S. Luo, S. Greenfield, D. Paisley, R. Johnson, T. Shimada, D. Byler, E. Loomis, S. DiGiacomo, B. Patterson, K. McClellan, R. Dickerson, P. Peralta, A. Koskelo, D. Tonks
We present two laser driven shock wave loading techniques utilizing long pulse lasers, laser-launched flyer plate
and confined laser ablation, and their applications to shock physics. The full width at half maximum of the drive
laser pulse ranges from 100 ns to 10 μs, and its energy, from 10 J to 1000 J. The drive pulse is smoothed with
a holographic optical element to achieve spatial homogeneity in loading. We characterize the flyer plate during
flight and dynamically loaded target with temporally and spatially resolved diagnostics. The long duration
and high energy of the drive pulse allow for shockless acceleration of thick flyer plates with 8 mm diameter
and 0.1-2 mm thickness. With transient imaging displacement interferometry and line-imaging velocimetry, we
demonstrate that the planarity (bow and tilt) of the loading is within 2-7 mrad (with an average of 4±1 mrad),
similar to that in conventional techniques including gas gun loading. Plasma heating of target is negligible in
particular when a plasma shield is adopted. For flyer plate loading, supported shock waves can be achieved.
Temporal shaping of the drive pulse in confined laser ablation enables flexible loading, e.g., quasi-isentropic,
Taylor-wave, and off-Hugoniot loading. These dynamic loading techniques using long pulse lasers (0.1-10 μs)
along with short pulse lasers (1-10 ns) can be an accurate, versatile and efficient complement to conventional
shock wave loading for investigating such dynamic responses of materials as Hugoniot elastic limit, plasticity,
spall, shock roughness, equation of state, phase transition, and metallurgical characteristics of shock-recovered
samples, in a wide range of strain rates and pressures at meso- and macroscopic scales.
Confined plasma ablation for shock physics, plate launch, and material dynamics has several unique advantages over direct-drive laser plasma that allows the freely expanding plasma away from the ablated surface, thus reducing energy coupling to the surface of the specimen. By confining the plasma with a laser-transparent, high-impedance window, energy coupling from the plasma to the specimen under test can be increased by several orders. However, for confined ablation, the laser power density and energy fluence must be kept below the break down threshold of the confining window material. Confined ablation to accelerate flyer plates decouples the laser parameters from the shock profile imparted in a target by the plasma-accelerated flyer plate.
An understanding of the timing and dynamics of hohlraum filling by laser-induced gold wall ablation is critical to the performance of indirectly-driven fusion ignition designs for the National Ignition Facility [E. Moses and C. Wuest, Fusion Science and Technology, 43, 420 (2003)]. Hohlraum wall ablation negatively affects ignition hohlraum performance by (1) reducing laser coupling by increasing backscatter by laser plasma instabilities, e.g., stimulated Brillouin scattering, (2) altering where lasers couple by moving the critical surface away from the walls and changing the refractive index, and (3), in the case of vacuum hohlraums, ablating directly into contact with the ablation layer of the fuel capsule. We report on measurements of gold-filling of hohlraums from a series of OMEGA laser [T.R. Boehly, R.L. McCrory, C.P. Verdon et al., Fusion Engineering and Design, 44, 35 (1999)] experiments involving vacuum and gas-filled hohlraums. On-axis x-ray imaging of gold self-emission shows delayed filling for gas-filled hohlraums, as expected. In addition, we present data on the hohlraum temperature penalty incurred with the use of a 1-atmosphere methane-fill. We discuss data and simulation predictions for 1-atmosphere neopentane filled hohlraums driven with a modified laser pulse.
Shocks extending across crystals' grain boundaries can nucleate and grow velocity fluctuations on the order of 5-10% when the shock speeds differ in the adjacent grains. Dynamic materials experiments at the Los Alamos National Laboratory Trident Laser Laboratory aim to examine this phenomenon by temporally- and spatially-resolving free surface velocity over a large region of interest. While line-imaged velocimetry can serve as a quantitative method for examining the velocity fluctuations across a single boundary, it is more desirable to resolve the velocity field around an entire embedded grain. We present a novel diagnostic design that utilizes a four-frame gated-optical-imaging interferometric velocimeter in combination with a streaked line-imaging interferometric velocimeter. This diagnostic will provide high-spatial resolution velocigraphs of a shock as it hits a free surface in multigrain crystals.
Confined plasma ablation is an efficient method to accelerate 1-D metal plates, with the impact of the plate resulting in a well-defined shock being generated in a target material. By using confined plasma to accelerate a plate, some details of the laser parameters are decoupled from the plate impact. Several types of experiments and related diagnostics to evaluate the performance parameters of the laser beam, flyer plate acceleration, and plate conditions are described. Several experiments using the flyer plates to generate shocks in materials to determine pressure-velocity relations, and dynamic spall strength of various metals are presented.
Optical diagnostic techniques including interferometry, electronic streak photography, and transient x-ray diffraction are used to study the dynamic material response to shock loading by direct laser irradiation and impact by laser- launched plates. The Los Alamos Trident laser is one of several lasers that have been used to generate shocks of 10 Kbar to several Mbar in single crystal and polycrystalline materials. Incorporating optical velocity interferometry (line-VISAR and point-VISAR) with transient x-ray diffraction can provide a complete understanding of the dynamic material response to shock compression and release. Laser-launched flyer plates provide an ideal method to generate one- dimensional shocks in materials. The quality of the one- dimensionality of the launch and acceleration of plates is evaluated by line-imaging VISAR. The line-imaging VISAR images the fringes along a line across the diameter of the plate. Each fringe maxima and minima provide acceleration and velocity information at the specific point on the plate. By varying the fringe constant, number of fringes and fringe spacing on the plate, detailed experimental data can be obtained. For our experiments, most plates are 3-mm diameter and accelerated to 0.2 - > 6 km/sec.
Laser-generated shocks can and have been used to study their effects on single crystal materials during shock compression. While a crystal undergoes shock compression and release, the transient x- ray diffraction (TXD) of the Bragg and Laue signals is indicative of the change in the crystal lattice spacing. The lattice spacing directly relates to the strain in the crystal. From the dynamic lattice data, strain, strain rate, and/or phase change in a material may be determined. Confined ablation plasmas can efficiently launch a flyer plate for direct impact on a target material imparting a well-characterized shock input and generate kilobar to megabar pressure pulses over a wide range of pulse duration (<EQ 1 - >= 20 ns). The laser-launched flyer plates are analogous to those launched by gas guns, but the smaller size provides an experimental method not easily accessible by larger gas gun experiments. With lasers, diagnostic equipment can be easily synchronized to study dynamic material parameters, i.e., single crystal shock dynamics, interfacial bond strengths of thin coatings, grain-interfaces, texture, and high strain rates (106 - 109 sec-1).
A high-power, high-energy laser is used to accelerate miniature metal plates (3 mm diameter X 50 micron thick, typ.) to high velocities of perform 1D impacts. Several plate conditions are critical to their performance. The plates must be flat and intact, and traveling at a known velocity and acceleration profile. Because the plates are small only optical diagnostic techniques can be used without perturbing the plate are small only optical diagnostic techniques can be used without perturbing the plate performance. We use pulsed laser stereo-photography to determine plate integrity and flatness, and velocity interferometry (VISAR) to evaluate plate acceleration and terminal velocity. Laser-launched plates can accelerate to over 109 m2/s2 and reach terminal velocities over 5 km/s. The high accelerations require temporal resolution of < 0.200-ns. The temporal resolution is accomplished by transferring the VISAR optical signals by optical fiber to an electronic streak camera for recording. Near 1D plate impacts are accomplished by converting the spatial Gaussian laser beam to a Tophat spatial profile. The pulsed laser stereo-photography system consists of a 10X stereo camera and a 5-ns pulsed doubled Nd:YAG laser for the light source. By incorporating these different optical techniques a complete understanding of the dynamic performance of miniature plates in flight is accomplished.
In recent years lasers have become a common tool for medical procedures. Lasers are typically used to deliver energy/power to a biological specimen to alter its characteristics, fuse tissue or destroy a particular structure. Under a Los Alamos CRADA, we have been working with a medical laser company and a laser medical center to study the laser interaction with pseudo-blood clots that are typical of those found in human coronary arteries. A 577-nm flash lamp pumped dye laser beam is pulsed through a 300- micron optical fiber to deliver the laser energy on the surface of a pseudo-clot material. The fiber and pseudo-clot are surrounded by water or x-ray contrast fluid transparent at 577 and 514 nm. The laser-pulse/clot interaction creates a bubble at the water-clot interface. The bubble expands out and collapses back on the pseudo-clot resulting part of the clot being removed. Using a backlight technique with an electronic framing camera we record the bubble growth, expansion, and collapse, and the debris generated by the interaction.
Miniature plates are laser-launched with a 10-Joule Nd:YAG for 1D impacts on to target materials much like gas gun experiments and explosive plane wave plate launch. By making the experiments small, flyer plates (3 mm diameter X 50 micron thick) and targets (10 mm diameter X 200 micron thick), 1D impact experiments can be performed in a standard laser-optical laboratory with minimum confinement and collateral damage. The laser-launched plates do not require the traditional sabot on gas guns nor the explosives needed for explosive planewave lenses, and as a result are much more amenable to a wide variety of materials and applications. Because of the small size very high pressure gradients can be generated with relative ease. The high pressure gradients result in very high strains and strain rates that are not easily generated by other experimental methods. The small size and short shock duration (1 - 20 ns) are ideal for dynamically measuring both strengths of micron-thick coatings. Experimental techniques, equipment, and dynamic material results are reported.
Pulsed laser ablation of blood clots in a fluid-filled blood vessel is accomplished by an explosive evaporation process. The resulting vapor bubble rapidly expands and collapses to disrupt the thrombus (blood clot). The hydrodynamic pressures following the bubble expansion and collapse can also be used as a driving force to deliver clot-dissolving agents into thrombus for enhancement of laser thrombolysis. Thus, the laser-induced bubble formation plays an important role in the thrombus removal process. In this study the effects of material properties on laser-induced cavitation bubbles formed in liquids and on submerged targets have been visualized with a microsecond strobe or high speed framing camera.
Miniature laser-launched plates have applications in shock wave physics, studying dynamic properties of materials and can be used to generate experimental data in a manner similar to a laboratory gas gun for 1D impact experiments. Laser-launched plates have the advantage of small size, low kinetic energy, and can be launched with ubiquitous laboratory lasers. Because of the small size and high accelerations (107 - 1010 g's), improved temporal resolution and optical non-contact methods to collect data are required. Traditional mechanical in-situ gauges would significantly impair the data quality and do not have the required time response.
Photoelectronic techniques provide the highest temporal resolution for dynamic material interaction studies. Optical signals from a velocity interferometer (VISAR) are coupled by fiber optics to the slit plane of an electronic streak camera. Temporal resolution of < 100 ps has been realized. Laser-launched miniature flat plates as thin as 1 micrometers have been launched to 4 km/s resulting in an impact pulse duration of approximately 0.3 ns at > 30 GPa. Miniature plates are being used to study grain and sub-grain size effects of metals and ceramics. The small size of the experiment permits evaluation of: (1) spall strength at strain rates > 107, (2) local grain size effects, (3) grain boundaries, (4) shock and release all within one grain thickness, (5) small samples of valuable or toxic materials, (6) bond strengths between similar or different materials, (7) Hugoniot elastic limits, and (8) separation of elastic-plastic shock velocities. Dynamic properties of metals at grain and sub-grain size affects are just now being investigated experimentally. This presentation discuses the status of work in progress and the application of photoelectronic techniques for data collection and analysis.
Several optical diagnostic techniques are used to evaluate the dynamic response of materials to intense dynamic loading and unloading, high stress and strain, and pressure. Velocity interferometry and electronic streak photography, each with sub-nanosecond time resolution, are used to record dynamic material response. Laser-launched flat plates are accelerated to 1012 m/s2 with terminal velocities > 5 km/s. By impacting these plates into target samples, high strain rates (108 sec-1) and pressures > 100 GPa have been generated for a duration of 0.8 - 5 nanoseconds. The efficacy and limitations of each technique are detailed and applications to other fields discussed.
Laser-launched, miniature, pseudo-one-dimensional flyer plates are evaluated by three distinct optical techniques that may be incorporated into an optical diagnostic system to give a complete understanding of the plate performance. These techniques are: velocity interferometry, streak photography, and pulsed laser stereo photography.
Optical diagnostics to evaluate the velocity, acceleration profiles, planarity, and integrity of miniature (≤5 μm thick x ≤600 μm diam) plates of aluminum and other metals will be reported. By correlating various optical techniques and collected data, a complete understanding of the flying plate can be determined. Velocity interferometer system for any reflector (VISAR), with ≤120 ps resolution per data point, is used to record plate acceleration and terminal velocity. Electronic-streak and pulsed-laser stereo photography can determine planarity and integrity. Flyer-plate performance data are related to the properties of the laser beam that accelerates the plate. Laser parameters, such as energy density, power density, and spatial profile, affect the flying-plate performance parameters, i.e., planarity, acceleration, and velocity. Flying-plate accelerations of ≥1010 G and terminal velocities of ≥6 mm/μs have been recorded, via a 10-ns-Nd:YAG laser pulse delivered to a tamped, 5-μm-aluminum plate.
Velocity interferomety has been used to determine velocities of miniature laser-driven flyer plates with sub-nanosecond (100 ps typical) time resolution. Since laser-driven flyer plates can have acceleration 101 0 m/s2 and attain 95 of terminal velocity within 20 nanoseconds the acceleration rate and terminal velocity cannot easily and accurately be resolved by current velocity interferometry techinques because of limitiations in temporal resolution of the interferometer and/or spatial resolution for conventional methods of recording raw data. By selecting an interferometer and recording system that is appropriate for the experiment sub-nanosecond time resolution is possible.
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