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On the Initiation of High Explosives by Laser Radiation

Description: The problem of laser initiation of high explosives in munitions is considered. In this situation, the laser illuminates a small spot on the casing, and lateral thermal transport affects the initiation temperature. We use a variational method to calculate the critical temperature for explosive initiation as a function the laser spot size, for common high explosives. The effect of the dwelling time of the irradiation is then evaluated. We demonstrate that in typical situations the critical temperature is determined by the dwelling time rather than by the laser spot size.
Date: March 28, 2006
Creator: Rubenchik, A M
Partner: UNT Libraries Government Documents Department

Modeling Antimortar Lethality by a Solid-State Heat-Capacity Laser

Description: We have studied the use of a solid-state heat-capacity laser (SSHCL) in mortar defense. This type of laser, as built at LLNL, produces high-energy pulses with a wavelength of about 1 {micro}m and a pulse repetition rate of 200 Hz. Currently, the average power is about 26 kW. Our model of target interactions includes optical absorption, two-dimensional heat transport in the metal casing and explosive, melting, wind effects (cooling and melt removal), high-explosive reactions, and mortar rotation. The simulations continue until HE initiation is reached. We first calculate the initiation time for a range of powers on target and spot sizes. Then we consider an engagement geometry in which a mortar is fired at an asset defended by a 100-kW SSHCL. Propagation effects such as diffraction, turbulent broadening, scattering, and absorption are calculated for points on the trajectory, by means of a validated model. We obtain kill times and fluences, as functions of the rotation rate. These appear quite feasible.
Date: February 15, 2005
Creator: Boley, C D & Rubenchik, A M
Partner: UNT Libraries Government Documents Department

Modeling of Laser-Induced Metal Combustion

Description: Experiments involving the interaction of a high-power laser beam with metal targets demonstrate that combustion plays an important role. This process depends on reactions within an oxide layer, together with oxygenation and removal of this layer by the wind. We present an analytical model of laser-induced combustion. The model predicts the threshold for initiation of combustion, the growth of the combustion layer with time, and the threshold for self-supported combustion. Solutions are compared with detailed numerical modeling as benchmarked by laboratory experiments.
Date: February 20, 2008
Creator: Boley, C D & Rubenchik, A M
Partner: UNT Libraries Government Documents Department

Laser intensity modulation by nonabsorbing defects

Description: Nonabsorbing bulk defects can initiate laser damage in transparent materials. Defects such as voids, microcracks and localized stress concentrations can serve as positive or negative lenses for the incident laser light. The resulting interference pattern between refracted and diffracted light can result in intensity increases on the order of a factor of 2 some distance away from a typical negative microlens, and even larger for a positive microlens. Thus, the initial damage site can be physically removed from the defect which initiates damage. The parameter that determines the strength of such lensing is (Ka){sup 2}{Delta}{epsilon}, where the wavenumber K is 2{pi}/{lambda} linear size of the defect and AF, is the difference in dielectric coefficient between matrix and scatterer. Thus, even a small change in refractive index results in a significant effect for a defect large compared to a wavelength. Geometry is also important. Three dimensional (eg. voids) as well as linear and planar (eg. cracks) microlenses can all have strong effects. The present paper evaluates the intensification due to spherical voids and high refractive index inclusions. We wish to particularly draw attention to the very large intensification that can occur at inclusions.
Date: January 1, 1997
Creator: Feit, M.D. & Rubenchik, A.M.
Partner: UNT Libraries Government Documents Department

Laser intensity modulation by nonabsorbing defects

Description: Nonabsorbing defects can lead to laser damage. Defects such as voids, microcracks, and localized stressed concentrations, even if they differ from the surrounding medium only by refractive index, can serve as positive or negative lenses for the incident laser light. The resulting interference pattern between refracted and diffracted light can result in intensity increases on the order of a factor of 2 some distance away from a typical negative microlens, and even larger for a positive microlens. Thus, the initial damage site can be physically removed from the defect which initiates damage. The parameter that determines the strength of such lensing is (Ka){sup 2}{Delta}{epsilon}, where the wavenumber K is 2{pi}/{lambda}, 2a is the linear size of the defect, and {Delta}{epsilon} is the difference in dielectric coefficient between matrix and scatterer. Thus, even a small change in refractive index results in a significant effect for a defect large compared to a wavelength. Geometry is also important. Three dimensional (e.g. voids) as well as linear and planar (e.g. cracks) microlenses can all have strong effects. This paper evaluates intensification due to spherical voids and high refractive index inclusions.
Date: November 20, 1996
Creator: Feit, M.D., Rubenchik, A.M.
Partner: UNT Libraries Government Documents Department

Feasibility of High-Power Diode Laser Array Surrogate to Support Development of Predictive Laser Lethality Model

Description: Predictive modeling and simulation of high power laser-target interactions is sufficiently undeveloped that full-scale, field testing is required to assess lethality of military directed-energy (DE) systems. The cost and complexity of such testing programs severely limit the ability to vary and optimize parameters of the interaction. Thus development of advanced simulation tools, validated by experiments under well-controlled and diagnosed laboratory conditions that are able to provide detailed physics insight into the laser-target interaction and reduce requirements for full-scale testing will accelerate development of DE weapon systems. The ultimate goal is a comprehensive end-to-end simulation capability, from targeting and firing the laser system through laser-target interaction and dispersal of target debris; a 'Stockpile Science' - like capability for DE weapon systems. To support development of advanced modeling and simulation tools requires laboratory experiments to generate laser-target interaction data. Until now, to make relevant measurements required construction and operation of very high power and complex lasers, which are themselves costly and often unique devices, operating in dedicated facilities that don't permit experiments on targets containing energetic materials. High power diode laser arrays, pioneered by LLNL, provide a way to circumvent this limitation, as such arrays capable of delivering irradiances characteristic of De weapon requires are self-contained, compact, light weight and thus easily transportable to facilities, such as the High Explosives Applications Facility (HEAF) at Lawrence Livermore National Laboratory (LLNL) where testing with energetic materials can be performed. The purpose of this study was to establish the feasibility of using such arrays to support future development of advanced laser lethality and vulnerability simulation codes through providing data for materials characterization and laser-material interaction models and to validate the accuracy of code predictions. This project was a Feasibility Study under the LLNL Laboratory Directed Research and Development (LDRD) Program.
Date: January 13, 2011
Creator: Lowdermilk, W H; Rubenchik, A M & Springer, H K
Partner: UNT Libraries Government Documents Department

Deterministic processing of alumina with ultra-short laser pulses

Description: Ultrashort pulsed lasers can accurately ablate materials which are refractory, transparent, or are otherwise difficult to machine by other methods. The typical method of machining surfaces with ultrashort laser pulses is by raster scanning, or the machining of sequentially overlapping linear trenches. Experiments in which linear trenches were machined in alumina at various pulse overlaps and incident fluences are presented, and the dependence of groove depth on these parameters established. A model for the machining of trenches based on experimental data in alumina is presented, which predicts and matches observed trench geometry. This model is then used to predict optimal process parameters for the machining of trenches for maximal material removal rate for a given laser.
Date: June 27, 2007
Creator: Furmanski, J; Rubenchik, A M; Shirk, M D & Stuart, B C
Partner: UNT Libraries Government Documents Department

Simple model of laser damage initiation and conditioning in frequency conversion crystals

Description: Laser conditioning, i.e. pre-exposure to less than damaging laser fluence, has been shown to improve the damage resistance of KDP/DKDP frequency conversion crystals. We have extended our damage model, small absorbing precursors with a distribution of sizes, to describe various damage related properties such as damage density and effects of laser conditioning in crystals. The model assumes the rate limiting process for both initiation and conditioning depends on temperature and that separate threshold temperatures exist for either conditioning or damage initiation to occur. This is reasonable in KDP/DKDP since the melting temperature is far below the temperatures associated with plasma formation and damage events. This model is capable of accounting for some recently observed damage-conditioning behaviors.
Date: October 28, 2005
Creator: Feit, M D; Rubenchik, A M & Trenholme, J B
Partner: UNT Libraries Government Documents Department

On the Theory of the Modulation Instability in Optical Fiber and Laser Amplifiers

Description: The modulation instability (MI) in optical fiber amplifiers and lasers with anomalous dispersion leads to CW beam breakup and the growth of multiple pulses. This can be both a detrimental effect, limiting the performance of amplifiers, and also an underlying physical mechanism in the operation of MI-based devices. Here we revisit the analytical theory of MI in fiber optical amplifiers. The results of the exact theory are compared with the previously used adiabatic approximation model, and the range of applicability of the latter is determined. The same technique is applicable to the study of spatial MI in solid state laser amplifiers and MI in non-uniform media.
Date: November 3, 2010
Creator: Rubenchik, A M; Turitsyn, S K & Fedoruk, M P
Partner: UNT Libraries Government Documents Department

Size-selection initiation model extended to include shape and random factors

Description: The Feit-Rubenchik size-selection damage model has been extended in a number of ways. More realistic thermal deposition profiles have been added. Non-spherical shapes (rods and plates) have been considered, with allowance for their orientation dependence. Random variations have been taken into account. An explicit form for the change of absorptivity with precursor size has been added. A simulation tool called GIDGET has been built to allow adjustment of the many possible parameters in order to fit experimental data of initiation density as a function of fluence and pulse duration. The result is a set of constraints on the possible properties of initiation precursors.
Date: November 2, 2005
Creator: Trenholme, J B; Feit, M D & Rubenchik, A M
Partner: UNT Libraries Government Documents Department

SOLAR PUMPED LASER MICROTHRUSTER

Description: The development of microsatellites requires the development of engines to modify their orbit. It is natural to use solar energy to drive such engines. For an unlimited energy source the optimal thruster must use a minimal amount of expendable material to minimize launch costs. This requires the ejected material to have the maximal velocity and, hence, the ejected atoms must be as light as possible and be ejected by as high an energy density source as possible. Such a propulsion can be induced by pulses from an ultra-short laser. The ultra-short laser provides the high-energy concentration and high-ejected velocity. We suggest a microthruster system comprised of an inflatable solar concentrator, a solar panel, and a diode-pumped fiber laser. We will describe the system design and give weight estimates.
Date: February 5, 2010
Creator: Rubenchik, A M; Beach, R; Dawson, J & Siders, C W
Partner: UNT Libraries Government Documents Department

MICRON-SCALE DEEP HOLE DRILLING FOR BERYLLIUM CAPSULE FILL APPLICATIONS

Description: A laser processing system has been developed to drill high aspect ratio holes through the impermeable beryllium capsules envisioned for ignition shots on NIF. The drilling system was designed to produce holes with an entrance and exit diameter of approximately 5 {micro}m through the full 175 {micro}m thickness of the capsule. To meet these requirements, a frequency doubled femtosecond-class Ti:Sapphire laser is directed through a high numerical aperture lens to provide the spot geometry needed to drill the hole. The laser pulse is confined by the metallic walls of the hole, thereby maintaining the diameter of the channel well beyond the Rayleigh range of the optical system. Presented is the current state of this work-in-progress, including descriptions of the device and the technique used to produce the holes. The various means of characterizing the laser-drilled channels are also discussed.
Date: November 29, 2005
Creator: Armstrong, J P; Rubenchik, A M; Gunther, J & Stuart, B C
Partner: UNT Libraries Government Documents Department

On the Theory of the Modulation Instability in Optical Fibre Amplifiers

Description: The modulation instability (MI) in optical fiber amplifiers and lasers with anomalous dispersion leads to CW radiation break-up and growth of multiple pulses. This can be both a detrimental effect limiting the performance of amplifiers, and also an underlying physical mechanism in the operation of MI-based devices. Here we revisit the analytical theory of MI in fiber optical amplifiers. The results of the exact theory are compared with the previously used adiabatic approximation model and the range of applicability of the later is determined.
Date: May 10, 2010
Creator: Turitsyn, S K; Rubenchik, A M & Fedoruk, M P
Partner: UNT Libraries Government Documents Department

Unique aspects of laser energy deposition in the fs pulse regime

Description: Ultrashort laser pulse tissue ablation has demonstrated advantages of greatly reduced required energy and collateral damage. These advantages stem directly from the fact that laser energy is absorbed nonlinearly in a time too hsort for significant thermal dn hydrodynamic response. The high peak power and short pulse duration both have implications for practical fiber delivery systems.
Date: February 2, 1996
Creator: Feit, M.D.; Rubenchik, A.M. & Shore, B.W.
Partner: UNT Libraries Government Documents Department

Increased damage thresholds due to laser pulse modulation

Description: Nonlinear self-focusing in laser glass imposes limits on the energy fluence that can be safely transmitted without risking damage. For this reason, it is desirable to strictly limit the peak to average spatial variations of fluence by smoothing schemes such as Smoothing by Spectral Dispersion (SSD). While spatial variations are problematic, the same is not necessarily true of temporal variations since normal group velocity dispersion tends to smooth out temporal peaks caused by spatial self-focusing. Earlier work indicated that increased bandwidth can delay the onset of self focusing. The present work re-examines the question of self focusing threshold increases due to high bandwidth by investigating another source of such increase in three dimensional beam breakup--the bending instability. For simplicity, the authors consider the behavior of a single space-time speckle. Normal dispersion can lead to splitting of the pulse and delay of self focusing for short enough pulses as noted above. In addition to the self focusing instability, the laser beam is also subject to the so-called bending (sausage like) instability which can spatially disperse the field maxima over time. Because the bending instability breaks an initial axial symmetry, a full three dimensional numerical simulation is required to study it accurately. Such calculations are possible, but costly. The authors have used a modified 2D nonlinear Schroedinger equation with a high power nonlinearity since this mimics the 3D behavior of the competition between self focusing and bending. This study is relevant for inertial confinement conditions.
Date: May 30, 1995
Creator: Feit, M.D.; Musher, S.L.; Shapiro, E.G. & Rubenchik, A.M.
Partner: UNT Libraries Government Documents Department

Laser-induced damage in dielectrics with nanosecond to subpicosecond pulses. II. Theory

Description: The authors have reported extensive measurements of damage thresholds for fused silica and several fluorides (LiF, CaF, MgF, and BaF) at 1053 and 526 nm for pulse durations, {tau}, ranging from 275 fs to 1 ns. A theoretical model based on electron production via multiphoton ionization, Joule heating, and collisional (avalanche) ionization is in good agreement with experimental results.
Date: December 1, 1994
Creator: Feit, M.D.; Rubenchik, A.M. & Shore, B.W.
Partner: UNT Libraries Government Documents Department

Effect of the stimulated Brillouin backscattering on selffocusing threshold

Description: In many physical problems stimulated Brillouin scattering (SBS) and selffocusing are manifested simultaneously. We consider effect of the stimulated Brillouin backscattering (SBS) on self-focusing of laser radiation in plasmas. It was found that the self-focusing may be supressed substantionally by the SBS effect.
Date: March 1, 1994
Creator: Rubenchik, A. M.; Shapiro, E. G. & Turitsyn, S. K.
Partner: UNT Libraries Government Documents Department

Investigation of laser-induced damage in DKDP under multi-color irradiation

Description: Laser-induced initiation of bulk damage sites in DKDP crystals is investigated under simultaneous exposure to 532- and 355-nm nanosecond laser pulses in order to simulate the operational conditions during harmonic conversion as well as probe the damage mechanisms. The results demonstrate synergetic damage effects under the dual-wavelength excitation. Furthermore, the damage performance is directly related to and can be predicted from the damage performance at each wavelength separately. The measured relative effective absorption coefficients at the two wavelengths are found to depend on the laser fluence. Laser-induced damage sites initiated within the bulk of optical components is a key limiting factor in the development of high power laser systems. Potassium dihydrogen phosphate (KH{sub 2}PO{sub 4} or KDP) and its deuterated analog (KD{sub 2-x}H{sub x}PO{sub 4} or DKDP) have been widely used for over three decades as Pockels cells and frequency converters and are still the only nonlinear materials suitable for large-aperture laser systems [1,2]. Damage thresholds in these materials have increased over time, primarily due to purer raw materials and improvement in growth processes, though localized damage sites still arise from laser intensities far below that necessary for intrinsic dielectric breakdown [3]. The damage precursors and their absorption mechanism leading to damage initiation are still unknown despite more than four decades of research [4,5]. In the case of KDP and DKDP crystals, recent work has highlighted the importance of synergetic effects between the second and third harmonics in Nd:glass lasers present during harmonic conversion to the observed damage density [6]. In this work, we quantitatively assess the damage performance of KDP/DKDP crystals under simultaneous exposure to the second and third harmonics of a nanosecond Nd:YAG laser system in order to (a) probe the underlying mechanism of damage initiation and (b) simulate the conditions taking place during harmonic conversion towards developing ...
Date: August 21, 2006
Creator: DeMange, P; Negres, R A; Rubenchik, A M; Radousky, H B; Feit, M D & Demos, S G
Partner: UNT Libraries Government Documents Department

The energy coupling efficiency of multi-wavelength laser pulses to damage initiating defects in DKDP nonlinear crystals

Description: The bulk damage performance of potassium dihydrogen phosphate crystals under simultaneous exposure to 1064-, 532-, and 355-nm nanosecond-laser pulses is investigated in order to probe the laser-induced defect reactions leading to damage initiation during frequency conversion. The results provide insight into the mechanisms governing the behavior of the damage initiating defects under exposure to high power laser light. In addition, it is suggested that the damage performance can be directly related to and predicted from the damage behavior of the crystal at each wavelength separately.
Date: September 25, 2007
Creator: DeMange, P; Negres, R A; Rubenchik, A M; Radousky, H B; Feit, M D & Demos, S G
Partner: UNT Libraries Government Documents Department

Does complex absorption behavior leading to conditioning and damage in KDP/DKDP reflect the electronic structure of initiators?

Description: Currently, most of our thinking about the defects responsible for initiating laser damage considers them as featureless absorbers. However, an increasing body of evidence, particularly involving multi-wavelength irradiation, suggests electronic structure of damage initiators is important in determining both initiation and conditioning behaviors in KDP. The effective absorption coefficient of energy under multi-wavelength irradiation cannot be accounted for by a structureless absorber, but is consistent with an initiator with a multi-level structure. We outline the evidence and assess the ability of such a simple multi-level model to explain these and other experimentally observed behaviors.
Date: October 24, 2007
Creator: Feit, M D; DeMange, P P; Negres, R A; Rubenchik, A M & Demos, S G
Partner: UNT Libraries Government Documents Department

Laser Systems for Orbital Debris Removal

Description: The use of a ground based laser for space debris cleaning was investigated by the ORION project in 1996. Since that study the greatest technological advance in the development of high energy pulsed laser systems has taken place within the NIF project at LLNL. The proposed next laser system to follow the NIF at LLNL will be a high rep rate version of the NIF based on diode-pumping rather than flashlamp excitation; the so called 'LIFE' laser system. Because a single 'LIFE' beamline could be built up in a few year time frame, and has performance characteristics relevant to the space debris clearing problem, such a beamline could enable a near term demonstration of space debris cleaning. Moreover, the specifics of debris cleaning make it possible to simplify the LIFE laser beyond what is required for a fusion drive laser, and so substantially reduce its cost. Starting with the requirements for laser intensity on the target, and then considering beam delivery, we will flow back the laser requirements needed for space debris cleaning. Using these derived requirements we will then optimize the pulse duration, the operational regime, and the output pulse energy of the laser with a focus of simplifying its overall design. Anticipated simplifications include operation in the heat capacity regime, eliminating cooling requirements on the laser gain slabs, and relaxing B-integral and birefrigence requirements.
Date: February 5, 2010
Creator: Rubenchik, A M; Barty, C P; Beach, R J; Erlandson, A C & Caird, J A
Partner: UNT Libraries Government Documents Department