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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.

The effects of short pulsed lasers in ocular tissues involve both thermal and mechanical damage. In the past year of study we have examined the literature values for the threshold radiant exposure causing 'minimal visible lesion'. In particular, the threshold exposures for short pulses in the sub-100-ps regime were examined. Two possible mechanisms of mechanical damage in the retina were considered: (1) melanosomal disruption, and (2) shock front development.

The objective of this basic research effort is to provide the theoretical foundations for modeling/simulating the interaction of aerosols and ice at high altitudes (near and above the tropopause). The AF, together with the ABL SPO (through the MDA), anticipate that aerosols/ice will impact the efficacy of the ABL and the ability to predict possible impacts is of great value. The PI and his colleagues will interrogate a number of conceptual schemes which incorporate cloud microphysics (with 6 classes of moisture species together with actual ice concentrations). Particular attention will be paid to ice nucleation scenarios.

A two-dimensional laser Doppler anemometer was designed to take measurements in a ducted coaxial flow system having a centerbody separating the jets. The measurements, made in isothermal flow, can be categorized as: central jet only (C02); annular jet only (air); and, combined central and annular jet flow. Radial profiles of the axial and radial/azimuthal velocity were obtained. The central jet only condition approximates a free jet since the central jet diameter is much smaller than the centerbody or duct diameter (1:29.3: 53.2). Measurements of the jet development (diameter, turbulence intensity, etc.) were compared with previous free jet studies. In a study of the isotropy of the turbulent fluctuations it was determined that fully developed turbulent flow begins further downstream than anticipated. Velocity profiles were combined with gas sampling measurements to ascertain jet entrainment. Results show a linear growth of the entrained mass starting at the entrance plane. Downstream (20 diameters) the rate of growth increases to match past studies in that region. The annular jet flow creates a recirculation vortex in the wake of the centerbody. The flowfield was mapped and the streamlines describing the flow were obtained throughout the recirculating flow and further downstream.

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)

The recombination of CO and O2 formed by the dissociation of CO2 in a sealed CO2 laser discharge zone is examined. Conventional base-metal-oxide catalysts and conventional noble-metal catalysts are not effective in recombining the low O2/CO ratio at the low temperatures used by the lasers. The use of Pt/SnO2 as the noble-metal reducible-oxide (NMRO), or other related materials from Group VIIIA and IB and SnO2 interact synergistically to produce a catalytic activity that is substantially higher than either componet separately. The Pt/SnO2 and Pd/SnO2 were reported to have significant reaction rates at temperatures as low as -27 C, conditions under which conventional catalysts are inactive. The gas temperature range of lasers is 0 + or - 40 C. There are three general ways in which the NMRO composite materials can interact synergistically: one component altering the properties of another component; the two components each providing independent catalytic functions in a complex reaction mechanism; and the formation of catalytic sites through the combination of two components at the atomic level. All three of these interactions may be important in low temperature CO oxidation over NMRO catalysts. The effect of the noble metal on the oxide is discussed first, followed by the effect of the oxide on the noble metal, the interaction of the noble metal and oxide to form catalytic sites, and the possible ways in which the CO oxidation reaction is catalyzed by the NMRO materials.

Flow in a pressurized, vapor condensation (PVC) boron nitride nanotube (BNNT) production rig is modeled. A laser provides a thermal energy source to the tip of a boron ber bundle in a high pressure nitrogen chamber causing a plume of boron-rich gas to rise. The buoyancy driven flow is modeled as a mixture of thermally perfect gases (B, B2, N, N2, BN) in either thermochemical equilibrium or chemical nonequilibrium assuming steady-state melt and vaporization from a 1 mm radius spot at the axis of an axisymmetric chamber. The simulation is intended to define the macroscopic thermochemical environment from which boron-rich species, including nanotubes, condense out of the plume. Simulations indicate a high temperature environment (T > 4400K) for elevated pressures within 1 mm of the surface sufficient to dissociate molecular nitrogen and form BN at the base of the plume. Modifications to Program LAURA, a finite-volume based solver for hypersonic flows including coupled radiation and ablation, are described to enable this simulation. Simulations indicate that high pressure synthesis conditions enable formation of BN vapor in the plume that may serve to enhance formation of exceptionally long nanotubes in the PVC process.

Journal of Research of the National Institute of Standards and Technology

Volume I of this report describes and documents a comprehensive and reasonably general computer analysis applicable to a broad class of transient, electrically excited laser systems. The theoretical model is based on a coupled analysis of molecular kinetics, electron kinetics, external driving circuit, and radiative extraction. A complete discussion has been given in Volume I. Volume II contains the FORTRAN listing for these programs and their subroutines, which were developed for the CDC 6000 and CYBER series computer system. (Author)

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.

The project initiated a study of a mathematical model of a tunable Ti:sapphire solid-state laser. A general mathematical model was developed for the purpose of identifying design parameters which will optimize the system, and serve as a useful predictor of the system's behavior.

A fusion technique which combines two different types of sensory data for 3-D modeling of a navigation space is presented. The sensory data is generated by a vision camera and a laser scanner. The problem of different resolutions for these sensory data was solved by reduced image resolution, fusion of different data, and use of a fuzzy image segmentation technique.

The chemical kinetic aspects of a transient increase in OH and HO2 by several orders of magnitude are explored in three model tropospheres. This chemical kinetic modeling effort was undertaken to support the operation of a pump-and-probe LIDAR instrument. A powerful excimer laser pulse perturbs the troposphere after which its relaxation back to steady state is examined by remote sensing, for example by DIAL or LIF. Instead of probing ambient levels of key free radicals, a study of the relaxation kinetics in real time enables chemical mechanistic studies in situ.

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.

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.

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.

The optical forces in optical tweezers can be robustly modeled over a broad range of parameters using generalsed Lorenz-Mie theory. We describe the procedure, and show how the combination of experimental measurement of properties of the trap coupled with computational modeling, can allow unknown parameters of the particle - in this case, the refractive index - to be determined.

We describe how a new framework for coupling a full-PIC algorithm with a reduced PIC algorithm has been implemented into the code OSIRIS. We show that OSIRIS with this new hybrid-PIC algorithm can efficiently and accurately model high energy density scenarios such as ion acceleration in laser-solid interactions and fast ignition of fusion targets. We model for the first time the full density range of a fast ignition target in a fully self-consistent hybrid-PIC simulation, illustrating the possibility of stopping the laser generated electron flux at the core region with relatively high efficiencies. Computational speedups greater than 1000 times are demonstrated, opening the way for full-scale multi-dimensional modeling of high energy density scenarios and for the guiding of future experiments.

We test numerical filamentation models against experimental data about the peak intensity and filament density in laser filaments. We show that the consideration of the higher-order Kerr effect (HOKE) improves the quantitative agreement without the need of adjustable parameters.

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.

Modeling of laser-plasma wakefield accelerators in an optimal frame of reference \cite{VayPRL07} is shown to produce orders of magnitude speed-up of calculations from first principles. Obtaining these speedups requires mitigation of a high-frequency instability that otherwise limits effectiveness in addition to solutions for handling data input and output in a relativistically boosted frame of reference. The observed high-frequency instability is mitigated using methods including an electromagnetic solver with tunable coefficients, its extension to accomodate Perfectly Matched Layers and Friedman's damping algorithms, as well as an efficient large bandwidth digital filter. It is shown that choosing the frame of the wake as the frame of reference allows for higher levels of filtering and damping than is possible in other frames for the same accuracy. Detailed testing also revealed serendipitously the existence of a singular time step at which the instability level is minimized, independently of numerical dispersion, thus indicating that the observed instability may not be due primarily to Numerical Cerenkov as has been conjectured. The techniques developed for Cerenkov mitigation prove nonetheless to be very efficient at controlling the instability. Using these techniques, agreement at the percentage level is demonstrated between simulations using different frames of reference, with speedups reaching two orders of magnitude for a 0.1 GeV class stages. The method then allows direct and efficient full-scale modeling of deeply depleted laser-plasma stages of 10 GeV-1 TeV for the first time, verifying the scaling of plasma accelerators to very high energies. Over 4, 5 and 6 orders of magnitude speedup is achieved for the modeling of 10 GeV, 100 GeV and 1 TeV class stages, respectively.

A computer simulation of a pulsed HF laser pumped by the H2 + F2 chain reaction is presented. A chemical kinetic model encompassing 68 reactions is used to approximate the reacting mixture contained within an optical cavity. For each vibrational level, a Boltzmann distribution for the rotational levels is assumed with lasing on the vibrational band at line center of the transition having maximum gain. An analysis of cavity and chemical mechanisms yields a simple relationship between the pumping and depletion rates of the vibrational levels. This relationship is used to make a comprehensive study of the effects of cavity and chemical parameters on the laser pulse. The effect of changes in uncertain chemical reaction rate coefficients is assessed.

A laser deposition process involves the supply of metallic powders into a laser-heated spot where the powder is melted and forms a melt puddle which quickly solidifies into a bead. In order to design an effective system, the laser beam, the powder beam, and their interactions need to be fully understood. In this paper, the laser-material interaction within the melt pool is reported using a multi-scale model: a macroscopic model to model mass, heat, and momentum transfer. Experiments were also conducted to validate the simulation model.

As lasers become more pervasive, the dangers posed by laser radiation to the human retina have increased. Computer aided modeling of laser tissue interaction in the retina allows for researchers to simulate parameter ranges where exposure can cause damage. While this approach has been studied using a variety of methods, one of the most referenced models has been the Thompson-Gerstman melanin granule model. Due to limited computing resources at the time, the original FORTRAN version of this model was implemented as a stand-alone serial code and was only able to model single-pulse exposures. A new C++ version of the Thompson-Gerstman model has been implemented, which expands both the functionality and portability of the method. The source type has been expanded to include multi-pulse as well as single pulse. Additionally, the functions developed by Thompson and Gerstman are now part of a library that can be accessed through other modeling software.

Thermal neutrons are of considerable interest to the Department of Defense and for commercial applications. Unlike high energy photons, neutrons easily penetrate high density targets, but get effectively absorbed by low density materials like paraffin, nylon or explosives. This makes them attractive complements to X-rays for radiographic applications, e.g. for the detection or inspection of explosives inside steel casings. The key challenge is to develop a compact generator for thermal neutrons with large enough flux. The limited availability of radio-isotopes, combined with the relatively short half-life, safety constraints and regulatory requirements make them unattractive for wide-spread use. An alternative design exploits the Deuterium-Tritium (D-T) fusion, which generates Alpha particles and fast neutrons. In these sources, Deuterium ions are accelerated to about 130 keV and hit a Tritium target. The acceleration of Deuterium ions is usually accomplished in a diode configuration. Recently, considerable success has been achieved in the acceleration of ions via laser-matter interaction. In this project we investigated whether laser-accelerated ions could undergo nuclear fusion in an adequately designed target and could be used for neutron sources. We therefore enhanced our proprietary plasma simulation code VORPAL with a model for fusion reactions and investigated the generation of neutrons in shaped D-T targets. We find that neutron fluxes large enough for radiographic applications can be generated by utilizing moderate ( 10(exp 17)-10(exp 18) W/sq cm) laser intensities.

A multiple quantum well (MQW) transistor vertical-cavity surface-emitting laser (T-VCSEL) is designed and numerically modeled. The important physical models and parameters are discussed and validated by modeling a conventional VCSEL and comparing the results with the experiment. The quantum capture/escape process is simulated using the quantum-trap model and shows a significant effect on the electrical output of the T-VCSEL. The parameters extracted from the numerical simulation are imported into the analytic modeling to predict the frequency response and simulate the large-signal modulation up to 40 Gbps.

We have analyzed the image formation and dynamic properties in laser speckle imaging (LSI) both experimentally and with Monte-Carlo simulation. We show for the case of a liquid inclusion that the spatial resolution and the signal itself are both significantly affected by scattering from the turbid environment. Multiple scattering leads to blurring of the dynamic inhomogeneity as detected by LSI. The presence of a non-fluctuating component of scattered light results in the significant increase in the measured image contrast and complicates the estimation of the relaxation time. We present a refined processing scheme that allows a correct estimation of the relaxation time from LSI data.

We present a novel laser mode-locking scheme and discuss its unusual properties and feasibility using a theoretical model. A large set of single-frequency continuous-wave lasers oscillate by amplification in spatially separated gain media. They are mutually phase-locked by nonlinear feedback from a common saturable absorber. As a result, ultra short pulses are generated. The new scheme offers three significant benefits: the light that is amplified in each medium is continuous wave, thereby avoiding issues related to group velocity dispersion and nonlinear effects that can perturb the pulse shape. The set of frequencies on which the laser oscillates, and therefore the pulse repetition rate, is controlled by the geometry of resonator-internal optical elements, not by the cavity length. Finally, the bandwidth of the laser can be controlled by switching gain modules on and off. This scheme offers a route to mode-locked lasers with high average output power, repetition rates that can be scaled into the THz range, and a bandwidth that can be dynamically controlled. The approach is particularly suited for implementation using semiconductor diode laser arrays.

Ultrafast shadowgraphy is a new experimental technique that uses few cycle laser pulses to image density gradients in a rapidly evolving plasma. It enables structures that move at speeds close to the speed of light, such as laser driven wakes, to be visualized. Here we study the process of shadowgraphic image formation during the propagation of a few cycle probe pulse transversely through a laser-driven wake using three-dimensional particle-in-cell simulations. In order to construct synthetic shadowgrams a near-field snapshot of the ultrashort probe pulse is analyzed by means of Fourier optics, taking into account the effect of a typical imaging setup. By comparing synthetic and experimental shadowgrams we show that the generation of synthetic data is crucial for the correct interpretation of experiments. Moreover, we study the dependence of synthetic shadowgrams on various parameters such as the imaging system aperture, the position of the object plane and the probe pulse delay, duration and wavelength. Finally, we show that time-dependent information from the interaction can be recovered from a single shot by using a broadband, chirped probe pulse and subsequent spectral filtering.

The results of a survey of pressure-broadening data in the literature, pertaining to species present in H2-F2 chemical lasers are described. Pressure-broadened linewidths of the HF vibration/rotation transitions were found to depend strongly on the vibrational and rotational quantum numbers, as well as on the perturbing species. The experimental data were incorporated into a rate equation model of the pulsed H2-F2 laser; where experimental data were unavailable, theoretical values were used. The calculations made from this model demonstrate the importance of including such detailed pressure-broadening information. Recent experimental results of Kwok and Cohen for HF V yeilds R, T deactivation were also included in the model and were taken to be a multiquantum process. Comparisons of model predictions for pulse energy and pulse duration with several atmospheric-pressure laser measurements showed good agreement. (Author)

This thesis developed a finite element solution of the Schrodinger wave equation. This technique is used by a computer program to calculate the energy levels and wave functions of a diatomic molecular for a particular potential energy model. The potential energy model is a function of a set of parameters which a non-linear minimization routine varies before solving the wave equation. This is done in an iterative manner until the calculated energy levels agree in a least squares sense with the observed energy levels. Then the transition probabilities (Franck-Condon factors) between the wave functions are calculated by another program developed for this thesis. Finally, two programs were written to determine the energy levels observed in spectroscopic data. One uses Dunham coefficients and the Dunham equation while the second uses a least square fit to the data directly. The four programs were tested and appear to work correctly. The numeric solutions were compared with the analytic solutions of the single harmonic oscillator. Originator-supplied keywords include: Franck-Condon factors, Diatomic molecules, Molecular vibration, Schrodinger equation, and Numerical analysis.

The coupled partial differential equations describing stimulated Brillouin scattering (SBS) in a fiber are solved numerically. The SBS builds up from random thermal phonons when a laser beam of sufficient power is incident upon the fiber. We show that the SBS can be suppressed by linearly ramping the laser frequency at a rate of up to 1016 Hz/s. High chirp rates lead to an increased Brillouin spectral bandwidth and decreased gain. The resulting SBS suppression agrees well with an adiabatic model and with experimental results.

Laser cladding is an additive manufacturing process that a laser generates a melt-pool on the substrate material while a second material, as a powder or a wire form, is injected into that melt-pool. Among all laser manufacture processes, laser cladding offers the most extensive variety of possibilities to alter a component at its surface. Despite immense potentials and advancements, the process model of microstructure evolution and its coupling with macro parameter of laser cladding process has not been fully developed. To address this issue, a process model of microstructure evolution has been studied by utilizing a phase-field method. Phase-field method has become a widely used computation tool for the modeling of microstructure evolution with the advantage of avoiding of tracking the interface explicitly and satisfying interfacial boundary conditions. In present work, the numerical solutions of a phase-field model are analyzed. The connection of macro-process and microstructure evolution is examined by considering the relationship of macro- and micro- parameters. The effects of thermal noise and melt undercooling on the final microstructure have also been studied. The prediction results are compared with other researchers' results and good agreement was found. Different solidification morphologies of different locations in the melt pool are also investigated. It was found that not the mass transfer but the heat transfer in the melt pool dominates the solidification process.

Laser cladding is an additive manufacturing process that a laser generates a melt-pool on the substrate material while a second material, as a powder or a wire form, is injected into that melt-pool. Among all laser manufacture processes, laser cladding offers the most extensive variety of possibilities to alter a component at its surface. Despite immense potentials and advancements, the process model of microstructure evolution and its coupling with macro parameter of laser cladding process has not been fully developed. To address this issue, a process model of microstructure evolution has been studied by utilizing a phase-field method. Phase-field method has become a widely used computation tool for the modeling of microstructure evolution with the advantage of avoiding of tracking the interface explicitly and satisfying interfacial boundary conditions. In present work, the numerical solutions of a phase-field model are analyzed. The connection of macro-process and microstructure evolution is examined by considering the relationship of macro- and micro- parameters. The effects of thermal noise and melt undercooling on the final microstructure have also been studied. The prediction results are compared with other researchers' results and good agreement was found. Different solidification morphologies of different locations in the melt pool are also investigated. It was found that not the mass transfer but the heat transfer in the melt pool dominates the solidification process.

Experiments have been performed which measured the global absorption and thermal efficiency of laser sustained hydrogen plasmas for the first time. Results include global absorption as high as 90% and thermal efficiency as high as 80%. These results validate laser propulsion as a feasible orbital transfer technology. A kinetic nonequilibrium model of laser sustained hydrogen plasmas has been formulated and solved. This model is the first of its kind and includes a discretized beam raytrace with a variable index of refraction based upon plasma electron number density. Model results have compared favorably with experimental results and the model has been used to provide predictions of LSP performance well outside the realm of experiments. Multiple model solutions have been obtained which are dependent upon initial conditions. No significant kinetic nonequilibrium was observed in LSP core regions for incident powers up to 700 kW. Beam refraction by the LSP has been observed to have a major effect on LSP performance. The methodology formulated in this document has direct applicability to two temperature modeling of arcjet plasmas, work which is currently underway at UIUC.... Beamed energy propulsion, Two temperature modeling, Laser sustained hydrogen plasmas.

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.

A detailed numerical model has been developed for characterizing the important energy transfer processes operative in the CO/N2 direct current discharge laser system. The model is based upon a rate equation formulation which includes 30 levels of both CO and N2 and treats multi-quantum electron- molecule excitation processes, single quantum vibration-vibration exchange and vibration-translation energy transfer processes and both harmonic and overtone spontaneous emission terms. In this paper the time evolution of the CO vibrational distributions with and without N2 is calculated and the associated small signal gain is predicted and compared to the measurements of Jeffers and Wiswall.

New Mexico State University (NMSU) proposed acquisition of equipment to establish an integrated system for laser-assisted, non-intrusive experimentation and data-driven reduced-order modeling of multidisciplinary phenomena in structural and fluid dynamics. The system consists of three components: (i) a 1D scanning laser vibrometer for structural vibration tests; (ii) a 3D particle image velocimetry (PIV) system for experimental fluid dynamics; and (iii) a 3D scanner for computational mesh data. All these three components have been acquired and installed respectively in the designated laboratories, and are currently tested to build up an integrated system as proposed. This integrated system assists the PI and key personnel in promoting a novel systematic methodology for data-driven (yet physics-based) reduced-order models of strongly nonlinear, unsteady, multidisciplinary and multiscale dynamics by means of analytical / empirical decomposition methods. The experimental system has been utilized to promote STEM education at NMSU by encouraging students participation in research-oriented projects. The courses that benefited include ME 456 Experimental Modal Analysis, AE 447 / ME510 Aero/Fluids Laboratory, and ME 533 Computational Fluid Dynamics.

The mathematical modeling of reaction systems for chemical generation of atomic iodine is presented. This process can be applied in the chemical oxygen-iodine laser (COIL), where it can save a substantial part of energy of singlet oxygen and so increase the laser output power. The parametric study of the production of atomic fluorine and subsequently atomic iodine in dependence on the pressure and dilution with inert gas was made. The calculation of the interaction between produced atomic iodine and singlet oxygen was made with four different mixing/reacting schemes.

Laser Doppler Flowmetery (LDF) is a noninvasive method to assess tissue blood flow. Previously published works have proposed a mathematical model for LDF and showed that there is a relationship between first moment of power spectrum and the velocity of moving blood cells (MBC's). Also researchers have studied this method by mathematical analyses in various aspects. In this paper a new model for LDF based on Monte Carlo method is proposed. We have shown that this new model is more flexible and provides a better agreement with the experimental results. For example this model represents the nonlinear relationship between the first moment of power spectrum and the blood cells velocity which is not seen in previous models.

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.

We are modeling the propagation of high intensity laser pulses through nonlinear optical materials including interactions of two-photon absorption, excited-state absorption and nonlinear refraction including thermal refraction. We have developed a preliminary code written in C++ applicable to Pentium-based PC's that is currently running and being tested against known results over a large range of input parameters. In particular, this code is being used to model optical limiting devices for sensor protection applications. While agreement is excellent for most nonlinearities at relatively low input energies, at high inputs, where transmittance values can drop to low levels, deviations are observed. It is thought that acoustic effects arising from thermal transients may be responsible. This is currently under investigation. We have recently developed an approximate solution for these photoacoustic nonlinearities that is computationally much faster than our previous code which was so computationally intensive that practical problems were prohibitive. This code is now being tested to verify its range of validity.

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.

Experimental and chemical modeling studies of a 60 Torr, NH3/N2O/Ar flame are performed in order to provide further testing and refinements of a detailed chemical mechanism developed previously in our laboratory. This mechanism, which is denoted as VS, consists of 87 reactions and 20 species. Flame temperatures are measured with a coated thin-wire thermocouple and by rotational analyses of OH and NH laser-induced fluorescence (LIF) spectra. Species concentration profiles of NH3, N2O, N2, H2O, NO, O2, NH, O, and OH are recorded using molecular beam-mass spectrometry, LIF, or both. The experimental species concentrations are compared to those obtained with both equilibrium and PREMIX flame code calculations. The NH3/N2O mixture equilibrium calculations agree very well with both the PREMIX and measured postflame N2 and H2O concentrations but underpredict the postflame NO concentration and overpredict the O2 concentration. The PREMIX calculations predict very well the shapes of the experimental NH3, N2O, N2, H2O, NO, OH, and NH profiles throughout the flame but do not adequately predict the shape of the O-atom profile and overpredict the O2 concentration by 60% at 16.25 mm, suggesting that the VS mechanism requires some refinement. A VS modified mechanism, which provides better agreement with the experimental results, is proposed. In this mechanism, the rate constants of the following reactions are altered to the limit of their experimental or calculated uncertainties: (1) N2O + H = OH + N2, (2) NH2 + O = HNO + H, (3) NH2 + OH = NH + HO, (4) NH + H = H + NH, (5) NH3 + OH = NH2 + H2O, and (6) NO + H + M = HNO + M.

Theoretical predictions based on temperature dependent rate equations are made of the pulse response of the individual axial modes of high power laser diodes. The temperature transients in the active area of the lasers are simulated with a finite element code and entered into the rate equations which describe the time evolution of both the carrier population and the photon number inside the laser cavity. Experimentally, the laser diodes are triggered by a fast electrical pulse and the modes of gain-guided AlGaAs laser diodes are dispersed with a echelle grating and individually imaged onto a scanning avalanche photodiode (ADP). The results of this study showed that modal instabilities during pulsed operation of the laser diodes can be separated into two types of transients: mode buildup transients and thermally-induced transients. Mode buildup transients, due to feedback from the cavity gradually causing power to concentrate in a few modes, were found to last up to 60 ns depending on laser length, whereas thermally-induced transients were found to last several hundred nanoseconds and are dependent on the overall efficiency of the laser. Our experimental results show that broad-stripe lasers, which minimize current spreading, have much smaller temperature transients as compared to narrow single-stripe lasers.

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.

Accurate numerical predictions are essential to the design of scramjet combustors, and in order to develop and properly validate these predictions accurate temperature, species concentration, and velocity measurements must be obtained. Large eddy simulation (LES) promises to give improved predictions of turbulent, supersonic combustion relative to CFD RANS models, and therefore, a LES approach employing a subgrid turbulent mixing model was selected to model both reacting and non-reacting flow in a cavity-stabilized supersonic combustor. Hydroxyl tagging velocimetry (HTV) measurements were made in a non-reacting Mach 2 air flow over the cavity with and without an upstream strut. Velocity profiles were compared to LES simulations with good agreement in both configurations. In the flow with an upstream strut, there were some model-data discrepancies in the strut wake region where better resolution of the shear layer compression wave interactions were needed. UV Raman scattering was used to obtain temperature and major species concentrations in a Mach 2 flow over a cavity fueled with a 30% H2/70% CH4 mixture. UV laser-induced fluorescence obscured the Raman spectra in rich regions but data was obtained in most of the cavity. The Raman composition and temperature data were in reasonable agreement with the LES simulations.

A microscopic quantum-kinetic theory based on density matrix formalism is formulated to describe the processes of short pulse laser interaction with materials such as semiconductors accounting for arbitrary spatial inhomogeneities in the excitation conditions and other spatial phenomena such as filamentation of tightly focused femtosecond laser pulses, structural modification and catastrophic optical damage. A system of Boltzmann-Bloch transport equations are established that include both space and momentum dependence of the electron and hole distribution functions and the polarization. Microscopic electronphonon and electron-electron scattering terms as well as scattering terms that lead to transitions between valence and conduction bands, i.e. impact ionization and recombination terms, are included explicitly in the equations. The formulated theory describes the spatio-temporal dynamics of electrons and holes in inhomogeneously excited materials including the coherent interactions of carriers and the laser light field as well as transport due to spatial gradients and electrostatic forces.

A microscopic quantum-kinetic theory based on density matrix formalism is formulated to describe the processes of short pulse laser interaction with materials such as semiconductors accounting for arbitrary spatial inhomogeneities in the excitation conditions and other spatial phenomena such as filamentation of tightly focused femtosecond laser pulses, structural modification and catastrophic optical damage. A system of Boltzmann-Bloch transport equations are established that include both space and momentum dependence of the electron and hole distribution functions and the polarization. Microscopic electronphonon and electron-electron scattering terms as well as scattering terms that lead to transitions between valence and conduction bands, i.e. impact ionization and recombination terms, are included explicitly in the equations. The formulated theory describes the spatio-temporal dynamics of electrons and holes in inhomogeneously excited materials including the coherent interactions of carriers and the laser light field as well as transport due to spatial gradients and electrostatic forces.