WorldCat Identities

Obenschain, S. P.

Works: 37 works in 41 publications in 1 language and 41 library holdings
Roles: Author
Publication Timeline
Most widely held works by S. P Obenschain
Flash X Radiography of Laser-Accelerated Targets by R. R Whitlock( Book )

2 editions published in 1982 in English and held by 2 WorldCat member libraries worldwide

Flash x radiography of ablatively accelerated planar foils has provided quantitative measurements and qualitative observations regarding several parameters of critical interest to direct illumination laser fusion. A 1.05 micron, 3.3 nsec driver beam was focused onto carbon foils in a large (0.7-1 mm diameter) spot to reduce edge effects. From images produced by a backlighting x-ray flash, we have measured overall coupling efficiency, smoothing of laser nonuniformities, target velocity, and ablation pressure. The high velocity targets maintain a localized, high density (> 3% of solid). In contrast to other workers' recent measurement of pressure from x-ray imaging, our x-radiographic results, including pressure, are in general agreement with earlier NRL studies. Our results have also provided further insights into double foil interactions and planar target preheat measurements. (Author)
Doppler Velocimeter for Laser Accelerated Targets by S. P Obenschain( Book )

2 editions published in 1980 in English and held by 2 WorldCat member libraries worldwide

A doppler velocimeter utilizing a spectrograph and a short pulse laser probe is described which provides good spatial (20 microns) and subnanosecond temporal resolution. This system has been used to measure the velocity profiles of targets ablatively accelerated to very high velocities by a high power laser beam. A unique aspect of this velocimeter is that the doppler wavelength shifts are measured directly rather than employing interferometric techniques. (Author)
Beam Nonuniformity Effects on Laser Ablatively Accelerated Targets by S. P Obenschain( Book )

2 editions published in 1980 in English and held by 2 WorldCat member libraries worldwide

A laser doppler shift technique gives the velocity profiles of ablatively accelerated targets. Measurements of the effects of laser beam nonuniformity on the target acceleration are presented and interpreted in the context of laser pellet fusion
Uniformity of Laser-Driven, Ablatively Accelerated Targets( Book )

2 editions published in 1981 in English and held by 2 WorldCat member libraries worldwide

A double-target technique is used to measure the uniformity with which laser irradiated thin foil targets are ablatively accelerated. Studies of the effects of laser nonuniformities on the target acceleration reveal an increase in smoothing (symmetrization) with increasing laser intensity. Measurements of targets accelerated to velocities (160 km/sec) sufficient for pellet fusion implosions are presented and discussed
Ablative acceleration of planar targets to high velocities( Book )

1 edition published in 1982 in English and held by 1 WorldCat member library worldwide

Laser irradiated targets are ablatively accelerated to velocities near those required for fusion pellet implosions while remaining relatively cool and uniform. The target velocities and velocity profiles are measured using a double-foil method, which is described in detail. Also, the ablation plasma flow from the target surface is spatially resolved, and the scalings with absorbed irradiance of the ablation pressure, ablation velocity, and mass ablation rate are determined. Results are compared with hydrodynamic code calculations. (Author)
Efficient Second Harmonic Conversion of Broadband High-Peak-Power Nd:Glass Laser Radiation Using Large-Aperture KDP Crystals in Quadrature( Book )

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

The second harmonic conversion efficiency broadband Nd: glass Laser Light (Delta nu/c <30/cm FWHM). Using two KDP crystals in a quadrature arrangement we obtained energy conversion efficiencies 55% with an initial bandwidth for the fundamental of Delta nu/c about 17 cm FWHM. For these conditions, we observed a modest increase (about 70%) in the harmonic bandwidth (FWHM) relative to the fundamental. The usual theory of three wave mixing a dispersive birefringent nonlinear crystals is extended to described the broadband harmonic conversion process; the generalized theory includes the statistical properties of the Light and phase mismatch effects on the spectral components in the complex field amplitudes. Good agreement is shown between the code calculations and the measurements. Keywords: Full width at half maximum
Experimental Methods for Studying the Rayleigh-Taylor Instability of Ablatively Accelerated Targets( Book )

1 edition published in 1983 in English and held by 1 WorldCat member library worldwide

The authors describe new diagnostic methods for the study of hydrodynamic instabilities in ablatively accelerated targets. These methods include face-on x-ray backlighting that does not require a backlighting laser beam (for growth rate measurement), and a tracer dot technique (for tracking ablation plasma flow). The targets in the experiments are periodically perturbed to provide initial conditions for the growth of the Rayleigh-Taylor instability. (Author)
Observation of Rayleigh-Taylor-Like Structures in a Laser-Accelerated Foil( Book )

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

Laser-accelerated targets have been predicted to be subject to the Rayleigh-Taylor hydrodynamic instability. The development of the instability was studied by introducing mass thickness variations in foil targets and observing the development of the target nonuniformities by side-on flash x radiography. Observations were made of target structures and mass redistribution effects which resemble Rayleigh-Taylor bubbles and spikes, including not only advanced broadening of the spike tips on the laser-irradiated side of the foil but also projections of mass on the unirradiated side. The observations compare well with numerical simulations. (Author)
Laser interaction in long-scalelength plasmas( Book )

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

Absorption of a short-pulse, high-intensity Nd-laser beam (vacuum irradiance of 10 to the 14th power to 10 to the 15th power W/sq cm) by preformed plasmas of different density scalelengths is investigated. Increased effects of plasma instabilities are found at longer scalelengths. The amount of backscattered light increases with plasma scalelength and limits the absorption fraction at the longest scalelength. The onset of suprathermal electron production, deduced from observations of energetic (20 to 50 keV) x rays, occurs at lower laser irrradiance for longer scalelength plasmas. A correlation between energetic x rays and 3 omega sub 0/2 emission suggests that the suprathermal electrons are produced by a plasma instability at quarter-critical density. At higher intensities there is evidence for severe perturbations of the preformed plasma and for self-focusing of the incident beam. Keywords: Long scalelength; Laser plasma interaction; Stimulated brillouin backscatter; Laser absorption
Use of Induced Spatial Incoherence for Uniform Illumination on Laser Fusion Targets( Book )

1 edition published in 1983 in English and held by 1 WorldCat member library worldwide

We report a technique to achieve very uniform illumination of laser irradiated targets by inducing a controlled amount of transverse spatial incoherence in the output beam of a broadband laser. The spatial incoherence is created by imposing different optical delays upon different transverse sections of the beam, and choosing the delay increments to be larger than the laser coherence time tc = 1 delta. This technique may allow easy attainment of the pellet illumination uniformity required for laser fusion
Uniform laser ablative acceleration of targets at 1014 W/cm2( Book )

1 edition published in 1982 in English 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 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
High-Gain Direct-Drive Target Design for Laser Fusion (Preprint)( )

1 edition 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
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 Richtmyer-Meshkov (RM) growth and the Rayleigh-Taylor (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 ramp-up of the laser intensity, where RT formulas predict more growth than seen in the simulations
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
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
Development of Electron Beam Pumped KrF Lasers for Fusion Energy( )

1 edition published in 2008 in English and held by 0 WorldCat member libraries worldwide

Direct drive with krypton fluoride (KrF) lasers is an attractive approach to inertial fusion energy (IFE): KrF lasers have outstanding beam spatial uniformity, which reduces the seed for hydrodynamic instabilities; they have short wavelength (248 nm) that increases the rocket efficiency and raises the threshold for deleterious laser-plasma instabilities; they have the capability for zooming, i.e. decreasing the spot size to follow an imploding pellet and thereby increase efficiency; and they have a modular architecture, which reduces development costs. Numerical 1-D simulations have shown that a target driven by a KrF laser can have a gain above 125 [1,2], which is ample for a fusion system. Simulations of the pellet burn in 2-D and 3-D are underway. In addition to these laser-target advantages, the Sombrero Power Plant study showed a KrF based system could lead to an economically attractive power plant [3]. In view of these advances, several world-wide programs are underway to develop KrF lasers for fusion energy. These include programs in Japan [4, 5], China [6], Russia [7], and The United Kingdom [8]. There was also a large program in the United States [9]. The paper here concentrates on current research in the US with two lasers at the Naval Research Laboratory: The Electra laser [10] is a 400-700 J repetitively pulsed system that is being used to develop the technologies that meet the fusion requirements for rep-rate, durability, efficiency and cost. The Nike laser [11] is a 3-5 kJ single shot device that is used to study KrF issues with full-scale electron beam diodes
Dark-field study of rear-side density structure in laser-accelerated foils( )

1 edition published in 1981 in English and held by 0 WorldCat member libraries worldwide

A dark-field, laser-probing diagnostic has produced the first high- resolution photographs of density structure on the rear side of laser- accelerated foils. This diagnostic allows the preferential sampling of the steep-gradient region of an expanding plasma and permits two-dimensional, multiple-time recordings on a single photograph. The studies are aimed at understanding the early-time physics of target implosions for inertial- confinement fusion. Both long (500 psec) and short (150 psec) probe pulses were used to study the rear-side plasmas of thin foils accelerated by the rocket-like reaction to a hot plasma ablated from the front side by the laser radiation. The longer pulse results, both for angular scatter and the life-time of small, transverse structure, imply a relatively cold (1 eV) rear side plasma. The short pulses provide high resolution photographs of the complete structure. One of these was a vortex-like structure, suggestive of the remnants of a hydrodynamic instability. These observations are relevant to two of the basic requirements of inertial-confinement fusion: cold fuel isentrope and implosion symmetry
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