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Scaling of Pressure with Intensity in Laser-Driven Shocks and Effects of Hot X-ray Preheat

Description: To drive shocks into solids with a laser we either illuminate the material directly, or to get higher pressures, illuminate a plastic ablator that overlays the material of interest. In both cases the illumination intensity is low, <<10{sup 13} W/cm{sup 2}, compared to that for traditional laser fusion targets. In this regime, the laser beam creates and interacts with a collisional, rather than a collisionless, plasma. We present scaling relationships for shock pressure with intensity derived from simulations for this low-intensity collisional plasma regime. In addition, sometimes the plastic-ablator targets have a thin flashcoating of Al on the plastic surface as a shine-through barrier; this Al layer can be a source of hot x-ray preheat. We discuss how the preheat affects the shock pressure, with application to simulating VISAR measurements from experiments conducted on various lasers on shock compression of Fe.
Date: August 29, 2005
Creator: Colvin, J D & Kalantar, D H
Partner: UNT Libraries Government Documents Department

XUV radiography measurements of direct drive imprint in thin aluminum foils using a Ge x-ray laser on Vulcan

Description: One key aspect for high gain direct drive inertial confinement fusion is the imprint of perturbations in the outer surface of a capsule due to nonuniformities in the direct laser illumination of the capsule. Direct drive implosions are achieved by uniformly irradiating the outside surface of a hollow spherical capsule that contains a layer of fusionable D-T on its inner surface. The intensity of laser irradiation is down with a low intensity ``foot`` at 10{sup 13} W/cm{sup 2} for several nanoseconds before it builds up to more than 10{sup 15} W/cm{sup 2} during the main drive portion of the pulse. Laser ablation of the capsule surface produces a high pressure that accelerates the capsule shell radially inward in a spherical implosion. During this acceleration, perturbations due to surface roughness and due to imprint from spatial nonuniformities in the laser irradiation undergo Rayleigh-Taylor growth, potentially severely degrading performance. Our interest is in studying the imprint process and subsequent Rayleigh-Taylor growth of perturbations in a foil target that is irradiated by a low intensity laser speckle pattern. Previous experiments have been done to study laser imprint with an x-ray laser backlighter at the Nova laser using 0.35 micrometer laser irradiation of a 3 micrometer Si foil. In these experiments we irradiated a 2 micrometer thick Al foil with 0.53 micrometer laser light at 2-8 {times} 10{sup 12} W/cm{sup 2} using the Vulcan laser. We used a Ge x-ray laser as an XUV backlighter to measure the modulation in optical depth of the foil on a CCD during the initial imprint phase and after Rayleigh-Taylor growth with different laser smoothing schemes. 4 refs., 6 figs.
Date: March 29, 1996
Creator: Kalantar, D.H.; Demir, A. & Key, M.H.
Partner: UNT Libraries Government Documents Department

Measurements of laser imprint by XUV radiography using an x-ray laser

Description: We have developed a technique for studying the imprint of a laser beam on a thin foil using an x-ray laser as an XUV backlighter and XUV multilayer optics. This technique allows us to measure small fractional variations in the foil thickness due to hydrodynamics imprinted by direct laser irradiation. We present results of imprinted modulation and growth due to a low intensity 0.53 {mu}m drive beam incident on a 2 {mu}m Al foil using a germanium x-ray laser at the Vulcan facility. We present measurements of the modulation due to static RPP, SSD smoothed, and ISI smoothed speckle patterns at 0.53 {mu}m irradiation.
Date: May 30, 1996
Creator: Kalantar, D.H.; DaSilva, L.B. & Glendinning, S.
Partner: UNT Libraries Government Documents Department

Measurements of direct drive laser imprint in thin foils by XUV radiography using an X-ray laser backlighter

Description: In direct drive inertial confinement fusion, the residual speckle pattern remaining after beam smoothing plays an important role in the seeding of instabilities at the ablation front. We have used an x-ray laser as an XUV backlighter to characterize the imprinted modulation in thin foils for smoothing by random phase plate and spectral dispersion at both 0.35 pm and 0.53 pm irradiation, and induced spatial incoherence at 0.53 pm irradiation. We also demonstrate measurements of the modulation due to a single mode optical imprint generated by a narrow slit interference pattern, and modification of the imprint with a superposed smooth irradiation to study time dependence of the imprinting process. 8 refs., 10 figs.
Date: November 1, 1996
Creator: Kalantar, D.H.; Key, M.H. & DaSilva, L.B.
Partner: UNT Libraries Government Documents Department

X-ray laser measurements of direct drive imprint on vulcan

Description: High gain direct drive inertial confinement fusion requires very uniform irradiation of a hollow spherical shell with a layer of fusionable deuterium and tritium on its inner surface. The intensity of laser irradiation builds up in several nanoseconds from an initial `foot` at {approximately}10{sup 13} W/cm{sup 2} to more than 10{sup 15} W/cm{sup 2} during the main drive pulse. Laser ablation of the capsule surface produces a high pressure, accelerating the shell radially inward, and resulting in Rayleigh-Taylor (R-T) growth of surface perturbations originating from both the initial surface roughness of the capsule and from imprint of spatial non- uniformities in the laser drive intensity early in the laser pulse. The uniformity of illumination on a direct drive implosion capsule is determined on a large scale by the multi-beam irradiation geometry, and on a small scale by beam smoothing techniques. By using a large number of beams (such as the 60 beams of the Omega laser or 48 beam clusters for the NIF), large scale non-uniformities due to the overlap of the laser focal spots are adequately reduced. Random phase plates (RPPs) are introduced to smooth the individual beam focal spots. The spatial intensity variations of the individual beam speckle patterns may be smoothed by spectral dispersion (SD) with induced spatial incoherence (ISI) or by using partially coherent light. We performed experiments to study the imprint under conditions simulating the low intensity foot of the pulse on an ignition target, such as designed for the NIF. We used a 0.53 micrometer laser wavelength, and considered the imprint in thin Al foils due to both a broadband distribution of modes such as those in smoothed speckle patterns, and a single mode optical intensity variation. We characterized the laser imprint using a Ge x-ray laser and multilayer imaging optics, as described previously. ...
Date: July 1, 1997
Creator: Kalantar, D.H.; Wolfrum, E. & Zhang, J.
Partner: UNT Libraries Government Documents Department

Hydrodynamic instability experiments on the Nova laser

Description: Hydrodynamic instabilities in compressible plasmas play a critical role in the fields of inertial confinement fusion (ICF), astrophysics, and high energy-density physics. We are, investigating hydrodynamic instabilities such as the Rayleigh-Taylor (RT) instability, at high compression at the Nova laser in a series of experiments, both in planar and in spherical geometry. In the indirect drive approach, a thermal x-ray drive is generated by focusing the Nova laser beams into a Au cylindrical radiation cavity (hohlraum). Issues in the instability evolution that we are examining are shock propagation and foil compression, RT growth of 2D versus 3D single-mode perturbations, drive pulse shape, perturbation location at the ablation front versus at an embedded interface, and multimode perturbation growth and nonlinear saturation. The effects of convergence on RT growth are being investigated both with hemispherical implosions of packages mounted on the hohlraum wall and with spherical implosions of capsules at the center of the hohlraum. Single-mode perturbations are pre-imposed at the ablation front of these capsules as a seed for the RT growth. In our direct drive experiments, we are investigating the effect of laser imprinting and subsequent RT growth on planar foils, both at {lambda}{sub Laser} = 1/3 {mu}m and 1/2 {mu}m. An overview is given describing recent progress in each of these areas.
Date: August 1, 1996
Creator: Remington, B.A.; Glendinning, S.G. & Kalantar, D.H.
Partner: UNT Libraries Government Documents Department

X-ray backlit imaging measurement of in-flight pusher density for an indirect drive capsule implosion

Description: Both the efficiency of an implosion and the growth rate of hydrodynamic instability increase with the aspect ratio of an implosion. In order to study the physics of implosions with high Rayleigh-Taylor growth factors, we use doped ablators which should minimize x-ray preheat and shell decompression, and hence increase in-flight aspect ratio. We use x-ray backlighting techniques to image the indirectly-driven capsules. We record backlit 4.7 keV images of the full capsule throughout the implosion phase with 55 ps and 15 {mu}m resolution. We use these images to measure the in-flight aspect ratios for doped ablators, and we inferred the radial density profile as a function of time by Abel inverting the x-ray transmission profiles.
Date: May 6, 1996
Creator: Kalantar, D.H.; Haan, S.W. & Hammel, B.A.
Partner: UNT Libraries Government Documents Department

XUV probing of laser imprint in a thin foil using an x-ray laser backlighter

Description: For direct drive ICF, a capsule is imploded by directly illuminating the surface with laser light. Beam smoothing and uniformity of illumination affect the seeding of instabilities at the ablation front. We have developed a technique for studying the imprint of a laser beam on a thin foil using an x-ray laser as an XUV backlighter. We use multilayer XUV optics to relay the x-ray laser onto the directly driven foil, and then to image the foil modulation onto a CCD camera. This technique allows us to measure small fractional variations in the foil thickness. We have measured the modulation due to imprint from a low intensity 0.35 pm drive beam incident on a 3 {mu}m Si foil using an yttrium x-ray laser on Nova. We present results from a similar technique to measure the imprinted modulation due to a low intensity 0.53 {mu}m drive beam incident on a 2 {mu}m Al foil using a germanium x-ray laser at the Vulcan facility.
Date: May 6, 1996
Creator: Kalantar, D.H.; DaSilva, L.B. & Demir, A.
Partner: UNT Libraries Government Documents Department

Measurement by XUV laser radiography of hydrodynamic perturbations in laser accelerated thin foil targets

Description: A novel diagnostic application of XUV lasers has been developed for the study of the hydrodynamic imprinting of laser speckle pattern on directly driven laser fusion targets. A neon-like Yttrium laser operating at 15.5 nm is used to probe thin foils of Si irradiated with an SSD smoothed laser at 0.35 mm wavelength and 6 10{sup 12} Wcm{sup {minus}2} intensity, simulating the initial phase of irradiation a laser fusion capsule. Measurements of the perturbations in target opacity are made by XUV radiography through the foil. The magnitude and Fourier composition of the perturbations has been determined both before and after Rayleigh Taylor growth showing the mode spectra of both the initial imprint and the subsequent RT growth.
Date: July 11, 1995
Creator: Key, M.H.; Kalantar, D.H. & Barbee, T.W. Jr.
Partner: UNT Libraries Government Documents Department

Laser imprint and implications for direct drive ignition with the National Ignition Facility

Description: For direct drive ICF, nonuniformities in laser illumination can seed ripples at the ablation front in a process called imprint. Such nonuniformities will grow during the capsule implosion and can penetrate the capsule shell impede ignition, or degrade burn. We have simulated imprint for a number of experiments on tile Nova laser. Results are in generally good agreement with experimental data. We leave also simulated imprint upon National Ignition Facility (NIF) direct drive ignition capsules. Imprint modulation amplitude comparable to the intrinsic surface finish of {approximately}40 nm is predicted for a laser bandwidth of 0.5 THz. Ablation front modulations experience growth factors up to several thousand, carrying modulation well into the nonlinear regime. Saturation modeling predicts that the shell should remain intact at the time of peak velocity, but penetration at earlier times appears more marginal.
Date: July 9, 1996
Creator: Weber, S.V.; Glendinning, S.G.; Kalantar, D.H.; Remington, B.A. & Rothenberg, J.E.
Partner: UNT Libraries Government Documents Department

Dispersion relationship for solid state instability growth and sensitivity to equation of state

Description: We have derived an approximate analytical dispersion relation for solid state instability growth following the method of Mikaelian. I He starts with the general eigenvalue equation for the velocity of a perturbation on a finite-thickness fluid layer with surface tension and viscosity, and derives an exact solution numerically from det(M)=0, where M is an 8x8 matrix. He then derives an approximate solution analytically by substituting the inviscid eigenfunctions into the exact eigenvalue equation. The integrations yield a dispersion relation which is a polynomial in the growth rate.
Date: June 1, 1997
Creator: Colvin, J.D.; Wiley, L.G.; Chandler, E.A.; Remington, B.A. & Kalantar, D.H.
Partner: UNT Libraries Government Documents Department

X-ray framing cameras for > 5 keV imaging

Description: Recent and proposed improvements in spatial resolution, temporal resolution, contrast, and detection efficiency for x-ray framing cameras are discussed in light of present and future laser-plasma diagnostic needs. In particular, improvements in image contrast above hard x-ray background levels is demonstrated by using high aspect ratio tapered pinholes.
Date: July 20, 1995
Creator: Landen, O.L.; Bell, P.M.; Costa, R.; Kalantar, D.H. & Bradley, D.K.
Partner: UNT Libraries Government Documents Department

DEFORMATION SUBSTRUCTURES AND THEIR TRANSITIONS IN LASER SHOCK-COMPRESSED COPPER-ALUMINUM ALLOYS

Description: It is shown that the short pulse durations (0.1-10 ns) in laser shock compression ensure a rapid decay of the pulse and quenching of the shocked sample in times that are orders of magnitude lower than in conventional explosively driven plate impact experiments. Thus, laser compression, by virtue of a much more rapid cooling, enables the retention of a deformation structure closer to the one existing during shock. The smaller pulse length also decreases the propensity for localization. Copper and copper aluminum (2 and 6 wt% Al) with orientations [001] and [{bar 1}34] were subjected to high intensity laser pulses with energy levels of 70 to 300 J delivered in an initial pulse duration of approximately 3 ns. The [001] and [{bar 1}34] orientations were chosen since they respectively maximize and minimize the number of slip systems with highest resolved shear stresses. Systematic differences of the defect substructure were observed as a function of pressure, stacking-fault energy and crystalline orientation. The changes in the mechanical properties for each condition were compared using micro- and nano-hardness measurements and correlated well with observations of the defect substructure. Three regimes of plastic deformation were identified and their transitions modeled: dislocation cells, stacking-faults, and twins. An existing constitutive description of the slip to twinning transition, based on the critical shear stress, was expanded to incorporate the effect of stacking-fault energy. A new physically-based criterion accounting for stacking-fault energy was developed that describes the transition from perfect loop to partial loop homogeneous nucleation, and consequently from cells to stacking-faults. These calculations predict transitions that are in qualitative agreement with the effect of SFE.
Date: October 17, 2007
Creator: Meyers, M A; Schneider, M S; Jarmakani, H; Kad, B; Remington, B A; Kalantar, D H et al.
Partner: UNT Libraries Government Documents Department

Measurement of the in-flight pusher density of an indirect drive capsule implosion core using x-ray backlighting

Description: Both the efficiency of an implosion and the growth rate of hydrodynamic instability increase with the aspect ratio of an implosion. In order to study the physics of implosions with high Rayleigh-Taylor growth factors, we use doped ablators which should minimize x-ray preheat and shell decompression, and hence increase in- flight aspect ratio. We use x-ray backlighting techniques to image the indirectly-driven capsules. We record backlit 4.7 KeV images of the full capsule throughout the implosion phase with 55 ps and 15{mu}m resolution. We use these images to measure the in-flight aspect ratios for doped ablators, and we infer the radial density profile as a function of time by Abel inverting the transmission profiles.
Date: May 30, 1996
Creator: Kalantar, D.H.; Haan, S.W.; Hammel, B.A.; Keane, C.J.; Landen, O.L. & Munro, D.H.
Partner: UNT Libraries Government Documents Department

Developing beam phasing on the Nova laser

Description: We are presently adding the capability to irradiate indirectly-driven Nova targets with two rings of illumination inside each end of the hohlraum for studies of time-dependent second Legendre (P2) and time- integrated fourth Legendre (P4) flux asymmetry control. The rings will be formed with specially designed kinoform phase plates (KPPs), which will direct each half of each beam into two separate rings that are nearly uniform azimuthally. The timing and temporal pulse shape of the outer rings will be controlled independently from those of the inner rings, allowing for phasing of the pulse shapes to control time dependent asymmetry. Modifications to the incident beam diagnostics (IBDS) will enable us to verify that acceptable levels of power balance among the contributing segments of each ring have been achieved on each shot. Current techniques for precision beam pointing and timing are expected to be sufficiently accurate for these experiments. We present a design for an affordable retrofit to achieve beam phasing on Nova, results of a simplified demonstration, and calculations highlighting the anticipated benefits.
Date: March 10, 1997
Creator: Ehrlich, R.B.; Amendt, P.A.; Dixit, S.N.; Hammel, B.A.; Kalantar, D.H.; Pennington, D.M. et al.
Partner: UNT Libraries Government Documents Department

Nova experiments to investigate hydrodynamic instabilities in the solid state

Description: Experiments were done to shock compress and accelerate copper foils at peak presssures of {approximately}3 Mbar above and below the melt temperature to study the effects of material strength on hydrodynamic instabilities. An x-ray drive generated in a hohlraum target was used to generate the shock wave profiles. The growth of a preimposed perturbation at an embedded interface is diagnosed by x-ray radiography. Results obtained using a high contrastshaped laser pulse show that the growth of the modulation is delayed compared to fluid simulations,which could be due to material strength stabilization. In contrast, when a copper foil is placed above the melt temperature at {gt}3 Mbar with a single shock, it melts upon compression and the modulation growth is consistent with fluid modeling. Experimental results from copper shocked to 3 Mbar both below and above the melt temperature are presented and compared with simulation.
Date: July 8, 1997
Creator: Kalantar, D.H.; Remington, B.A.; Chandler, E.A.; Colvin, J.D.; Griswold, D.L.; Turner, R.E. et al.
Partner: UNT Libraries Government Documents Department

Shock compressed solids on the Nova laser

Description: Experiments are being developed to shock compress metal foils in the solid state to study the material strength under high compression. The x-ray drive has been characterized and hydrodynamics experiments performed to study growth of the Rayleigh-Taylor (RT) instability in Al foils at a peak pressure of about 1.8 Mbar. Pre-imposed modulations with an initial wavelength of lo-50 pm, and amplitude of 0.5 pm show growth. Variation in the growth factors may be a result of shot-shot variation in preheating of the Al sample due to emission from the plasma in the hohlraum target
Date: August 3, 1999
Creator: Colvin, J D; Gold, D M; Kalantar, D H; Mikaelian, K O; Remington, B A; Weber, S V et al.
Partner: UNT Libraries Government Documents Department

Characterizing high energy spectra of NIF ignition hohlraums using a differentially filtered high energy multi-pinhole X-ray imager

Description: Understanding hot electron distributions generated inside hohlraums is important to the ignition campaign for controlling implosion symmetry and sources of preheat. While direct imaging of hot electrons is difficult, their spatial distribution and spectrum can be deduced by detecting high energy x-rays generated as they interact with the target materials. We used an array of 18 pinholes, with four independent filter combinations, to image entire hohlraums with a magnification of 0.87x during the hohlraum energetics campaign on NIF. Comparing our results with hohlraum simulations indicates that the characteristic 30 keV hot electrons are mainly generated from backscattered laser plasma interactions rather than from hohlraum hydrodynamics.
Date: May 11, 2010
Creator: Park, H; Dewald, E D; Glenzer, S; Kalantar, D H; Kilkenny, J D; MacGowan, B J et al.
Partner: UNT Libraries Government Documents Department

DIRECT OBSERVATION OF THE ALPHA-EPSILON TRANSITION IN SHOCKED SINGLE CRYSTAL IRON

Description: In-situ x-ray diffraction was used to study the response of single crystal iron under shock conditions. Measurements of the response of [001] iron showed a uniaxial compression of the initially bcc lattice along the shock direction by up to 6% at 13 GPa. Above this pressure, the lattice responded with a further collapse of the lattice by 15-18% and a transformation to a hcp structure. The in-situ measurements are discussed and results summarized.
Date: August 23, 2005
Creator: Kalantar, D H; Collins, G W; Colvin, J D; Davies, H M; Eggert, J H; Hawreliak, J et al.
Partner: UNT Libraries Government Documents Department

EFFECT OF SHOCK COMPRESSION METHOD ON THE DEFECT SUBSTRUCTURE IN MONOCRYSTALLINE COPPER

Description: Monocrystalline copper samples with orientations of [001] and [221] were shocked at pressures ranging from 20 GPa to 60 GPa using two techniques: direct drive lasers and explosively driven flyer plates. The pulse duration for these techniques differed substantially: 40 ns for the laser experiments at 0.5 mm into the sample and 1.1 {approx} 1.4 {micro}s for the flyer-plate experiments at 5 mm into the sample. The residual microstructures were dependent on orientation, pressure, and shocking method. The much shorter pulse duration in the laser driven shock yielded microstructures closer to the ones generated at the shock front. For the flyer-plate experiments, the longer pulse duration allows shock-generated defects to reorganize into lower energy configurations. Calculations show that the post-shock cooling for the laser driven shock is 10{sup 3} {approx} 10{sup 4} faster than that for plate-impact shock, propitiating recovery and recrystallization conditions for the latter. At the higher pressure level, extensive recrystallization was observed in the plate-impact samples, while it was absent in the laser driven shock. An effect that is proposed to contribute significantly to the formation of recrystallized regions is the existence of micro-shear-bands, which increase the local temperature beyond the prediction from adiabatic compression.
Date: September 23, 2005
Creator: Cao, B Y; Lassila, D H; Schneider, M S; Kad, B K; Huang, C X; Xu, Y B et al.
Partner: UNT Libraries Government Documents Department

An Analysis of the X-Ray Diffraction Signal for the (alpha) - (epsilon) Transition in Shock-Compressed Iron: Simulation and Experiment

Description: Recent published work has shown that the phase change of shock compressed iron along the [001] direction does transform to the {epsilon} (HCP) phase similar to the case for static measurements. This article provides an indepth analysis of the experiment and NEMD simulations, using x-ray diffraction in both cases to study the crystal structure upon transition. Both simulation and experiment are consistent with a compression and shuffle mechanism responsible for the phase change from BCC to HCP. Also both show a polycrystalline structure upon the phase transition, due to the four degenerate directions the phase change can occur on, with grain sizes measured of 4nm in the NEMD simulations and {approx} 2nm in the experiment. And looking at the time scale of the transition the NEMD shows the transition from the compressed BCC to HCP is less then 1.2 ps where the experimental data places an upper limit on the transition of 80 ps.
Date: April 10, 2006
Creator: Hawreliak, J; Colvin, J D; Kalantar, D H; Lorenzana, H E; Stolken, J S; Davies, H M et al.
Partner: UNT Libraries Government Documents Department