. Several scenarios concerning specific naval missions are investigated using simulation software to understand the impact and limitations on the electric system using these new electric weapons.

A nonreactive kinetics model of the flowing gaseous medium of a cw HF optical resonance transfer laser (ORTL) has been developed and used to examine the prospects of this type of laser for efficient performance. Model development has required the generation of HF(v1, J1) + HF(v2, J2) V-V, R-R, T, and V-R cross section by classical trajectory calculations and by surprisal methods. These cross section packages have been validated using non-ORTL HF kinetics experiments, and the entire ORTL medium model has been validated by modeling the reported experimental results of past ORTL experiments without any adjustment of the kinetics packages. The subsequent study observing the effect on small signal gain by varying controllable parameters, such as pumping laser radiative flux and flux distribution, flow velocity, flow density, and HF mole fractions, has suggested a regime for most efficient ORTL operation, in which overall efficiency could reach 0.50. For this regime., pumping laser fluxes should exceed 5000 W/per sq cm with a spectral pattern in which most of the power is in P1(5), p1(6), p2(5), and p2(6). The medium should be operated at slightly elevated temperatures to enhance kinetic-pumping rates with characteristic velocities allowing any ORTL fluid element to remain in the pump beam no longer than 200 microsec.

A theoretical model is developed for the interaction of intense femtosecond laser pulses with solid targets on the basis of the two-temperature equation of state for an irradiated substance. It allows the description of the dynamics of the plasma formation and expansion. Comparison of available experimental data on the amplitude and phase of the complex reflection coefficient of aluminum with the simulation results provides new information on the transport coefficients and absorption capacity of the strongly coupled Al plasma over a wide range of temperatures and pressures.

The overall goal of the project was to develop advanced computational models for analysis of the effect of external flow on material removal in laser ablation of metals in oxidizing environment and under the influence of air flow. The models are applied for investigation of the combined effect of the laser energy deposition, chemical reactions, and air flow on the material removal rate and the nature of laser damage/modification of irradiated targets. The results of this project include the elucidation of (1) the relative contributions of the recoil vapor pressure and Marangoni effects on the melt dynamics and melt through time in the laser melting of free-standing aluminum films, (2) the effect of the external gas pressure on the flow structures and the mechanisms of the alumina and oxygen transport to the target surface, (3) the distinct characteristics of short pulse laser interactions with a metal target under conditions of spatial confinement by a solid transparent overlayer, (4) the conditions for the formation of nanocrystalline surface layer, sub-surface voids, and frozen surface nanospikes in surface nanostructuring by short laser pulses, (5) the effect of sample porosity on laser-induced carbon pyrolysis and oxidation.

A combined experimental and modeling study of neat and NH3-doped (Phi = 1), 30-Torr flames is reported. The major species concentrations are measured by molecular beam mass spectrometry (MB/MS), whereas the minor species OH, NH, and O-atom concentrations are measured by laser-induced fluorescence (LIF). The species NO is measured both by LIF and MB/MS, and O2 by MB/MS. The flame temperatures are measured both by OH and NH LIF and by thin-wire thermometry. The flames are modeled with PREMIX using the temperature profiles and several detailed chemical mechanisms as input. The mechanisms include the GRI 2.11, SSLA, and their derivatives. The SSLA mechanism was developed previously in our laboratory from a critical literature review. Calculations using all the mechanisms predict fairly well the profiles of the major species for both neat and doped flames. However, both the SSLA and GRI 2.11 calculations fail to predict the postflame O2 concentration in the neat flame, the drop in the O2 concentration with the addition of NH3, and the NH3 decay in the doped flame. Sensitivity analyses suggest refinements to the SSLA and GRI 2.11 mechanisms. The experimental results are predicted rather well using a modified SSLA mechanism in which the NH+NO=N2O+H reaction rate is decreased and the N2O+M=N2+O+M reaction rate and/or H2O third-body efficiency is increased to the limit of their uncertainty. Rate analyses performed on the modeled calculations reveal the reactions important to NO, O2, NH, OH, and O-atom production and consumption and NH3 consumption. These reactions are presented and discussed.

The Satellite Survivability Module (SSM) is an end-to-end, physics-based, performance prediction model for directed energy engagement of orbiting spacecraft. Two engagement types are currently supported: laser engagement of the focal plane array of an imaging spacecraft and Radio Frequency (RF) engagement of spacecraft components. For laser engagements, the user creates a spacecraft, its optical system, any protection techniques used by the optical system, a laser threat, and an atmosphere through which the laser will pass. For RF engagements, the user creates a spacecraft (as a set of subsystem components), any protection techniques, and an RF source. SSM then models the engagement and its impact on the spacecraft using four impact levels: degradation, saturation, damage, and destruction. Protection techniques, if employed, will mitigate engagement effects. SSM currently supports two laser and three RF protection techniques. SSM allows the user to create and implement a variety of what if scenarios. Satellites can be placed in a variety of orbits. Threats can be placed anywhere on the Earth. Satellites and threats can be mixed and matched to examine possibilities. Protection techniques for a particular spacecraft can be turned on or off individually and can be arranged in any order to simulate more complicated protection schemes. Results can be displayed as 2-D or 3-D visualizations, or as textual reports.

The linewidth and Einstein coefficient values required for modeling the HCl non-reacting mixing laser (NRML) have been reviewed and new linewidth calculations prepared. The results have been fit to a simple functional form suitable for inclusion in detailed laser modeling codes. Further development was also carried out of an efficient non-equilibrium chemistry subroutine suitable for inclusion in laser modeling codes. Listings of the detailed results and the chemistry code are given in appendices of the report. (Author)

In this paper, the propagation of ultrasonic waves generated by a laser pulse in thermoelastic regime up to the evaporation threshold of the material is modeled in two-dimensions by using the Code-Aster finite element solver. While in previous studies, this problem is solved in one step, in the new approach, we use different meshes and different increment times for modeling each of the thermal and mechanical parts of the problem. This new approach results in significant reduction in computation time. The propagation of ultrasonic waves generated by laser powers of up to 10 mJ with 17 ns pulse duration are modeled as a function of time. Computation time for separate thermal and mechanical analyses on the computer used for this purpose was 1 hour and 55 minutes; while combined analysis of the problem on the same computer takes at least 11 hours and 29 minutes. The resulting longitudinal and transverse waves, as well as the thermal behaviour of the material are then analysed and the results are compared with experimental and numerical data available in the literature. Very good agreement is observed between our simulation results and experimental and numerical results available in the literature which indicates that the laser ultrasonics process is accurately modeled by using the new approach and new finite element solver.

This article is from Optics Express , volume 18 . Abstract To better understand the limitations of high-resolution adaptive optics scanning laser ophthalmoscopy (AOSLO), we describe an imaging model that examines the smallest cone photoreceptors in the fovea of normal human subjects and analyze how different factors contribute to their resolution. The model includes basic optical factors such as wavelength and pupil size, and defines limits caused by source coherence which are specific to the AOSLO imaging modality as well as foveal cone structure. The details of the model, its implications for imaging, and potential techniques to circumvent the limitations are discussed in this paper.

Ultra-strong laser pulses can be so intense that an electron in the focused beam loses significant energy due to gamma-photon emission while its motion deviates via the radiation back-reaction. Numerical methods and tools designed to simulate radiation-dominated and QED-strong laser-plasma interactions are summarized here.

An experiment and numerical investigation is presented of a lifted turbulent H2/N2 jet flame in a coflow of hot, vitiated gases. The vitiated coflow burner emulates the coupling of turbulent mixing and chemical kinetics exemplary of the reacting flow in the recirculation region of advanced combustors. It also simplifies numerical investigation of this coupled problem by removing the complexity of recirculating flow. Scalar measurements are reported for a lifted turbulent jet flame of H2/N2 (Re = 23,600, H/d = 10) in a coflow of hot combustion products from a lean H2/Air flame ((empty set) = 0.25, T = 1,045 K). The combination of Rayleigh scattering, Raman scattering, and laser-induced fluorescence is used to obtain simultaneous measurements of temperature and concentrations of the major species, OH, and NO. The data attest to the success of the experimental design in providing a uniform vitiated coflow throughout the entire test region. Two combustion models (PDF: joint scalar Probability Density Function and EDC: Eddy Dissipation Concept) are used in conjunction with various turbulence models to predict the lift-off height (H(sub PDF)/d = 7,H(sub EDC)/d = 8.5). Kalghatgi's classic phenomenological theory, which is based on scaling arguments, yields a reasonably accurate prediction (H(sub K)/d = 11.4) of the lift-off height for the present flame. The vitiated coflow admits the possibility of auto-ignition of mixed fluid, and the success of the present parabolic implementation of the PDF model in predicting a stable lifted flame is attributable to such ignition. The measurements indicate a thickened turbulent reaction zone at the flame base. Experimental results and numerical investigations support the plausibility of turbulent premixed flame propagation by small scale (on the order of the flame thickness) recirculation and mixing of hot products into reactants and subsequent rapid ignition of the mixture.

http://uf.catalog.fcla.edu/uf.jsp?st=UF002641240&ix=pm&I=0&V=D&pm=1

An analytical expression for approximating the time-dependent thermal focal length in finite solid state laser rods was derived. The analysis is based on the temperature variation of the material refractive index caused by optical pumping of these rods. Several quantities were found to be relevant to this analysis. These quantities were the specific thermal profiles of the rods, type of optical pumping employed, type of cooling scheme employed (side and end-cooling parameters), and the specific material characteristics of the rods. The Thermal Lensing Model was formulated using the geometric ray tracing approach. The focal lengths are then approximated, by calculating the phase shift in the index of refraction, as the different rays of an incident plane wave are tracked through a lens-like crystal medium. The approach also applies in the case of Gaussian or parabolic pump beams. It is shown that the prediction of thermal focal length is in good quantitative agreement with experimentally obtained data.

We have developed the first laser damage simulation algorithm capable of determining crater and surface modification morphology from microscopic physics. Rapid progress in the field of high intensity ultrafast lasers has enabled its utility in a myriad of applications. Simulation plays an important role in this research by allowing for closer analysis of the physical mechanisms involved, but current techniques struggle to meet both the spatial scope or resolution requirements for modeling such dynamics, typically specializing in one or the other. Consequently, it is difficult to extract the physical form of the laser induced surface modification, hampering direct comparison of simulation to experimental results. Our algorithm offers a compromise to existing simulation techniques and enables the production of a complete density profile in addition to the simulation of intermediate dynamics. We use this capability to directly test our simulation against experimentally produced copper craters. Additionally, we show how our algorithm can be used to model the formation of surface roughness and nanoparticles.

We have developed the first laser damage simulation algorithm capable of determining crater and surface modification morphology from microscopic physics. Rapid progress in the field of high intensity ultrafast lasers has enabled its utility in a myriad of applications. Simulation plays an important role in this research by allowing for closer analysis of the physical mechanisms involved, but current techniques struggle to meet both the spatial scope or resolution requirements for modeling such dynamics, typically specializing in one or the other. Consequently, it is difficult to extract the physical form of the laser induced surface modification, hampering direct comparison of simulation to experimental results. Our algorithm offers a compromise to existing simulation techniques and enables the production of a complete density profile in addition to the simulation of intermediate dynamics. We use this capability to directly test our simulation against experimentally produced copper craters. Additionally, we show how our algorithm can be used to model the formation of surface roughness and nanoparticles.

The topics covered include the following: (1) CO2 laser kinetics modeling; (2) gas lifetimes in pulsed CO2 lasers; (3) frequency chirp and laser pulse spectral analysis; (4) LAWS A' Design Study; and (5) discharge circuit components for LAWS. The appendices include LAWS Memos, computer modeling of pulsed CO2 lasers for lidar applications, discharge circuit considerations for pulsed CO2 lidars, and presentation made at the Code RC Review.

The topics covered include the following: (1) CO2 laser kinetics modeling; (2) gas lifetimes in pulsed CO2 lasers; (3) frequency chirp and laser pulse spectral analysis; (4) LAWS A' Design Study; and (5) discharge circuit components for LAWS. The appendices include LAWS Memos, computer modeling of pulsed CO2 lasers for lidar applications, discharge circuit considerations for pulsed CO2 lidars, and presentation made at the Code RC Review.

The Stratton-Chu integral representation of electromagnetic fields is used to study the spatio-temporal properties of large bandwidth laser pulses focused by high numerical aperture mirrors. We review the formal aspects of the derivation of diffraction integrals from the Stratton-Chu representation and discuss the use of the Hadamard finite part in the derivation of the physical optics approximation. By analyzing the formulation we show that, for the specific case of a parabolic mirror, the integrands involved in the description of the reflected field near the focal spot do not possess the strong oscillations characteristic of diffraction integrals. Consequently, the integrals can be evaluated with simple and efficient quadrature methods rather than with specialized, more costly approaches. We report on the development of an efficiently parallelized algorithm that evaluates the Stratton-Chu diffraction integrals for incident fields of arbitrary temporal and spatial dependence. This method has the advantage that its input is the unfocused field coming from the laser chain, which is experimentally known with high accuracy. We use our method to show that the reflection of a linearly polarized Gaussian beam of femtosecond duration off a high numerical aperture parabolic mirror induces ellipticity in the dominant field components and generates strong longitudinal components. We also estimate that future high-power laser facilities may reach intensities of $10^{24}\,\text{W/cm}^2$.

A computational plasma aerodynamics model is developed to study the performance of a laser propelled Lightcraft. The computational methodology is based on a time-accurate, three-dimensional, finite-difference, chemically reacting, unstructured grid, pressure-based formulation. The underlying physics are added and tested systematically using a building-block approach. The physics modeled include non-equilibrium thermodynamics, non-equilibrium air-plasma finite-rate kinetics, specular ray tracing, laser beam energy absorption and refraction by plasma, non-equilibrium plasma radiation, and plasma resonance. A series of transient computations are performed at several laser pulse energy levels and the simulated physics are discussed and compared with those of tests and literatures. The predicted coupling coefficients for the Lightcraft compared reasonably well with those of tests conducted on a pendulum apparatus.

Theoretical models for the laser heated thruster are reviewed. The requirements for an improved laser heated thruster theoretical model are discussed. The application of a time dependent finite difference Navier-Stokes equation solution to the laser heated thruster problem is described, along with a simple closed form solution which was developed in order to gain insight into the difficulties encountered in the pursuit of the numerical solution.

This report is a summary of research activities funded by AFOSR Lab Task 13RXCOR. An integrated computational materials science and engineering (ICMSE) approach was used to investigate the role of fiber-matrix interfaces in controlling optical and thermal energy flow in structural composite materials subject to high energy laser irradiation. Modeling tools, including molecular dynamics and finite elements methods, were used to characterize optical and thermal transport at multiple length scales spanning from nanometers to millimeters. Advanced experimental characterization techniques were used to parameterize and validate these models. Two new experimental techniques were developed at AFRL as part of this project: lateral thermal transport measurements using scanning thermal microscopy (SThM), and microscale temperature sensing via gold nanorods. Modeling and experimental techniques are explained, and important considerations for future work are outlined. Peer-reviewed articles containing detailed results and analysis are included as an appendix.

This report is a summary of research activities funded by AFOSR Lab Task 13RXCOR. An integrated computational materials science and engineering (ICMSE) approach was used to investigate the role of fiber-matrix interfaces in controlling optical and thermal energy flow in structural composite materials subject to high energy laser irradiation. Modeling tools, including molecular dynamics and finite elements methods, were used to characterize optical and thermal transport at multiple length scales spanning from nanometers to millimeters. Advanced experimental characterization techniques were used to parameterize and validate these models. Two new experimental techniques were developed at AFRL as part of this project: lateral thermal transport measurements using scanning thermal microscopy (SThM), and microscale temperature sensing via gold nanorods. Modeling and experimental techniques are explained, and important considerations for future work are outlined. Peer-reviewed articles containing detailed results and analysis are included as an appendix.

This report is a summary of research activities funded by AFOSR Lab Task 13RXCOR. An integrated computational materials science and engineering (ICMSE) approach was used to investigate the role of fiber-matrix interfaces in controlling optical and thermal energy flow in structural composite materials subject to high energy laser irradiation. Modeling tools, including molecular dynamics and finite elements methods, were used to characterize optical and thermal transport at multiple length scales spanning from nanometers to millimeters. Advanced experimental characterization techniques were used to parameterize and validate these models. Two new experimental techniques were developed at AFRL as part of this project: lateral thermal transport measurements using scanning thermal microscopy (SThM), and microscale temperature sensing via gold nanorods. Modeling and experimental techniques are explained, and important considerations for future work are outlined. Peer-reviewed articles containing detailed results and analysis are included as an appendix.

We discuss the dynamical behavior of the entanglement between the internal and the external degrees of freedom of a trapped atom in electromagnetically-induced transparency (EIT) laser cooling. It is shown that essential features of the intricate entanglement dynamics observed in full numerical simulations of the underlying quantum master equation can be understood in terms of a two-state model on the basis of Landau-Zener splittings in the atom-laser field Hamiltonian. An extension of this model to an effective non-Hermitian Hamiltonian is constructed which describes the decay of entanglement by spontaneous emission processes. We also discuss schemes for the control of entanglement and demonstrate that a permanent entanglement can be imprinted on trapped atoms through a rapid switch off of the driving fields. Finally, we point out fundamental distinctions between the entanglement created in EIT cooling and in the cooling scheme based on velocity-selective coherent population trapping.