Schmitt, Andrew J.
Overview
Works:  11 works in 11 publications in 1 language and 11 library holdings 

Publication Timeline
.
Most widely held works by
Andrew J Schmitt
The Effects of Optical Smoothing Techniques on Filamentation in Laser Plasmas(
Book
)
1 edition published in 1987 in English and held by 1 WorldCat member library worldwide
The effect of the induced spatial incoherence (ISI) and the random phase screen (RPS) optical smoothing techniques on the filamentation instability in laser plasmas has been investigated numerically and analytically. A two dimensional time dependent laser plasma propagation code, including both ponderomotive and thermal condition dominated filamentation, is used to simulate the laser plasma interaction. The results of these simulations are compared to the predictions of a simple theory that describes the filamentation of both coherent light and spatially and temporally incoherent light. It is shown that filaments driven by the thermal mechanisms tend to cluster together and produce greater largescale nonuniformities in the laser illumination than the ponderomotively driven filaments. The RPS optical smoothing technique is found to reduce filamentation only if fast focusing optics (F <or = 5) are used. The ISI smoothing method suppresses filamentation for fast or slow focusing optics, and requires only moderate laser bandwidth (delta omega/omega approx, = 0.1%). In general, the ISI smoothing method provides the best suppression of filamentation. Under common laboratory conditions, filamentation is pronounced at longer laser wavelengths (1.06 micrometers  0.53 micrometers), suggesting that current experiments may be dominated by filamentation effects at these wavelengths. The optical smoothing methods are most effective in short wavelength (0.25 micrometers) laser driven plasmas. At 0.25 micrometers laser wavelength, ISI is found to completely eliminate filamentation effects in both time averaged and instantaneous intensity distribution
1 edition published in 1987 in English and held by 1 WorldCat member library worldwide
The effect of the induced spatial incoherence (ISI) and the random phase screen (RPS) optical smoothing techniques on the filamentation instability in laser plasmas has been investigated numerically and analytically. A two dimensional time dependent laser plasma propagation code, including both ponderomotive and thermal condition dominated filamentation, is used to simulate the laser plasma interaction. The results of these simulations are compared to the predictions of a simple theory that describes the filamentation of both coherent light and spatially and temporally incoherent light. It is shown that filaments driven by the thermal mechanisms tend to cluster together and produce greater largescale nonuniformities in the laser illumination than the ponderomotively driven filaments. The RPS optical smoothing technique is found to reduce filamentation only if fast focusing optics (F <or = 5) are used. The ISI smoothing method suppresses filamentation for fast or slow focusing optics, and requires only moderate laser bandwidth (delta omega/omega approx, = 0.1%). In general, the ISI smoothing method provides the best suppression of filamentation. Under common laboratory conditions, filamentation is pronounced at longer laser wavelengths (1.06 micrometers  0.53 micrometers), suggesting that current experiments may be dominated by filamentation effects at these wavelengths. The optical smoothing methods are most effective in short wavelength (0.25 micrometers) laser driven plasmas. At 0.25 micrometers laser wavelength, ISI is found to completely eliminate filamentation effects in both time averaged and instantaneous intensity distribution
Feedout and RichtmyerMeshkov Instability at Large Density Difference(
)
1 edition published in 2001 in English and held by 0 WorldCat member libraries worldwide
The feedout process transfers mass perturbations from the rear to the front surface of a driven target, producing the seed for the RayleighTaylor (RT) instability growth. The feedout mechanism is investigated analytically and numerically for the case of perturbation wavelength comparable to or less than the shockcompressed target thickness. The lateral mass flow in the target leads to oscillations of the initial mass nonuniformity before the reflected rippled rarefaction wave breaks out, which may result in RT bubbles produced at locations where the areal mass was initially higher. This process is determined by the evolution of hydrodynamic perturbations in the rippled rarefaction wave, which is not the same as the RichtmyerMeshkov (RM) interfacial instability. An exact analytical formula is derived for the timedependent mass variation in a rippled rarefaction wave, and explicit estimates are given for the time of first phase reversal and frequency of the oscillations. The limiting transition from the case of RM perturbation growth at large density difference "low ambient density behind the rear surface" to the case of feedout "zero density" is studied, and it is shown that the latter limit is approached only if the ambient density is extremely low, less than 1/1000 of the preshock target density
1 edition published in 2001 in English and held by 0 WorldCat member libraries worldwide
The feedout process transfers mass perturbations from the rear to the front surface of a driven target, producing the seed for the RayleighTaylor (RT) instability growth. The feedout mechanism is investigated analytically and numerically for the case of perturbation wavelength comparable to or less than the shockcompressed target thickness. The lateral mass flow in the target leads to oscillations of the initial mass nonuniformity before the reflected rippled rarefaction wave breaks out, which may result in RT bubbles produced at locations where the areal mass was initially higher. This process is determined by the evolution of hydrodynamic perturbations in the rippled rarefaction wave, which is not the same as the RichtmyerMeshkov (RM) interfacial instability. An exact analytical formula is derived for the timedependent mass variation in a rippled rarefaction wave, and explicit estimates are given for the time of first phase reversal and frequency of the oscillations. The limiting transition from the case of RM perturbation growth at large density difference "low ambient density behind the rear surface" to the case of feedout "zero density" is studied, and it is shown that the latter limit is approached only if the ambient density is extremely low, less than 1/1000 of the preshock target density
DirectDrive Laser Fusion; Status and Prospects(
)
1 edition published in 1998 in English and held by 0 WorldCat member libraries worldwide
Techniques have been developed to improve the uniformity of the laser focal profile to reduce the ablative Rayleigh Taylor instability, and to suppress the various laser plasma instabilities. There are now three ignition target design concepts that utilize these techniques. Evaluation of these designs is still ongoing. Some of them may achieve the gains above 100 that are necessary for a fusion reactor. Two laser systems have been proposed that may meet all of the requirements for a fusion reactor
1 edition published in 1998 in English and held by 0 WorldCat member libraries worldwide
Techniques have been developed to improve the uniformity of the laser focal profile to reduce the ablative Rayleigh Taylor instability, and to suppress the various laser plasma instabilities. There are now three ignition target design concepts that utilize these techniques. Evaluation of these designs is still ongoing. Some of them may achieve the gains above 100 that are necessary for a fusion reactor. Two laser systems have been proposed that may meet all of the requirements for a fusion reactor
Analysis of Intensity Structure of the ISI Model in the FAST2D Hydrocode(
)
1 edition published in 2001 in English and held by 0 WorldCat member libraries worldwide
The intensity fluctuation spectrum due to spatially and temporally incoherent laser light is examined in detail. Details of the implementation of an optically smoothed light model in numerical codes are discussed
1 edition published in 2001 in English and held by 0 WorldCat member libraries worldwide
The intensity fluctuation spectrum due to spatially and temporally incoherent laser light is examined in detail. Details of the implementation of an optically smoothed light model in numerical codes are discussed
Enhanced DirectDrive Implosions with Thin HighZ Ablation Layers(
)
1 edition published in 2008 in English and held by 0 WorldCat member libraries worldwide
New directdrive spherical implosion experiments with deuterium filled plastic shells have demonstrated significant and absolute (2x) improvements in neutron yield when the shells are coated with a very thin layer (~200400 Angstron) of highZ material such as palladium. This improvement is interpreted as resulting from increased stability of the imploding shell. These results provide for a possible path to control laser imprint and stability in laserfusionenergy target designs
1 edition published in 2008 in English and held by 0 WorldCat member libraries worldwide
New directdrive spherical implosion experiments with deuterium filled plastic shells have demonstrated significant and absolute (2x) improvements in neutron yield when the shells are coated with a very thin layer (~200400 Angstron) of highZ material such as palladium. This improvement is interpreted as resulting from increased stability of the imploding shell. These results provide for a possible path to control laser imprint and stability in laserfusionenergy target designs
LargeScale HighResolution Simulations of High Gain DirectDrive Inertial Confinement Fusion Targets(
)
1 edition published in 2004 in English and held by 0 WorldCat member libraries worldwide
Targets have been designed that produce moderate to high gain when directly driven by lasers. The intrinsic sensitivity of these targets to hydro instabilities is found using the FAST(2D) multidimensional radiation hydrocode [J.H. Gardner, A.J. Schmitt, J.P. Dahlburg, et al., Phys. Plasmas 5, 1935 (1998)], which simulates the simultaneous behavior of a large bandwidth (e.g., l = 2256) of perturbations from compression to acceleration, and then to stagnation and burn. The development of the structure in these multimode simulations is benchmarked to theoretical analysis and singlemode calculations, which reveals the need to "renormalize" the simulation after compression. The simulations predict that a direct drive point design is expected to degrade significantly from its 1D clean yield, yet still ignite and give appreciable gain. Simulations of highgain pellets using a spike prepulse to inhibit RichtmyerMeshkov growth show a considerable robustness, with high (> 100) gains possible even with nominal surface finishes and laser imprint
1 edition published in 2004 in English and held by 0 WorldCat member libraries worldwide
Targets have been designed that produce moderate to high gain when directly driven by lasers. The intrinsic sensitivity of these targets to hydro instabilities is found using the FAST(2D) multidimensional radiation hydrocode [J.H. Gardner, A.J. Schmitt, J.P. Dahlburg, et al., Phys. Plasmas 5, 1935 (1998)], which simulates the simultaneous behavior of a large bandwidth (e.g., l = 2256) of perturbations from compression to acceleration, and then to stagnation and burn. The development of the structure in these multimode simulations is benchmarked to theoretical analysis and singlemode calculations, which reveals the need to "renormalize" the simulation after compression. The simulations predict that a direct drive point design is expected to degrade significantly from its 1D clean yield, yet still ignite and give appreciable gain. Simulations of highgain pellets using a spike prepulse to inhibit RichtmyerMeshkov growth show a considerable robustness, with high (> 100) gains possible even with nominal surface finishes and laser imprint
Time Dependent Filamentation and Stimulated Brillouin Forward Scattering in Inertial Confinement Fusion Plasmas(
)
1 edition published in 1997 in English and held by 0 WorldCat member libraries worldwide
Numerical simulations of the temporal evolution of laser light filamentation and stimulated Brillouin forward scattering (SBFS) in plasmas, under conditions that are relevant to laser fusion, are presented and analyzed. Long term unsteady behavior of filaments is observed to be the norm. Temporal and spatial incoherence due to filamentation and SBFS are impressed upon timeindependent incident laser beams. The bandwidth and angular divergence imposed upon the beam increase with the strength of the interaction. In addition, the spectrum of the transmitted light is redshifted by an amount that increases with the interaction strength. Spectral analysis of the transmitted light reveals that SBFS plays a role in the generation of the observed temporal incoherence. Incident beams with some spatial incoherence but no temporal smoothing are compared to those with ab initio temporal beam smoothing (TBS). Under typical conditions, TBS beams will undergo far less angular and spectral spreading and far less SBFS than unsmoothed beams
1 edition published in 1997 in English and held by 0 WorldCat member libraries worldwide
Numerical simulations of the temporal evolution of laser light filamentation and stimulated Brillouin forward scattering (SBFS) in plasmas, under conditions that are relevant to laser fusion, are presented and analyzed. Long term unsteady behavior of filaments is observed to be the norm. Temporal and spatial incoherence due to filamentation and SBFS are impressed upon timeindependent incident laser beams. The bandwidth and angular divergence imposed upon the beam increase with the strength of the interaction. In addition, the spectrum of the transmitted light is redshifted by an amount that increases with the interaction strength. Spectral analysis of the transmitted light reveals that SBFS plays a role in the generation of the observed temporal incoherence. Incident beams with some spatial incoherence but no temporal smoothing are compared to those with ab initio temporal beam smoothing (TBS). Under typical conditions, TBS beams will undergo far less angular and spectral spreading and far less SBFS than unsmoothed beams
Laser Imprint Reduction with a Short Shaping Laser Pulse Incident Upon a FoamPlastic Target(
)
1 edition published in 2002 in English and held by 0 WorldCat member libraries worldwide
In the previous work [Metzler et al., Phys. Plasmas 6, 3283 "1999"] it was shown that a tailored density profile could be very effective in smoothing out the laser beam nonuniformities imprinted into a laseraccelerated target. However, a target with a smoothly graded density is difficult to manufacture. A method of dynamically producing a graded density profile with a short shaping laser pulse irradiating a foam layer on top of the payload prior to the drive pulse is proposed. It is demonstrated that the intensity and the duration of the shaping pulse, the time interval between the shaping pulse and the drive pulse, and the density ratio between the foam and the payload can be selected so that the laser imprint of the drive pulse is considerably suppressed without increasing the entropy of the payload. The use of the foamplastic target and a shaping pulse reduces the imprinted mass perturbation amplitude by more than an order of magnitude compared to a solid plastic target. The requirements to the smoothing of the drive and shaping laser beams and to the surface finish of the foamplastic sandwich target are discussed
1 edition published in 2002 in English and held by 0 WorldCat member libraries worldwide
In the previous work [Metzler et al., Phys. Plasmas 6, 3283 "1999"] it was shown that a tailored density profile could be very effective in smoothing out the laser beam nonuniformities imprinted into a laseraccelerated target. However, a target with a smoothly graded density is difficult to manufacture. A method of dynamically producing a graded density profile with a short shaping laser pulse irradiating a foam layer on top of the payload prior to the drive pulse is proposed. It is demonstrated that the intensity and the duration of the shaping pulse, the time interval between the shaping pulse and the drive pulse, and the density ratio between the foam and the payload can be selected so that the laser imprint of the drive pulse is considerably suppressed without increasing the entropy of the payload. The use of the foamplastic target and a shaping pulse reduces the imprinted mass perturbation amplitude by more than an order of magnitude compared to a solid plastic target. The requirements to the smoothing of the drive and shaping laser beams and to the surface finish of the foamplastic sandwich target are discussed
Modeling Fluid Instabilities in Inertial Confinement Fusion Hydrodynamics Codes(
)
1 edition published in 2005 in English and held by 0 WorldCat member libraries worldwide
The numerical tools typically used to model the evolution of fluid instabilities in inertial confinement fusion (ICF) hydrodynamics codes are examined, and some are found to have properties which would seem to be incompatible with the accurate modeling of smallamplitude perturbations, i.e., perturbations in the linear stage of evolution. In particular a "differentiability condition" which is satisfied by the physics in such situations is not necessarily satisfied by the numerical algorithms in typical use. It is demonstrated that it is possible to remove much of the nondifferentiability in many cases, and that substantial improvement in one's ability to accurately model the evolution of small amplitude perturbations can result. First a simple example involving a nondifferentiable radiation transport algorithm is shown, and then the non differentiabilities introduced by the use of upwind and "high resolution" hydrodynamics algorithms are analyzed
1 edition published in 2005 in English and held by 0 WorldCat member libraries worldwide
The numerical tools typically used to model the evolution of fluid instabilities in inertial confinement fusion (ICF) hydrodynamics codes are examined, and some are found to have properties which would seem to be incompatible with the accurate modeling of smallamplitude perturbations, i.e., perturbations in the linear stage of evolution. In particular a "differentiability condition" which is satisfied by the physics in such situations is not necessarily satisfied by the numerical algorithms in typical use. It is demonstrated that it is possible to remove much of the nondifferentiability in many cases, and that substantial improvement in one's ability to accurately model the evolution of small amplitude perturbations can result. First a simple example involving a nondifferentiable radiation transport algorithm is shown, and then the non differentiabilities introduced by the use of upwind and "high resolution" hydrodynamics algorithms are analyzed
ThreeDimensional Filamentation of Light in Laser Plasmas(
)
1 edition published in 1991 in English and held by 0 WorldCat member libraries worldwide
The first calculations of timedependent laserplasma filamentation in three dimensions are reported. These calculations are done with a threedimensional laser propagation code based on a previous twodimensional code. The effect of incident beam structure, and in particular optical smoothing techniques, on the behavior of filamentation is studied. Both ponderomotive and thermal conduction dominated nonlinearities are considered, and calculations are done simulating both homogeneous nonabsorbing plasmas and inhomogeneous laboratory plasmas. Random phase screen (RPS) and induced spatial incoherence (ISI) optical smoothing techniques are investigated and compared to generic unsmoothed laser beams. Qualitative examples are presented and scaling studies are done and compared to a simple theoretical analysis, In typical laserplasma interactions without optical smoothing, threedimensional effects lead to greatly increased filament intensities, as expected. Peak filament intensities of order 100500 times the average intensity are routinely observed (without optical smoothing), as compared to earlier two dimensional calculations where peak intensities were of order 1050 times average. In spite of this tendency to create stronger filaments, threedimensional filamentation (when measured on a timeaveraged basis) can be suppressed by using ISI smoothing. Under the same conditions, instantaneous ISI intensities can show considerable enhancement, although much less than the unsmoothed beams. RPS smoothing exhibits less filamentation suppression. Under laserfusion reactor conditions, calculations indicate that ISI suppression can completely eliminate filamentation
1 edition published in 1991 in English and held by 0 WorldCat member libraries worldwide
The first calculations of timedependent laserplasma filamentation in three dimensions are reported. These calculations are done with a threedimensional laser propagation code based on a previous twodimensional code. The effect of incident beam structure, and in particular optical smoothing techniques, on the behavior of filamentation is studied. Both ponderomotive and thermal conduction dominated nonlinearities are considered, and calculations are done simulating both homogeneous nonabsorbing plasmas and inhomogeneous laboratory plasmas. Random phase screen (RPS) and induced spatial incoherence (ISI) optical smoothing techniques are investigated and compared to generic unsmoothed laser beams. Qualitative examples are presented and scaling studies are done and compared to a simple theoretical analysis, In typical laserplasma interactions without optical smoothing, threedimensional effects lead to greatly increased filament intensities, as expected. Peak filament intensities of order 100500 times the average intensity are routinely observed (without optical smoothing), as compared to earlier two dimensional calculations where peak intensities were of order 1050 times average. In spite of this tendency to create stronger filaments, threedimensional filamentation (when measured on a timeaveraged basis) can be suppressed by using ISI smoothing. Under the same conditions, instantaneous ISI intensities can show considerable enhancement, although much less than the unsmoothed beams. RPS smoothing exhibits less filamentation suppression. Under laserfusion reactor conditions, calculations indicate that ISI suppression can completely eliminate filamentation
Growth of Pellet Imperfections and Laser Imprint in Direct Drive Inertial Confinement Fusion Targets(
)
1 edition published in 2001 in English and held by 0 WorldCat member libraries worldwide
Simple hydrodynamic models for describing the RichtmyerMeshkov (RM) growth and the RayleighTaylor (RT) instability are tested by simulation. The RM sharp boundary model predictions are compared with numerical simulations of targets with surface perturbations or stationary intensity perturbations. Agreement is found in the overall trends, but the specific behavior can be significantly different. RM growth of imprint from optically smoothed lasers is also simulated and quantified. The results are used to calculate surface perturbations, growth factors, and laser imprint efficiencies. These in turn are used with standard RT growth formulas to predict perturbation growth in multimode simulations of compression and acceleration of planar and spherical targets. The largest differences between prediction and theory occur during rampup of the laser intensity, where RT formulas predict more growth than seen in the simulations
1 edition published in 2001 in English and held by 0 WorldCat member libraries worldwide
Simple hydrodynamic models for describing the RichtmyerMeshkov (RM) growth and the RayleighTaylor (RT) instability are tested by simulation. The RM sharp boundary model predictions are compared with numerical simulations of targets with surface perturbations or stationary intensity perturbations. Agreement is found in the overall trends, but the specific behavior can be significantly different. RM growth of imprint from optically smoothed lasers is also simulated and quantified. The results are used to calculate surface perturbations, growth factors, and laser imprint efficiencies. These in turn are used with standard RT growth formulas to predict perturbation growth in multimode simulations of compression and acceleration of planar and spherical targets. The largest differences between prediction and theory occur during rampup of the laser intensity, where RT formulas predict more growth than seen in the simulations
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