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The European Spallation Source (ESS), Lund, Sweden will be a 5MW long-pulse neutron spallation research facility and will enable new opportunities for researchers in the fields of life sciences, energy, environmental technology, cultural heritage and fundamental physics. Neutrons are produced by accelerating a high-energy proton beam into a rotating helium-cooled tungsten target. These neutrons pass through moderators to reduce their energy to an appropriate range (< 5 meV for cold neutrons); two of which will use liquid hydrogen at 17 K as the moderating and cooling medium. There are several technical challenges to overcome in the design of a robust system that will operate under such conditions, not least the 20 kW of deposited heat. These challenges and the associated design solutions will be detailed in this paper. [ABSTRACT FROM AUTHOR]

We present molecular dynamics simulations of shock-induced plasticity and spall damage in single crystal Ta described by a recently developed embedded-atom-method (EAM) potential and a volume-dependent qEAM potential. We use impact or Hugoniotstat simulations to investigate the Hugoniots, deformation and spallation. Both EAM and qEAM are accurate in predicting, e.g., the Hugoniots and γ-surfaces. Deformation and spall damage are anisotropic for Ta single crystals. Our preliminary results show that twinning is dominant for [100] and [110] shock loading, and dislocation, for [111]. Spallation initiates with void nucleation at defective sites from remnant compressional deformation or tensile plasticity. Spall strength decreases with increasing shock strength, while its rate dependence remains to be explored. [ABSTRACT FROM AUTHOR]

Laser driven shocks allow an investigation of materials behavior at very high strain rate (107s-1) and present a great interest for research applications. Microscopic simulations of ultra-short laser driven shock on micrometric Tantalum single-crystals have been performed by using the CEA-DAM Classical Molecular Dynamics code. This method, complementary to continuum models, provides an analysis the microscopic processes related to damage (ductile pore nucleation and growth) which occurs during spallation. These results are compared to spallation experimental data (VISAR signals, micro-tomography) obtained with the LULI100TW femtosecond laser in order to validate the MD behavior. Moreover, in the framework of a multi-scale approach, we show the possibility to use MD simulation to fit macroscopic damage models. This method is illustrated with an application to the parameters determination of Kanel damage model parameters. This also shows the high strain rates involved during damage process, around 109s-1, can approach the inter-atomic theoretical cohesion stress threshold. [ABSTRACT FROM AUTHOR]

The investigation of the dynamic tensile or spallation response of materials is generally characterized by the measurement of reload signals during rear surface velocity experiments, or postmortem by metallography of sectioned target assemblies. The latter can only reveal features in two-dimensions, although serial sectioning can generate a three-dimensional representation of the spalled region. By contrast, X-ray tomography can generate three-dimensional images without the need for sectioning. In this investigation, we combine rear surface heterodyne velocimetry measurements with X-ray tomography of recovered Ti-6Al-4V targets to identify tensile failure mechanisms during dynamic loading. [ABSTRACT FROM AUTHOR]

Damage in plate impacts of 50 kbar and 30 kbar Cu, has been simulated with the traditional square wave loading profile of plate impacts and a triangular loading profile to study the time dependence of the damage evolution. At 30 kbar loading the square wave loading produces complete fracture while the triangular wave loading does not, similar to experiments by Koller et al. [1]. The calculated stress and strain loading histories are compared to provide insight for this result. The damage process in the triangular loading simply stops before fracture due to lack of kinetic energy. [ABSTRACT FROM AUTHOR]

Complementary techniques are combined to investigate dynamic fragmentation and shrapnel generation in laser shock-loaded samples of aluminium and gold, which will be two constituents of the target assemblies designed for the inertial confinement fusion experiments to be performed on large scale laser facilities. Fast optical transverse shadowgraphy is used to observe and analyze fragment ejection while Photonic Doppler Velocimetry provides time-resolved measurements of the free surface velocity. Experimental results are compared with two-dimensional numerical simulations involving a phenomenological fragmentation model based on a probabilistic distribution of material tensile strength within the sample. Although not physically-based at this preliminary stage, the model is shown to provide consistent predictions over the explored range of sample thickness and laser intensity. [ABSTRACT FROM AUTHOR]

We describe laser-driven spall experiments on aluminum, tantalum, steel and gold targets. The free-surface velocity is measured by using a VISAR diagnostic and is compared with numerical simulations at early spallation stage based on the Johnson fracture model. For each material, we first launch one-dimensional simulations with different values of the model parameters and determine the ones which reproduce correctly the experimental data. Then we use two-dimensional simulations to take into account spatial inhomogeneity of the loading pressure over the laser spot. We show non planar ejected spalls which are in agreement with experimental results. We finally study the fragmentation of thin targets and show differences in material behavior for steel in comparison to gold, tantalum and aluminum. [ABSTRACT FROM AUTHOR]

Fission of highly excited nuclei is affected by the viscose character of the system motion in deformation coordinates as is reported for very heavy nuclei with Z≥90. The long-time-scale fission was proved for such systems formed in heavy-ion induced reactions. The overdamped diabatic motion may influence also fission of the spallation-residue products in reactions with protons at intermediate energy. In the present work, the experimental results on blocking lifetime experiments are analyzed together with the independent data on fission-to-spallation ratios for medium-mass targets. Cross examination of the fission probability, time-scale, and pre-fission neutron multiplicity allows a new conclusion on fission mechanism. The novel approach is specified for estimate of the characteristic fission time and for the consistent interpretation of whole set of experimental results. The standard statistical approach fails for understanding of them without contradictions, though some partial data at restricted range of the encounter parameters could be described, as known from previous publications. [ABSTRACT FROM AUTHOR]

High pressure melting of iron conditions the understanding of the Earth core constitution. Shock compression has been used for many years to complement the data obtained under quasi-static loading. Still, shock-induced melting is not easy to detect. Here, we investigate how dynamic fragmentation of laser shock-loaded iron can be affected by melting. Iron samples are irradiated by a high power pulsed laser. The motion of the fragments ejected from the free surface is recorded by transverse shadowgraphy and soft recovery of the ejecta is performed in a low density gel. At low laser intensity, spalled layers can be seen in the shadowgraphs and solid fragments of mm-dimensions are recovered. At higher intensity, wide debris clouds are observed to expand from the free surface, and tiny spherical fragments are recovered in the gel. This evolution is qualitatively consistent with the predictions of one-dimensional hydrodynamic simulations accounting for laser-matter interaction and pulse decay during propagation. The results are discussed to infer whether such experiments may provide data on incipient melting of iron under high dynamic pressure loading. [ABSTRACT FROM AUTHOR]

With the development of high energy laser facilities dedicated to inertial confinement fusion, the question of debris ejection from metallic shells subjected to intense laser irradiation has become a key issue. We have used two diagnostics to investigate this phenomenon. Transverse shadowgraphy is an optical time-resolved diagnostic. It provides successive images that allow characterizing the motion of fragments generated by processes such as microjetting and spallation. Quasi-instantaneous pictures of the debris clouds are obtained and mean ejection velocities can be derived. Complementary data are provided by post-shock analysis of recovered fragments. Such recovery can be achieved in aerogels, but their brittleness and low transparency make the analysis difficult. Instead, we have used a new technique, based on a highly transparent gel of density 0.9 g/cm3, which allows soft recovery and easy observation of the fragments sizes, shapes and penetration depths, with a spatial resolution of μm-order. [ABSTRACT FROM AUTHOR]

Cavities of coalesced voids have been found in recovered samples of Tantalum in high-explosive-driven experiments. The boundaries of these cavities are imprinted with details of the coalescence and void growth processes. One way of quantifying these details is to measure the roughness of the surfaces. In this work, we calculate the roughness of 2D cross sections of such cavity surfaces from micrographs by analyzing the images with the box counting technique. Spall plane damage driven by flyer plates in Copper samples is also analyzed. The different length scale regimes found will be discussed. [ABSTRACT FROM AUTHOR]

Experimental investigations of dynamic tension of liquids (water, ethanol, glycerol, hexane, hexadecane, pentadecane, and transformer oil) under shock waves have been made. The method of spall strength measurements was applied and wave profiles were registered by laser interferometer VISAR. It was found that negative pressures in liquids were almost independent from the value of stain rate when the temperature was far from melting point. But near the melting point the spall strength of water, hexadecane, pentadecane, and glycerol is a strong function of strain rate and shock-wave amplitude. The process of cavitation in hexadecane and methanol is double-staged. At the first stage formation of cavities starts, and a kinked of free velocity profile is observed. At the second stage the cavity growth rate increases and the spall-pulse occurs. The theory of homogeneous bubble nucleation was used to explain the experimental results. It was observed for water that spall-pulse amplitude may be higher than the shock wave amplitude. To explain this phenomenon the model of failure kinetics, taking into account the inertial bubbles growth, has been proposed. [ABSTRACT FROM AUTHOR]

Dynamic fracture of ductile metals is attributed to the nucleation, growth, and coalescence of voids which eventually form the fracture surface. Large-scale MD simulations are used to model void growth behavior in single crystal Cu at high strain rates. Yielding is studied for systems with various values of void fractions as well as combinations of loading conditions. The calculated yield surface of single crystal Cu for varying void fractions indicates the inability of traditional continuum models to predict void growth at high strain rates. [ABSTRACT FROM AUTHOR]

Shock wave propagation and the spallation within materials induced by laser shock have been investigated for roughly two decades. With the latest laser technologies evolution, one can access to shorter regimes in durations, going below the picosecond range. Shots performed with the LULI 100 TW facility evidence the possibility to obtain spallation in a few microns thick metallic target. Such conditions provide an experimental data layout which may be directly comparable with molecular dynamic simulations reachable to these scales. Molecular dynamic simulations on a single crystal of Tantalum have been performed with the CEA TERA 10 computer. First, the Hugoniot calculated by the equilibrium molecular dynamics has been compared with experimental data to check the potential (EAM) relevance to reproduce the shock wave propagation. Then, large scale simulations on a micrometric target have been performed. We have observed the microscopic ductile damage process, the pore apparition and their time and space evolution. The results are compared with experimental results and classical one-dimensional hydrodynamic simulations. [ABSTRACT FROM AUTHOR]

A detailed description of photofission process at intermediate energies (200 to 1000 MeV) is presented. The study of the reaction is performed by a Monte Carlo method which allows the investigation of properties of residual nuclei and fissioning nuclei. The information obtained indicate that multifragmentation is negligible at the photon energies studied here, and that the symmetrical fission is dominant. Energy and mass distributions of residual and fissioning nuclei were calculated. [ABSTRACT FROM AUTHOR]

Complementary gas-gun and electromagnetic pulse tests conducted in Sandia's Dynamic Integrated Compression Experimental (DICE) Facility have, respectively, probed the behavior of electronic-grade Kovar® samples under controlled impact and intermediate-strain-rate ICE (Isentropic Compression Experiment) loading. In all experiments, velocity interferometer (VISAR) diagnostics provided time-resolved measurements of sample response for conditions involving one-dimensional (i.e., uniaxial strain) compression and release. Wave-profile data from the gas-gun impact experiments have been analyzed to assess the Hugoniot Elastic Limit (HEL), Hugoniot equation of state, spall strength, and high-pressure yield strength of shocked Kovar®. The ICE wave-profile data have been interpreted to determine the locus of isentropic stress-strain states generated in Kovar® for deformation rates substantially lower than those associated with a shock process. The impact and ICE results have been compared to examine the influence of loading rate on high-pressure yield strength. [ABSTRACT FROM AUTHOR]

A numerical hydrodynamic study of femtosecond laser irradiation (800 nm, 100 fs, 1012–5×1013 W/cm2) of metal targets (Al and Au) is presented. A detailed analysis of laser induced phase transitions and material decomposition is performed using a thermodynamically complete two-temperature equation of state with separate stable and metastable phase states and phase boundaries. Investigation of the phase trajectories of different target layers shows the presence of the metastable states in rarefaction wave. The lifetime of the metastable liquid state is estimated by means of the theory of homogeneous nucleation. Mechanical fragmentation of the target material at high strain rates is modeled using the Grady criterion. As a result, several ablation mechanisms are observed. A major fraction of the ablated material, however, is found to originate from the metastable liquid region, which is decomposed either thermally in the vicinity of the critical point into a liquid-gas mixture, or mechanically at high strain rate and negative pressure into liquid droplets. [ABSTRACT FROM AUTHOR]

Laser shocks are most often used to produce uniaxial stress and strain into materials by irradiating a spot diameter conventionally admitted at least three times larger than the thickness of the shocked sample. By reducing the laser spot versus the sample thickness, 2D lateral waves are created earlier and their crossing during propagation stages generates traction which can yield to voids into materials (near the front loaded face). This phenomenon has been evidenced by an experimental study, including VISAR measurements which exhibit the signature of the fracture generated by these 2D effects. Numerical simulations with the explicit finite element code RADIOSS clearly evidence the origin of the 2D effects on VISAR measurements. This different mode of damaging materials by laser lateral waves can act simultaneously with the classical laser spallation produced by the uniaxial propagation (shock wave reverberation crossing the unloading). This opens new discussed prospects for the development of 2D damage models. [ABSTRACT FROM AUTHOR]

Iron is known to undergo a polymorphic phase transition when shocked to pressures above 13 GPa. The details of the resulting wave structure shed light on the kinetics of the transformation. To explore the early stages of this wave structure evolution, we investigate the propagation of short compressive pulses over small distances in laser shock-loaded iron foils of thicknesses ranging from 150 to 520 μm. Comparing time-resolved velocity measurements to the predictions of a non-equilibrium phase change model provides an estimate of the relaxation time governing the dynamics of the phase transition. This time is found to strongly condition the attenuation of the pressure pulse as well as the occurrence of spallation upon reflection of that pulse from the free surface. Metallographic examination of the recovered samples confirms the results inferred from the velocity profiles, in agreement with the model calculations. Finally, they show the very clear change in spallation response to the characteristic smooth spall expected above the phase transformation. [ABSTRACT FROM AUTHOR]

We consider modeling and simulation of dynamic atomistic phenomena and processes in condensed matter under high strain rate: intensive shock compression and release, uniaxial and hydrostatic stretching. An attempt is done to draft out the atomistic theory of the phenomena. The basic concepts for the theory are the multiscale approach, the analysis of thermodynamic paths of relaxation on phase diagrams, the explicit utilization of the stochastic features of the MD method. A number of “elementary processes” (cavitation of voids at negative pressures, voids growth, dislocation formation and motion, phase transformations, etc.) are briefly considered for both single and nanocrystals. A theoretical MD based multiscale approach is presented for the spall process which could be used to extend the MD results to the larger spatial and temporal scales. Examples are presented for Al, Cu and Fe. The EAM potentials are deployed. Comparisons with the experimental data available are given. A hierarchy of dynamic and stochastic processes is discussed. [ABSTRACT FROM AUTHOR]

3D molecular dynamics calculations of plasticity and fracture in nanocrystalline copper under high strain rate are carried out. Grain boundary sliding and dislocation motion are observed. Flow stress dependence on grain size is obtained. Three general properties of spall process are considered: existence of sites of stress concentration, two step process of microcrack formation and stochastic character of microcrack formation. Spall strength dependences on grain size and strain rate are obtained. [ABSTRACT FROM AUTHOR]

We have conducted a series of free surface shock experiments on cerium metal at peak shock pressures from 0.9–7.6 GPa. These experiments were done to examine the elastic-plastic behavior, the solid-solid phase transition occurring at 0.7–0.8 GPa, and the spall strength of the material as a function of peak stress. The elastic and low-pressure plastic waves exhibit long rise times while the post-transition plastic wave is sharp. Spallation wave profiles obtained using optical techniques are presented and discussed. [ABSTRACT FROM AUTHOR]

Phase transition behavior of FeMnNi alloy with α-[variant_greek_epsilon] phase transition threshold in range of 6 GPa–7 GPa and corresponding spall phenomena in symmetric impact were studied experimentally. Impact of z-cut sapphire flyer on specimen with z-cut sapphire window and reverse impact by using specimen driven as flyer on z-cut sapphire and LiF window were used for loading and release wave profile measurement by VISAR. Experiments were conducted in velocity range 500 m/s–2000 m/s. Pulse X-ray was used for spallation verification by reason of difficulty in specimen recovery. Spallation phenomena were observed as impact stress far exceeds α-[variant_greek_epsilon] transition threshold. Observed spall phenomena should be related to special release wave configuration formed during reverse phase transition when impact stress slight exceeds phase transition threshold and even typical rarefaction shock wave formation is not observed. [ABSTRACT FROM AUTHOR]

When a shock-loaded target is melted on compression or on release, the tensile stresses generated upon reflection of the pressure pulse from a free surface are induced in a liquid state. Instead of the well-known spallation process occurring in solid targets, cavitation takes place in the melted material and liquid fragments are ejected from the free surface. Although increasing interest is manifested on the subject, related data are still scarce. In a recent paper, we have reported an exploratory investigation of liquid spall in tin targets submitted to intense laser shocks. Here, we present new results obtained over a lower pressure range, where we focus on the progressive transition from the ductile fracture behaviour of solid tin to the cavitation expected above melting. Both time-resolved free surface velocity measurements and post-test examination of the recovered samples clearly indicate this transition. Incipient melting produces a decrease in tensile strength that can be evaluated from the velocity profiles. Scanning electron micrographs of the fracture surfaces confirm such melting and provide an insight into the cavitation process. [ABSTRACT FROM AUTHOR]

Cylindrical spallation damage experiments allow for analysis of physical effects not present in a planar configuration. The authors report on 6 experiments providing data about failure initiation in cylindrical geometry and the behavior of material recollected after damage initiation. [ABSTRACT FROM AUTHOR]

The TRIDENT laser at Los Alamos has been used to drive small plate impact experiments. Flyers are typically 8 mm in diameter and 0.1 to 1.0 mm thick, while the targets are 10 mm in diameters and 0.2 to 2.0 mm thick. One of the sample materials is copper composed of large columnar grains. These samples can reveal information about damage in single crystals and bi-crystals. This work will focus on simulating these experiments to reveal the stress loading histories. A phenomenological single crystal plasticity model in the FLAG hyrdocode will be used. At the time of first negative pressure, calculated stress and strain concentrations occur at regions of high damage seen by metallurgical examination of a recovered sample. These calculated concentrations help to reveal the driving conditions that cause damage formation. [ABSTRACT FROM AUTHOR]

3D molecular-dynamics simulations of planar shock experiments have been carried out to check for whether nano-particles act as stress concentrators, decreasing significantly the spall strength. The MD model set-up was constructed of two samples of different length colliding at a given velocity. As a result of an interaction of two release waves formed after the shock waves reached free surfaces, a region of negative pressure was created in the longer sample. This stretching led to fracture of the longer sample. The calculations have been carried out with carbon and lead foreign nano-particles introduced into a copper matrix. The effects of the nano-particles’ average size (1–2 nm and 2–4 nm cases have been considered) and their concentration upon the spall strength and the mechanism of spallation have been studied. © 2006 American Institute of Physics [ABSTRACT FROM AUTHOR]

A series of gas-gun experiments has probed the planar (uniaxial strain) impact response of six different commercial brazing alloys that were subjected to a peak shock stress in the range of 5.5 – 7.2 GPa. The alloys studied were copper + gold (65/35 wt%), copper + gold (50/50 wt%), Cusil®, Nicusil®-3, Nicoro® + titanium (98/2 wt%), and silver zirconate. Both as-received and annealed samples of each alloy were tested under ambient (room temperature) and preheated (100 C) initial conditions. Velocity interferometer data acquired during this investigation have been evaluated to determine the dynamic yield strength (i.e., Hugoniot Elastic Limit), shock Hugoniot state, and spall strength for baseline alloy samples (i.e., unheated, as-received material) and for samples whose initial condition involved annealing and/or preheating. © 2006 American Institute of Physics [ABSTRACT FROM AUTHOR]

To account for the physical mechanisms of failure, the concept of thermal activation of damage and failure has been adopted as basis for this material model development. This basic assumption makes the proposed approach compatible with the Mechanical Threshold Stress (MTS) model, which was used as the strength part of the proposed constitutive model. The developments were incorporated into public domain DYNA3D. In order to validate the model, a series of FE simulations of plate impact experiments were performed for OFHC Cu. The numerical analysis results clearly demonstrate the ability of the model to predict the spall process and experimentally observed tensile damage and failure. The model allows simulation of high strain rate deformation processes and dynamic failure in tension for wide range of temperatures. The model is able to reproduce typical longitudinal stress reloading observed in plate impact tests, which is caused by the creation of the internal free surface. Plate impact tests used for model validation were performed on a single-stage gas gun. Longitudinal stresses were measured with stress gauges. © 2006 American Institute of Physics [ABSTRACT FROM AUTHOR]

The damage in the “spall plane” of tantalum incipiently spalled in a gas gun or from HE loading consists of voids and/or regions of localized shear through which a long, continuous path can be traced. The geometry of this path has been analyzed using wavelets to show that it constitutes an affine fractal with a well defined “roughness”, i. e it scales statistically with the scaling factor in the shock direction being a power of the transverse scaling factor. © 2006 American Institute of Physics [ABSTRACT FROM AUTHOR]

8 fission micro chambers will be inserted inside the central rod of the 1 MW liquid Pb-Bi MEGAPIE target in order to study the transmutation of two major actinides and to measure the neutron flux at a level of 5%. These chambers were developed for high neutron fluxes and tested at Laue Langevin Institute. © 2005 American Institute of Physics [ABSTRACT FROM AUTHOR]

Cosmogenic-induced activity is an issue of great concern for many sensitive experiments sited underground. A variety of different arch-type experiments — such as those geared toward the detection of dark matter, neutrinoless double beta decay and solar neutrinos — have reached levels of cleanliness and sensitivity that warrant careful consideration of secondary activity induced by cosmic rays. This paper reviews some of the main issues associated with the modeling of cosmogenic activity underground. Comparison with data, when such data is available, is also presented. © 2005 American Institute of Physics [ABSTRACT FROM AUTHOR]