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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

Nanosecond Time Resolved Electron Diffraction Studies of the (Alpha) to (Beta) Transition in Pure Ti Thin Films using the Dynamic Transmission Electron Microscope (DTEM)

Description: The transient events of the {alpha}-{beta} martensitic transformation in nanocrystalline Ti films were explored via single shot electron diffraction patterns with 1.5 ns temporal resolution. The diffraction patterns were acquired with a newly constructed dynamic transmission electron microscope (DTEM), which combines nanosecond pulsed laser systems and pump-probe techniques with a conventional TEM. With the DTEM, the transient events of fundamental material processes, that are far too fast to be studied by conventional bulk techniques, can be captured in the form of electron diffraction patterns or images with nanosecond temporal resolution. The transient phenomena of the martensitic transformations in nanocrystalline Ti is ideally suited for study in the DTEM, with their rapid nucleation, characteristic interface velocities {approx}1 km/s, and significant irreversible microstructural changes. Free-standing 40-nm-thick Ti films were laser-heated at a rate of {approx}10{sup 10} K/s to a temperature above the 1155 K transition point, then probed at various time intervals with a 1.5-ns-long, intense electron pulse. Diffraction patterns show an almost complete transition to the {beta} phase within 500 ns. Postmortem analysis (after the sample is allowed to cool) shows a reversion to the {alpha} phase coupled with substantial grain growth, lath formation, and texture modification. The cooled material also shows a complete lack of apparent dislocations, suggesting the possible importance of a ''massive'' short-range diffusion transformation mechanism.
Date: December 9, 2005
Creator: LaGrange, T; Campbell, G H; Colvin, J D; Reed, B & King, W E
Partner: UNT Libraries Government Documents Department

Summary of Vulcan Calculations

Description: This is a summary of the results of my calculations compared to Elisabeth Wolfrum's data on the Vulcan imprint experiments. The material strength makes essentially no difference to the growth of perturbations seeded by the laser imprint. For the low-intensity case (30 J laser energy, beam intensity of 0.5 x 10{sup 12} W/cm{sup 2}) the thin (2 microns) Al foil melts quickly from the front (driven) surface and decompresses quickly from the back surface, so there is actually only a fraction of the foil that is solid and compressed, and then for only a short time. And this solid fraction is not accelerating much during this short time. In particular, the shock (which is at about 250 kbar when it is about half way through the foil) travels entirely through the foil in about 0.25 ns. At 0.3 ns the ablation front is 0.6 {micro}m in from the original position of the front surface, the next 0.6 {micro}m is melted, so only the back 0.8 {micro}m is solid and compressed. This solid portion, though, is not moving much; the place where the imprinted perturbations are growing is back at the ablation front, where the perturbations are clearly growing fluid-like. By 0.5 ns the entire foil is melted and decompressing from both ends. Thus, the actual foil distortion looks little different with and without strength.
Date: March 23, 2000
Creator: Colvin, J.D.
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

ACTIVATION ENERGY FOR GRAIN GROWTH IN BISMUTH COATINGS

Description: The knowledge of both activation energy and diffusion coefficient is needed for a predictive processing of grain size in coatings. However, for metals as Bismuth there is insufficient information available in the literature for these parameters. To determine these values, a method is adopted wherein an examination of the grain size is conducted for coatings deposited isothermally. The exponent for grain growth with time is determined, thereby enabling quantification of the activation energy and diffusion coefficient. Bismuth coatings that range from 10 {micro}m to 1 mm thick are deposited using electron-beam evaporation onto temperature-controlled substrate surfaces of glass and lithium fluoride. The grain size of each coating is measured upon examination of the microstructure in cross-section using the intercept method. Ideal grain growth is observed over the experimental range of deposition temperatures examined from 317 to 491 K. The activation energy (Q) for grain growth in bismuth is fit as 0.47 eV {center_dot} atom{sup -1} with a diffusion coefficient (D{sub 0}) of 3.3 x 10{sup -4} cm{sup 2} {center_dot} sec{sup -1}.
Date: September 9, 2005
Creator: Jankowski, A F; Hayes, J P; Smith, R F; Reed, B W; Kumar, M & Colvin, J D
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

MULTI-KEV X-RAY YIELDS FROM HIGH-Z GAS TARGETS FIELDED AT OMEGA

Description: The authors report on modeling of x-ray yield from gas-filled targets shot at the OMEGA laser facility. The OMEGA targets were 1.8 mm long, 1.95 mm in diameter Be cans filled with either a 50:50 Ar:Xe mixture, pure Ar, pure Kr or pure Xe at {approx} 1 atm. The OMEGA experiments heated the gas with 20 kJ of 3{omega} ({approx} 350 nm) laser energy delivered in a 1 ns square pulse. the emitted x-ray flux was monitored with the x-ray diode based DANTE instruments in the sub-keV range. Two-dimensional x-ray images (for energies 3-5 keV) of the targets were recorded with gated x-ray detectors. The x-ray spectra were recorded with the HENWAY crystal spectrometer at OMEGA. Predictions are 2D r-z cylindrical with DCA NLTE atomic physics. Models generally: (1) underpredict the Xe L-shell yields; (2) overpredict the Ar K-shell yields; (3) correctly predict the Xe thermal yields; and (4) greatly underpredict the Ar thermal yields. However, there are spreads within the data, e.g. the DMX Ar K-shell yields are correctly predicted. The predicted thermal yields show strong angular dependence.
Date: November 4, 2010
Creator: Kane, J O; Fournier, K B; May, M J; Colvin, J D; Thomas, C A; Marrs, R E et al.
Partner: UNT Libraries Government Documents Department

In-situ Studies of the Martensitic Transformation in Ti Thin Films using the Dynamic Transmission Microscope (DTEM)

Description: The {alpha} to {beta} transition in pure Ti occurs mainly by a 'martensitic type' phase transformation. In such transformations, growth rates and interface velocities tend to be very large, on the order of 10{sup 3} m/s, making it difficult to observe the transformation experimentally. With thin films, it becomes even more difficult to observe, since the large surface augments the nucleation and transformation rates to levels that require nanosecond temporal resolution for experimental observations. The elucidation of the transformational mechanisms in these materials yearns for an apparatus that has both high spatial and temporal resolution. We have constructed such an instrument at LLNL (the dynamical transmission electron microscope or DTEM) that combines pulsed lasers systems and optical pump-probe techniques with a conventional TEM. We have used the DTEM to observe the transient events of the {alpha}-{beta} transformation in nanocrystalline Ti films via single shot diffraction patterns with 1.5 ns resolution. With pulsed, nanosecond laser irradiation (pump laser), the films were heated at an extreme rate of 10{sup 10} K/s. was observed At 500 ns after the initial pump laser hit, the HCP, alpha phase was almost completely transformed to the BCC, beta phase. Post-mortem investigations of the laser treated films revealed that substantial grain growth occurred and lath microstructure, containing no apparent dislocations. The lack of dislocations may indicate that the {alpha} to {beta} transformation may also proceed by a 'massive' type mechanism (short range diffusion).
Date: November 21, 2005
Creator: LaGrange, T B; Campbell, G H; Colvin, J D; King, W E; Browning, N D; Armstrong, M R 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

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

High pressure solid state experiments on the NOVA laser

Description: An x-ray drive has been developed to shock compress metal foils in the solid state in order to study the material strength under high compression. The drive has been characterized and hydrodynamics experiments designed to study growth of the Rayleigh-Taylor (RT) instability in Cu foils at 3 Mbar peak pressures have been started. Pre-imposed modulations with an initial wavelength of 20-50 {micro}m, and amplitudes of 1.0-2.5 {micro}m show growth consistent with simulations. In this parameter regime, the fluid and solid states are expected to behave similarly for Cu. An analytic stability analysis is used to motivate an experimental design with an Al foil where the effects of material strength on the RT growth are significantly enhanced. Improved x-ray drive design will allow the material to stay solid under compression throughout the experiment, and dynamic diffraction techniques are being developed to verify the compressed state.
Date: November 1, 1998
Creator: Chandler, E A; Colvin, J D; Gold, D M; Hauer, A A; Kalantar, D H; Meyers, M A et al.
Partner: UNT Libraries Government Documents Department

Solid State Experiments at High Pressure and Strain Rates

Description: Experiments have been developed using high powered laser facilities to study the response of materials in the solid state under extreme pressures and strain rates. Details of the target and drive development required for solid state experiments and results from two separate experiments are presented. In the first, thin foils were compressed to a peak pressure of 180 GPa and accelerated. A pre-imposed modulation at the embedded RT unstable interface was observed to grow. The growth rates were fluid-like at early time, but suppressed at later time. This result is suggestive of the theory of localized heating in shear bands, followed by dissipation of the heat, allowing for recovery of the bulk material strength. In the second experiment, the response of Si was studied by dynamic x-ray diffraction. The crystal was observed to respond with uni-axial compression at a peak pressure 11.5-13.5 GPa.
Date: November 24, 1999
Creator: Kalantar, D.H.; Remington, B.A.; Colvin, J.D.; Mikaelian, K.O.; Weber, S.V.; Wiley, L.G. et al.
Partner: UNT Libraries Government Documents Department

Nova Experiments Examining Raleigh-Taylor Instability in Materials with Strength

Description: Material strength can affect the growth of the Rayleigh-Taylor instability in solid materials, where growth occurs through plastic flow. In order to study this effect at megabar pressures, we have shocked metal foils using hohlraum x-ray drive on Nova, and observed the growth of pre-imposed modulations with x-ray radiography. Previous experiments employing Cu foils did not conclusively show strength effects for resolvable wavelengths. Therefore, we have redesigned the experiment to use aluminum foils. As aluminum has higher specific strength at pressures {approx}1 Mbar, the new design is predicted to show growth reduction due to strength of at least a factor of two for some wavelengths in the observable range of 10 - 50 {micro}m. We have also modified the drive history to extend the interval of uniform acceleration and to reduce the risk of melting the foils with coalesced shocks. The design changes, as well as Nova operational constraints, limit peak pressures to 1-1.5 Mbar. Foil surface motion has been measured with high sensitivity by laser interferometry to look for thermal expansion due to preheat. We have continued to pursue dynamic x-ray diffraction as the most definitive measurement of crystal state.
Date: October 6, 1999
Creator: Weber, S.V.; Kalantar, D.H.; Colvin, J.D.; Gold, D.M.; Mikaelian, K.O.; Remington, B.A. et al.
Partner: UNT Libraries Government Documents Department

Practical Considerations for High Spatial and Temporal Resolution Dynamic Transmission Electron Microscopy

Description: Although recent years have seen significant advances in the spatial resolution possible in the transmission electron microscope (TEM), the temporal resolution of most microscopes is limited to video rate at best. This lack of temporal resolution means that our understanding of dynamic processes in materials is extremely limited. High temporal resolution in the TEM can be achieved, however, by replacing the normal thermionic or field emission source with a photoemission source. In this case the temporal resolution is limited only by the ability to create a short pulse of photoexcited electrons in the source, and this can be as short as a few femtoseconds. The operation of the photo-emission source and the control of the subsequent pulse of electrons (containing as many as 5 x 10{sup 7} electrons) create significant challenges for a standard microscope column that is designed to operate with a single electron in the column at any one time. In this paper, the generation and control of electron pulses in the TEM to obtain a temporal resolution <10{sup -6} s will be described and the effect of the pulse duration and current density on the spatial resolution of the instrument will be examined. The potential of these levels of temporal and spatial resolution for the study of dynamic materials processes will also be discussed.
Date: May 1, 2006
Creator: Armstrong, M; Boyden, K; Browning, N D; Campbell, G H; Colvin, J D; DeHope, B et al.
Partner: UNT Libraries Government Documents Department

PICOSECOND X-RAY DIFFRACTION FROM LASER-SHOCKED COPPER AND IRON

Description: In situ X-ray diffraction allows the determination of the structure of transient states of matter. We have used laser-plasma generated X-rays to study how single crystals of metals (copper and iron) react to uniaxial shock compression. We find that copper, as a face-centered-cubic material, allows rapid generation and motion of dislocations, allowing close to hydrostatic conditions to be achieved on sub-nanosecond timescales. Detailed molecular dynamics calculations provide novel information about the process, and point towards methods whereby the dislocation density might be measured during the passage of the shock wave itself. We also report on recent experiments where we have obtained diffraction images from shock-compressed single-crystal iron. The single crystal sample transforms to the hcp phase above a critical pressure, below which it appears to be uniaxially compressed bcc, with no evidence of plasticity. Above the transition threshold, clear evidence for the hcp phase can be seen in the diffraction images, and via a mechanism that is also consistent with recent multi-million atom molecular dynamics simulations that use the Voter-Chen potential. We believe these data to be of import, in that they constitute the first conclusive in situ evidence of the transformed structure of iron during the passage of a shock wave.
Date: August 23, 2005
Creator: Wark, J S; Belak, J F; Collins, G W; Colvin, J D; Davies, H M; Duchaineau, M et al.
Partner: UNT Libraries Government Documents Department

Developing solid state experiments on the Nova laser

Description: An x-ray drive has been developed to shock compress metal foils in the solid state using an internally shielded hohlraum with a high contrast shaped pulse from the Nova laser. The drive has been characterized and hydrodynamics experiments designed to study growth of the Rayleigh-Taylor (RT) instability in Cu foils at 3 Mbar peak pressures in the plastic flow regime have been started. Pre-imposed modulations with an initial wavelength of 20-50 {micro}m, and amplitudes of 1.0-2.5 {micro}m show growth consistent with simulations. In the Nova experiments, the fluid and solid states are expected to behave similarly for Cu. An analytic stability analysis is used to motivate an experimental design with an Al foil where the effects of material strength on the RT growth are significantly enhanced. The conditions reached in the metal foils at peak compression are similar to those predicted at the core of the earth.
Date: August 6, 1999
Creator: Chandler, E A; Colvin, J D; Failor, B H; Gold, D M; Hauer, A; Kalantar, D H et al.
Partner: UNT Libraries Government Documents Department

Direct Observation of the alpha-epsilon Transition in Shock-compressed Iron via Nanosecond X-ray Diffraction

Description: In-situ x-ray diffraction studies of iron under shock conditions confirm unambiguously a phase change from the bcc ({alpha}) to hcp ({var_epsilon}) structure. Previous identification of this transition in shock-loaded iron has been inferred from the correlation between shock wave-profile analyses and static high-pressure x-ray measurements. This correlation is intrinsically limited because dynamic loading can markedly affect the structural modifications of solids. The in-situ measurements are consistent with a uniaxial collapse along the [001] direction and shuffling of alternate (110) planes of atoms, and in good agreement with large-scale non-equilibrium molecular dynamics simulations.
Date: March 21, 2005
Creator: Kalantar, D. H.; Belak, J. F.; Collins, G. W.; Colvin, J. D.; Davies, H. M.; Eggert, J. H. et al.
Partner: UNT Libraries Government Documents Department