WorldCat Identities

Schmitt, A. J.

Works: 18 works in 20 publications in 2 languages and 19 library holdings
Roles: Author
Publication Timeline
Most widely held works by A. J Schmitt
Verkehrspolitik in Europa : (mit Exkurs: Verkehrspolitik und Raumordnungspolitik) by A. J Schmitt( Book )

1 edition published in 1975 in German and held by 1 WorldCat member library worldwide

Baseline Design of a 5-7 kJ KrF Laser Facility for Direct Illumination ICF Experiments by R. H Lehmberg( Book )

1 edition published in 1985 in English and held by 1 WorldCat member library worldwide

This report presents the baseline design for a 48 channel angularly-multiplexed KrF laser system, using amplifier modules similar to those in the AURORA laser at Los Alamos National Laboratory. With output energies of 5-7 kJ, pulsewidths approx. = 5 ns, and broadband (> 20 1/cm) capabilities, the proposed system is intended primarily for laser-plasma experiments related to direct-drive inertial confinement fusion. The design incorporates the NRL induced spatial incoherence (ISI) technique into each beam in order to achieve the smoothly-varying and controllable irradiation profiles essential to the direct-drive approach. The option of using stimulated Raman scattering to improve the beam quality prior to the ISI echelons is also discussed. (Author)
Absolutely Uniform Illumination of Laser Fusion Pellets by A. J Schmitt( Book )

2 editions published in 1984 in English and held by 1 WorldCat member library worldwide

Absolutely uniform illumination of spherical laser fusion pellets is possible when the energy deposition from a single laser beam is given by a simple cos 3 theta distribution. Conditions can be derived for which the laser beam targeting angles allow this absolute illumination uniformity. Configurations based upon the cube and higher order Platonic solids satisfy the constraints, as well as an infinite class of other less symmetric configurations. (Author)
Achieving Adequate Beam Quality for Commercial Laser Fusion Reactors( Book )

1 edition published in 1989 in English and held by 1 WorldCat member library worldwide

In this paper we discuss the laser beam smoothing techniques for obtaining the highly symmetric illumination required for high gain laser fusion. We review the various beam smoothing techniques and discuss the constraints placed on these techniques if they are to be implemented on commercial fusion reactors. Laser, Fusion, Laser fusion, Fusion reactors, Laser beam smoothing. (mjm)
Theory of Induced Spatial Incoherence( Book )

1 edition published in 1987 in English and held by 1 WorldCat member library worldwide

This paper describes theoretical and experimental investigations of induced spatial incoherence (ISI), a technique for achieving the smooth and controllable target beam profiles required for direct-drive laser fusion. Analytic and numerical calculations show that nonuniformities due to interference among the beamlets are smoothed by both thermal diffusion and temporal averaging. Under laser-plasma conditions of interest to ICF, average ablation pressure nonuniformities about 1% should be readily attainable. A partial ISI scheme, which allows widely-spaced beamlets to remain mutually-coherent is examined with the resulting high spatial frequency interference structure can be effectively smoothed by thermal diffusion alone. A perturbation analysis shows that the average target profile <I(x)> remains relatively insensitive to laser beam aberration when the scalelength of that aberration is larger than the initial beamlet width. This aberration will tend to broaden and smooth <I(x)>, rather than introduce any small-scale structure. The broadening is largely controllable because it depends only upon spatial averages of the aberrated quantities over the entire laser aperture; the uncontrollable perturbations can be reduced to about 1% in practical cases
Effects of Thin High-z Layers on the Hydrodynamics of Laser-Accelerated Plastic Targets( )

1 edition published in 2002 in English and held by 0 WorldCat member libraries worldwide

We present experimental results and simulations that study the effects of thin metallic layers with high atomic number (high-Z) on the hydrodynamics of laser accelerated plastic targets. These experiments employ a laser pulse with a low-intensity foot that rises into a high-intensity main pulse. This pulse shape simulates the generic shape needed for high-gain fusion implosions. Imprint of laser nonuniformity during start up of the low intensity foot is a well-known seed for hydrodynamic instability. We observe large reductions in hydrodynamic instability seeded by laser imprint when certain minimum thickness gold or palladium layers are applied to the laser-illuminated surface of the targets. The experiment indicates that the reduction in imprint is at least as large as that obtained by a 6 times improvement in the laser uniformity. We present simulations supported by experiments showing that during the low intensity foot the laser light can be nearly completely absorbed by the high-Z layer. X-rays originating from the high-Z layer heat the underlying lower-Z plastic target material and cause large buffering plasma to form between the layer and the accelerated target. This long-scale plasma apparently isolates the target from laser nonuniformity and accounts for the observed large reduction in laser imprint. With onset of the higher intensity main pulse, the high-Z layer expands and the laser light is transmitted. This technique will be useful in reducing laser imprint in pellet implosions and thereby allow the design of more robust targets for high-gain laser fusion
Nonlinear Modelling of Start-Up Phase Pressure Spectra from Optically Smoothed Induced Spatial Incoherence Laser Imprint( )

1 edition published in 2005 in English and held by 0 WorldCat member libraries worldwide

The spectrum of early time pressure perturbations, due to optically smoothed induced spatial incoherence (ISI) laser imprint, is computed for a planar target using a forced, dissipative model. The time-dependent ISI laser deposition is computed using a time-dependent electromagnetic full wave Maxwell code. It is found that the pressure spectrum evolves into a power law in which spectral power is transferred from large to smaller scales through a nonlinear cascade process. The model results are compared with experimental observations
Shock Front Distortion and Richtmyer-Meshkov-like Growth Caused by a Small Pre-Shock Non-Uniformity( )

1 edition published in 2007 in English and held by 0 WorldCat member libraries worldwide

Response of a shock front to small pre-shock non-uniformities of density, pressure and velocity is studied theoretically and numerically. These pre-shock nonuniformities emulate imperfections of a laser target, due either to its manufacturing, like joints or feeding tubes, or to pre-shock perturbation seeding/growth, as well as density fluctuations in foam targets, "thermal layers" near heated surfaces, etc. Similarly to the shock-wave interaction with a small non-uniformity localized at a material interface which triggers a classical Richtmyer-Meshkov (RM) instability, interaction of a shock wave with periodic or localized pre-shock perturbations distributed in the volume distorts the shape of the shock front and can cause a RM-type instability growth. Explicit asymptotic formulae describing distortion of the shock front and the rate of RM-type growth are presented. These formulae are favorably compared both to the exact solutions of the corresponding initial-boundary-value problem and to numerical simulations. It is demonstrated that a small density modulation localized sufficiently close to a flat target surface produces the same perturbation growth as an "equivalent" ripple on the surface of a uniform target, characterized by the same initial areal mass modulation amplitude
Classical and Ablative Richtmyer-Meshkov Instability and Other ICF-Relevant Plasma Flows Diagnosed With Monochromatic X-Ray Imaging( )

1 edition published in 2007 in English and held by 0 WorldCat member libraries worldwide

In inertial confinement fusion (ICF) and high-energy density physics (HEDP), the most important manifestations of the hydrodynamic instabilities and other mixing processes involve lateral motion of the accelerated plasmas. In order to understand the experimental observations and to advance the numerical simulation codes to the point of predictive capability, it is critically important to accurately diagnose the motion of the dense plasma mass. The most advanced diagnostic technique recently developed for this purpose is the monochromatic x-ray imaging that combines large field of view with high contrast, high spatial resolution and large throughput, ensuring high temporal resolution at large magnification. Its application made it possible for the experimentalists to observe for the first time important hydrodynamic effects that trigger compressible turbulent mixing in laser targets, such as ablative Richtmyer-Meshkov (RM) instability, feedout, interaction of a RM-unstable interface with rarefaction waves. It also helped to substantially improve the accuracy of diagnosing many other important plasma flows, ranging from laser-produced jets to electromagnetically driven wires in a Z-pinch, and to test various methods suggested for mitigation of the Rayleigh-Taylor instability. We will review the results obtained with the aid of this technique in ICF-HEDP studies at the Naval Research Laboratory and the prospects of its future applications
Direct Observation of Mass Oscillations due to Ablative Richtmyer-Meshkov Instability and Feedout in Planar Plastic Targets( )

1 edition published in 2002 in English and held by 0 WorldCat member libraries worldwide

Perturbations that seed Rayleigh-Taylor (RT) instability in laser-driven targets form during the early-time period. This time includes a shock wave transit from the front to the rear surface of the target, and a rarefaction wave transit in the opposite direction. During this time interval, areal mass perturbations caused by all sources of nonuniformity (laser imprint, surface ripple) are expected to oscillate. The first direct experimental observations of the areal mass oscillations due to ablative Richtmyer-Meshkov (RM) instability and feedout followed by the RT growth of areal mass modulation are discussed. The experiments were made with 40 to 99 mm thick planar plastic targets rippled either on the front or on the rear with a sine wave ripple with either 30 or 45 mm wavelength and with 0.5, 1 or 1.5 mm amplitude. Targets were irradiated with 4 ns long Nike KrF laser pulses at approximately 50 TW/cm2. The oscillations were observed with our novel diagnostic technique, a monochromatic x-ray imager coupled to a streak camera. For the ablative RM instability (front side ripple), the mass modulation amplitude was typically observed to grow, reach a peak, and then decrease, after which the exponential RT growth started. In some cases, one phase reversal due to the ablative RM instability was observed. For the feedout geometry (rear side ripple), in all cases two phase reversals were observed: a distinct half-oscillation was followed by the onset of the RT growth, resulting in a second phase reversal
High-Gain Direct-Drive Target Design for Laser Fusion (Preprint)( )

2 editions published in 2000 in English and held by 0 WorldCat member libraries worldwide

We have developed a new direct-drive target design that has a predicted energy gain of 127 using a 1.3 MJ KrF laser and a gain of 155 using 3.1 MJ. The DT fuel is surrounded by an ablator consisting of a low density CH foam filled with frozen DT. The ablator is then surrounded by a thin CH coating and a very thin high-Z overcoat. The energy gain of 127-155 is possible through the use of (1) direct-drive laser-target coupling; (2) controlled levels of radiative preheating that keeps the DT fuel on a low isentrope (3) a short 1/4 micrometer laser wavelength for maximum absorption and rocket efficiencies (4) reduction of the laser beam focal spot size during the implosion (zooming) so that the focal spot size better matches the imploding target size and (5) ISI optical smoothing to minimize the laser nonuniformities at both high and low mode numbers. In addition to its high energy gain this target design has several other attractive features: a low target fabrication cost through the use of a few simple target materials; the potential for a modest-size 300 MWe power plant the target's physical strength to withstand the acceleration into the chamber and a high infrared albedo to better protect the target from preheating during the injection into the chamber
Direct Observation of Mass Oscillations Due to Ablative Richtmyer-Meshkov Instability in Plastic Targets( )

1 edition published in 2002 in English and held by 0 WorldCat member libraries worldwide

We report the first direct experimental observation of the ablative Richtmyer-Meshkov instability. It manifests itself in oscillations of areal mass that occur during the shock transit time, which are caused by the rocket effect or dynamic overpressure characteristic of interaction between the laser absorption zone and the ablation front. With the 4 ns long Nike KrF laser pulse and our novel diagnostic technique (monochromatic x-ray imaging coupled to a streak camera) we were able to register a peak and a valley of the areal mass variation before the observed onset of the Rayleigh-Taylor growth
Rear Surface Light Emission Measurements from Laser-produced Shock Waves in Clear and Al-coated Polystyrene Targets( )

1 edition published in 1999 in English and held by 0 WorldCat member libraries worldwide

The Nike KrF laser, with its very uniform focal distributions, has been used at intensities near 10(exp. 14) W/cm2 to launch shock waves in polystyrene targets. The rear surface visible light emission differed between clear CH targets and targets with a thin (125 nm) Al coating on the rear side. The uncoated CH targets showed a relatively slowly rising emission followed by a sudden fall when the shock emerges, while the Al-coated targets showed a rapid rise in emission when the shock emerges followed by a slower fall -- allowing an unambiguous determination of the time the shock arrived at the rear surface. A half-aluminized target allowed us to observe this difference in a single shot. The brightness temperature of both the aluminized targets and the non-aluminized targets were slightly below but close to rear surface temperature predictions of a hydrodynamic code. A discussion of preheat effects is given
Direct-Drive Laser Target Designs for Sub-MegaJoule Energies( )

1 edition published in 2007 in English and held by 0 WorldCat member libraries worldwide

New direct-drive laser target designs with KrF laser light take advantage of the shorter wavelength to lower the laser energy required for substantial gain (>30x) to sub-MJ level. These low laser-energy pellets are useful in systems that could form an intermediate step towards fusion energy, such as the proposed Fusion Test Facility [S.P. Obenschain, et al, Phys. Plasmas 13, 056320 (2006)]. The short wavelength laser should allow higher intensity (and higher pressure) without increasing the risk of laser-plasma instabilities. The higher pressure in turn allows higher velocities to be achieved while keeping the low aspect ratios required for hydrodynamic stability. The canonical laser energy has been chosen to be 500 kJ. A target design is presented with various laser pulse shapes and both 1D and 2D simulation results are shown. The sensitivity of these targets to both low-mode and high-mode perturbations is examined. The analysis and simulations in this paper indicate that significant gain (G=57) can be achieved for these targets even in the presence of hydrodynamic instabilities
Observation of Rayleigh-Taylor Growth to Short Wavelengths on Nike( )

1 edition published in 1999 in English and held by 0 WorldCat member libraries worldwide

The uniform and smooth focal profile of the Nike KrF laser [S. Obenschain, et. al., Phys. Plasmas 3, 1996 (2098)] was used to ablatively accelerate 40 micrometer thick polystyrene planar targets with pulse shaping to minimize shock heating of the compressed material. The foils had imposed small amplitude sinusoidal wave perturbations of 60, 30, 20, and 12.5 micrometer wavelength. The shortest wavelength is near the ablative stabilization cutoff for Rayleigh-Taylor growth. Modification of saturated wave structure due to random laser imprint was observed. Excellent agreement was found between the two dimensional simulations and experimental data for most cases where laser imprint was not dominant
Direct Observation of Feedout-Related Mass Oscillations in Plastic Targets( )

1 edition published in 2001 in English and held by 0 WorldCat member libraries worldwide

Feedout means the transfer of mass perturbations from the rear to the front surface of a driven target. When a planar shock wave breaks out at a rippled rear surface of the target, a lateral pressure gradient drives sonic waves in a rippled rarefaction wave propagating back to the front surface. This process redistributes mass in the volume of the target, forming the feedout-generated seed for ablative Rayleigh-Taylor (RT) instability. We report the first direct experimental observation of areal mass oscillation associated with feedout, followed by the onset of exponential RT growth
Absolutely Calibrated Vacuum Ultraviolet Spectra in the 150 nm to 250 nm Range from Plasmas Generated by the NIKE KrF Laser( )

1 edition published in 2005 in English and held by 0 WorldCat member libraries worldwide

High resolution vacuum ultraviolet (VUV) spectra were recorded from plasmas generated by the NIKE KrF laser for the purpose of observing emission from the two-plasmon decay instability (TPDI) at 2/3 the NIKE wavelength (165 nm). The targets were irradiated by up to 43 overlapping beams with intensity up to approximately equal to 10(exp 14) W/sq cm and with beam smoothing by induced spatial incoherence (ISI). The targets consisted of planar foils of CH, BN, Al, Si, S, Ti, Pd, and Au. Titanium-doped silica aerogels in pyrex cylinders were also irradiated. Spectra of the target elements were observed from charge states ranging from the neutral atoms to 5 times ionized. The spectrometer was absolutely calibrated using synchrotron radiation, and absolute VUV plasma emission intensities were determined. Emission from the TPDI at 165 nm wavelength was not observed from any of the irradiated targets. An upper bound on the possible TPDI emission was less than 4x10(exp -8) the incident NIKE laser energy. The NIKE laser radiation backscattered from the silica aerogel targets at 248 nm was typically 6x10(exp -6) the incident NIKE laser energy, and the spectral broadening corresponded to the 1 THz bandwidth of the ISI smoothing. The spectra from the moderately charged plasma ions (up to 5 times ionized), spectral line widths, absolute continuum emission level, and slope of the continuum were consistent with plasma temperatures in the 100 eV to 300 eV range
Perturbation Evolution Started by Richtmyer-Meshkov Instability in Planar Laser Targets( )

1 edition published in 2006 in English and held by 0 WorldCat member libraries worldwide

The first observations of the interaction of the Richtmyer-Meshkov (RM) instability with reflected shock and rarefaction waves in laser-driven targets are reported. The RM growth is started by a shock wave incident upon a rippled interface between low-density foam and solid plastic. Subsequent interaction of secondary rarefaction and/or shock waves arriving from the ablation front and the rear surface of the target with the RM-unstable interface stops the perturbation growth and reverses its direction. The ensuing exponential Rayleigh-Taylor growth thus can sometimes proceed with an inverted phase
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English (19)

German (1)