Ejection Of Fragments Research Articles

Delayed fragmentation of weakly bound Kr2.

We report the experimental observation of the delayed fragmentation of the weakly bound dimer Kr2+ produced through the single ionization of Kr2 by a femtosecond laser field. The observed time delay between ionization and fragmentation, which reflects the survival time of the resulting Kr2+, is measured on the microsecond timescale. A detailed analysis of the kinetic energy releases of the ejected fragments and photoelectrons suggests that this delayed fragmentation arises from the radiative decay of the long-lived Kr2+, transitioning from the bound state II(1/2u) to the repulsive state I(1/2g).

Experimental study on the failure characteristics and mechanism between spalling failure and rockburst

To reveal the differences in the failure characteristics and mechanisms between rockburst and spalling failure, the comparison experiment between spalling failure and rockburst occurred in a circular cavity were carried out. Experiment results reveal that rockburst is accompanied by the intense ejection of fragments, whereas spalling failure features with the slow detachment of fragments. Additionally, besides the plate and flaky fragment, the rockburst fragment exhibits more pronounced blocky features. In terms of strength and deformation characteristics, the peak stress for rockburst is 1.04 times that of spalling failure, whereas the plastic deformation of spalling failure is 1.75 times that of rockburst. Furthermore, considering the micro failure and energy evolution process, three interpretations were proposed to elucidate the distinct mechanisms between rockburst and spalling failure, including: (i) The swift invasion of shear cracks triggers the buckling failure of pre-existing plate-like fractures, causing the fragment ejection observed in rockburst. When shear cracks not substantially contribute to the evolutionary process, the continuous penetrating of the pre-existing tensile cracks will culminate in the gradual delamination and spalling failure of the sidewall. (ii) The energy dissipating behavior determines the distinct mechanisms of rockburst and spalling failure. Almost all of the energy input preceding rockburst is converted into the elastic strain energy of rock units, whereas the energy input preceding spalling failure results in greater energy dissipation (about 1.85 times that of rockburst). (iii) under the framework of Mohr-Coulomb strength criterion, it has been revealed that the rock units of spalling failure adhere to the minimum failure energy criterion. However, the energy accumulated by the rock units prior to rockburst surpasses the minimum energy necessary for failure. This residual energy provides the fractured rock mass with kinetic energy, ultimately leading to a violent ejection phenomenon of rockburst.

Experimental investigation of the intensity and conditions for strainburst triggering in intact hard rocks

Strainburst occurs during deep hard rock excavations. A series of single-free-face true triaxial compression (TTC) tests were conducted on seven hard rock types to investigate strainburst intensity and triggering conditions. Five of the seven rock types failed by strainburst, and two failed by spalling. The differences between the strainburst intensities are attributed to the mineral composition and structure. High-hardness minerals and mosaic-type microstructures facilitate stress conduction, which promotes rock microcrack initiation and propagation, causing severe rock fragmentation and fragment ejection, as observed for the Jinzhou granite. However, if the proportions of high- and low-hardness rock minerals are high and the microstructure is stable, such as in the triangular microstructure of the Chifeng basalt, then microcrack initiation and propagation are inhibited, decreasing the likelihood of rock fragmentation. The triggering conditions for strainburst were further analysed by considering the source of kinetic energy. For intact rock specimens, the kinetic energy for rock fragment ejection mainly originates from the intrinsic potential energy (Wbk) released by the specimen itself; thus, strainburst triggering requires that the applied maximum principal stress exceeds the bearing capacity and that the rock Wbk exceeds zero. However, a high Wbk is needed to trigger strainburst for structural rock specimens. Based on these findings, the possibility of triggering strainburst during excavation is greater when the Wbk (>9.1 kJ/m3), rock quality designation (>75%), and in situ stress of the rock are all high.

Indication of a fast ejecta fragment in the atomic cloud interacting with the southwestern limb of SN 1006

Context. Supernova remnants interacting with molecular and atomic clouds are interesting X-ray sources for studies of broadband nonthermal emission. X-ray line emission in these systems can be produced by different processes, such as low-energy cosmic rays (LECRs) interacting with the cloud and fast ejecta fragments moving in the cloud. Aims. This paper is aimed at studying the origin of the non-thermal X-ray emission of the southwestern limb of SN 1006 beyond the main shock to determine whether the emission is due to LECRs diffusing in the cloud or to ejecta knots moving into the cloud. Methods. We analyzed the X-ray emission of the southwestern limb of SN 1006, where the remnant interacts with an atomic cloud, using three different X-ray telescopes: NuSTAR, Chandra, and XMM-Newton. We also performed a combined spectro-imaging analysis of this region. Results. Our analysis of the nonthermal X-ray emission of the southwestern limb of SN 1006 interacting with an atomic cloud has led to the detection of an extended X-ray source in the atomic cloud, approximately 2 pc upstream of the shock front. The source is characterized by a hard continuum (described by a power law with photon index Γ ∼ 1.4) and by Ne, Si, and Fe emission lines. The observed flux suggests that the origin of the X-ray emission is not associated with LECRs interacting with the cloud. On the other hand, the spectral properties of the source, together with the detection of an IR counterpart visible with Spitzer-MIPS at 24 μm, are in good agreement with the general expectations for a fast ejecta fragment moving within the atomic cloud. Conclusions. We detected X-ray and IR emission from a possible ejecta fragment, with an approximate radius of 1 × 1017 cm and approximate mass of 10−3 M⊙ at about 2 pc out of the shell of SN 1006, in the interaction region between the southwestern limb of the remnant and the atomic cloud.

Rockburst prevention by microwave destressing: a numerical investigation

Rockbursts are a complex phenomenon characterized by a violent ejection of rock fragments from the free face of deep underground mines. Due to the sudden, disruptive, and complex nature of these events, accurate predictions of rockbursts are difficult. Therefore, geotechnical intervention is imperative to prevent the occurrence of a rockburst. The scenarios of microwave destressing is introduced here as an application method to potentially prevent their occurrence. To understand the influences of microwave destressing, such as microwave exposure duration, spatial layout, and geometric parameters of the microwave preconditioned zone (MPZ), on the occurrence of a rockburst, numerical simulation was conducted based on an impact-induced rockburst model. The feasibility of microwave destressing technology was validated numerically. This paper also provides guidance for the utilization of microwave destressing in practice. Discontinuous MPZ is suitable for a known blasting source with a fixed position, and continuous MPZ can be applied for a working condition with multiple blasting sources with variable positions. A closer distance of MPZ from the free face results in a better microwave destressing effect. However, the closer distance of MPZ from the free face also introduces more negative influences on the static stability of surrounding rock.

Landing site of the Chang’e-6 lunar farside sample return mission from the Apollo basin

To address questions about the multiple lunar nearside–farside dichotomies and to provide new insights into both the early impact history of the Solar System and the geological evolution of the Moon, the Chang’e-6 (CE-6) landing zone has been selected to lie within the lunar farside South Pole–Aitken (SPA) basin in the southern part of the Apollo basin (150–158° W, 41–45° S), a site that provides access to a diversity of SPA material. Here, we describe the geomorphology, geology and chronology of three candidate sampling sites within this zone that are likely to ensure safe landing and sampling. The geological characteristics indicate that CE-6 is expected to collect lunar farside SPA ejecta fragments, possible mantle material and young (roughly 2.40 Gyr-year-old) and/or old (roughly 3.43 Gyr-year-old) basaltic material, all of which will provide important guidance for future in situ farside sample collection and deepen our understanding of the evolution of the Moon.

Dynamic mechanical properties of veined rocks under biaxial confinement

As the global demand for minerals continues to rise, mining operations deepen, and major risks arise from extreme geomechanical conditions. One risk of major concern is strainburst, a complex event characterized by the spontaneous release of energy at the limits of excavations triggered by dynamic disturbances such as blasting and induced seismicity. To study this phenomenon, we ran coupled static-dynamic tests performed under uniaxial and biaxial confinement in a Triaxial Hopkinson bar system. We used veined-rock samples from one of the largest underground mines in the world: El Teniente (Chile). Additionally, a high-speed camera was employed to study the role of dynamic loads and high strain rates (10-102 s−1) on the fracture pattern propagation. Our results show an increase in elastic modulus after increasing σ2 pre-stress. Two types of post-peak curves: I and II, were identified. While type I is related to fractures for which the sample does not lose the load capacity in a biaxial confinement state, type II is associated with the fragmentation and expulsion of fragments from the free faces of the tested samples. In addition, we observed that fracture propagation is controlled by veinlets that exhibit a favorable orientation in relation to the direction of the dynamic load. Moreover, two rockburst-related phenomena are observed: slab buckling, and rapid fragment ejection. Our findings suggest that fracture propagation occurs through the rock matrix when the mechanical properties exhibit a strong dependence on the strain rate. When this dependence is less evident, they are restricted to the veinlets. If a dynamic pulse generates deformation rates between 10 to 102 s−1, stiffness increases, and therefore energy dissipation increases as well. When combined with the predisposition of veined rocks to form blocks, these conditions create favorable environments for strainburst occurrences and may exacerbate the severity of this phenomenon in underground excavations.

Influence of water on rockburst proneness of sandstone: Insights from relative and absolute energy storage

To fully understand the influence of water on rockburst proneness from energy storage viewpoint of rock and to check the performance of energy-related rockburst proneness indexes, several groups of uniaxial compression tests at different stress levels are performed on three types of red sandstone specimens (i.e., the saturated, natural, and oven-dried specimens). The method of combining strain energy calculations and experimental observations is used to assess the rockburst proneness. The progressive failure of the three types of specimens is recorded by a high-speed (H-S) camera, and the rockburst proneness is comprehensively assessed by considering the failure features of the rock specimens (including the fracture sound, fragment ejection, fragment distribution, and far-field ejection mass ratio). The laboratory results reveal that the rockburst proneness of the natural and oven-dried sandstone specimens is similar. However, compared with the natural and oven-dried specimens, the water-saturated specimens exhibit a visible weaker rockburst proneness. It is also evident that the linear energy storage and dissipation laws are applicable to the saturated, natural, and oven-dried specimens. Most interestingly, the energy storage and energy dissipation coefficients are nearly immune to the presence of water in the red sandstone. Based on this, the definitions of absolute energy storage capacity and relative energy storage capacity are adopted to describe the influence of water on the energy storage performance of rocks. It is found that water dramatically weakens the absolute energy storage capacity, but has little and negligible effect on the relative energy storage capacity of sandstone. Considering the direct and indirect indexes, the accuracy of several representative energy-related rockburst proneness indexes is comprehensively compared in terms of the far-field ejection mass ratio, and their performance is discussed from the relative and absolute energy storage aspects. According to their performance, it is strongly recommended to use the direct index for determination of rockburst proneness when energy-related indexes are used.

Gas gun driven dynamic expansion of 3D-printed AlSi10Mg rings

This paper presents a new experimental setup to study dynamic fragmentation of metallic rings manufactured by 3D-printing technology. The ring is inserted over a ductile thin-walled tube which is impacted axially by a conical-nosed cylindrical projectile fired by a single-stage light–gas gun. The diameter of the projectile is larger than the internal diameter of the thin-walled tube, which expands as the projectile advances, pushing radially outwards the metallic ring, that eventually breaks into multiple fragments. The impact velocities considered in the present experimental campaign range from 197 m/s to 385 m/s. The AlSi10Mg ring specimens are printed by Selective Laser Melting technique, with an inner diameter of 14 mm and square cross section of 2 × 2 mm2. To obtain time-resolved information on the mechanics of fragmentation, the number of fractures, and the size of the fragments, the tests have been recorded with two-high speed cameras. A tunnel-shaped soft casing made of polymer foam is placed around the specimen to softly recover the ejected fragments, that have been weighted and sized to determine the statistics of the fragments size distribution. In addition, the predictions of the fragmentation theory of Kipp and Grady (1985) for the evolution of the number of fragments with the impact velocity have been compared with the experimental evidence, and an excellent quantitative agreement has been found within the whole range of loading rates investigated. Compared to other available techniques for dynamic fragmentation of rings, in which the expansion of the specimen is driven by electromagnetic or explosive loading, this experimental setup stands out for its simplicity, fast operation, quick assembly, and flexibility to test different engineering materials, which facilitates performing extensive experimental campaigns (34 successful tests have been carried out in this research). To the authors’ knowledge, this paper presents the most comprehensive data set to date on the fragmentation of dynamically expanded printed rings, including the first high-resolution video recordings of the formation of multiple cracks throughout the circumference of the samples, and scanning electron microscopy images of the fractures showing the porous microstructure on the cracks surface.

Mechanism of the coal bursts in the working face during mining of steeply inclined and extra thick coal seam

Coal bursts caused by mining-induced seismicity occur in the working face in steeply inclined and extra thick coal seams (SIETCSs). This article presents a case study in SIETCS through discrete element simulation as well as field investigation. The evolution of the coal burst was studied from the perspective of stress distribution, crack propagation and coal fragment ejection. The results showed that dynamic failure mainly occurred in the coal near the roof. The deep bottom coal and the top coal near the roof are under high static stress due to the clamping effect of the roof and floor and the rotation of the deep roof. The coal burst in the working face was triggered by mining-induced seismicity. The coal burst occurred when the sum of the static stress and the dynamic stress caused by the mining-induced seismicity exceeds the sum of the ultimate strength of the coal and the support strength, which was called the ‘clamping-rotation-dynamic loads’ mechanism. During the coal burst, the cracks developed rapidly into the deep bottom coal and the top coal on the roof side. The results can serve as an important reference for the coal burst failure mechanism in the working face in SIETCSs.

Characterization of true triaxial rock bursts in sandstones with different water contents

The rockburst phenomenon occurs in dry red sandstone under high in situ stress, and the rockburst effect is weaker for a water-bearing rock. The rockburst effect on red sandstone with different water contents is analyzed in this paper. A true triaxial testing machine is used to conduct the loading, and acoustic emission recording equipment and a high-speed camera are used to monitor the acoustic signal inside the rock and the rock-caving situation throughout the entire process in order to analyze the characteristics of the acoustic emissions and the ejection form of the rockburst. The results show that rockburst occurs in dry red sandstone and 50% saturated red sandstone but not in saturated red sandstone. The phrase characteristics of the stress–strain curve of the dry rock vary more significantly than those of the water-bearing rock, and the elastic strain energy inside the rock decreases gradually as the water content increases. The double peak of the acoustic emissions curve occurs during the failure process of the dry rock and gradually transitions to a stepped pattern as the water content increases. The ejected fragments of dry red sandstone during the rockburst are abundant and large. The true triaxial test results illustrate the characteristic effect of the rockburst on red sandstone with different water contents, reveal the failure mode and ejection characteristics of red sandstone with different water contents, and demonstrate the influence of the water content on the rockburst characteristics of red sandstone. The results of this study provide a theoretical reference for the study of the rockburst mechanisms of similar hard rocks.

Dissociative ionization and Coulomb explosion of CH4 in two-color asymmetric intense laser fields.

Directional fragment ejection from a tetrahedral molecule CH4 in linearly polarized two-color (ω and 2ω) asymmetric intense laser fields (50 fs, 1.4 × 1014 W cm-2, 800 nm and 400 nm) has been studied by three-dimensional ion coincidence momentum imaging. The H+ fragment produced from dissociative ionization, CH4 → H+ + CH3 + e-, is preferentially ejected on the larger amplitude side of the laser electric fields. Comparison with theoretical predictions by weak-field asymptotic theory shows that the observed asymmetry can be understood by the orientation selective tunneling ionization from the triply degenerated highest occupied molecular orbital (1t2) of CH4. A similar directional ejection of H+ was also observed for the low kinetic energy components of the two-body Coulomb explosion, CH4 → H+ + CH3+ + 2e-. On the other hand, the fragment ejection in the opposite direction were observed for the high energy component, as well as H2+ produced from the Coulomb explosion CH4 → H2+ + CH2+ + 2e-. Possible origins of the characteristic fragmentation are discussed.

Evaluation of rockburst proneness considering specimen shape by storable elastic strain energy

AbstractTo systematically assess the rockburst proneness considering specimen shape, multiple groups of laboratory tests were performed on 5 rock materials in cylindrical and cuboid shapes. The linear energy storage (LES) law of both cylindrical and cuboid rock specimens under uniaxial compressive load was confirmed, and the energy storage coefficient was found to be unrelated to specimen shape. On the basis of LES law, two rockburst proneness indexes, namely the strain energy storage index (Wet) and the potential energy of elastic strain (PES), were modified. Subsequently, the Wet, PES, peak‐strength strain energy storage index (, and peak‐strength potential energy of elastic strain (PESp) were used to assess the rockburst proneness of the cylindrical and cuboid specimens. In addition, the fragment ejection course of specimens under test was recorded by a high‐speed camera. Then, the rockburst proneness judgments obtained from the 4 indexes were compared with the qualitative data during rock destruction. The results show that, under similar stress conditions, specimen shape has an ignorable effect on the rockburst proneness as a whole. The judgment accuracy of the two modified indexes, especially that of the PESp, is favorably improved to evaluate the rockburst proneness of both cylindrical and cuboid specimens. However, misjudgment of and PESp may still occur in the assessment of rockburst proneness as these two indexes only consider the energy state before rock peak strength and the is formulated in a ratio form.

Effect of electrostatic interaction on impact breakage of agglomerates formed by charged dielectric particles.

In this paper, the prototypical process of the normal impact of dense agglomerates is investigated using the discrete element method-boundary element method coupled simulations. The agglomerate consists of 50 charged particles with the surface energy equal to 10mJ/m^{2}. The particles are assumed to be tribocharged and follow an exponential charge distribution, while the varying levels of coupled polarization are also considered. Simulation results reveal that the presence of the electrostatic interactions due to particle charging and polarization could drive more pronounced re-agglomeration after the collision, which effectively reduces the degree of agglomerate fragmentation. Moreover, when quantifying the collision outcomes using the fragmentation ratio, the influence of the electrostatic force is most significant at a moderate incident velocity. This is because, at such incident velocities, the impact is violent enough to break the agglomerate, but many ejected fragments are usually at low velocities and are attracted back by the long-range electrostatic force. Furthermore, the electrostatic force between same-sign particles even becomes attractive when particles are strongly polarized, leading to qualitative changes in particle dynamics. Finally, by comparing the collision outcomes under different incident velocities, the contact interactions are found to prevail when particles are still bounded in the agglomerate, while the electrostatic interaction becomes dominant after particles detach from each other.

Experimental study on rockburst fragment characteristic of granite under different loading rates in true triaxial condition

In order to study the effect of loading rate on rockburst, an indoor test of single-face fast unloading-three directions and five-face stress-vertical continuous loading is conducted under different loading rates using a new true-triaxial rockburst test system. The entire process of rockburst inoculation-occurrence-development is completely and accurately reproduced. The fragmentation degree, fractal dimension, and Weibull distribution characteristics of rockburst fragments under different loading rates are compared and analyzed. The results indicate that the rockburst ejection failure process can be summarized as four stages: grain ejection, rock spalling into plates, rock shearing into fragments, and rock fragment ejection. Rockburst fragments are mostly coarse-grained, medium-grained, and fine-grained. The macroscopic block characteristics of rockburst fragments reflect the fragmentation degree of rock, and the fragmentation degree of rockburst fragments increases with the loading rate. The fractal dimension shows the same trend with different loading rates, that is, the fractal dimension grows as the loading rate increases. As the loading rate increases, the crack expansion rate accelerates, which aggravates the damage to the rock specimen by decreasing its compressive strength and causing greater fragmentation when the specimen is failed. The loading rate has a significant effect on the energy consumption of rockburst fragments.

SimEx: A Tool for the Rapid Evaluation of the Effects of Explosions

The dynamic response of structural elements subjected to blast loading is a problem of growing interest in the field of defense and security. In this work, a novel computational tool for the rapid evaluation of the effects of explosions, hereafter referred to as SimEx, is presented and discussed. The classical correlations for the reference chemical (1 kg of TNT) and nuclear (106 kg of TNT) explosions, both spherical and hemispherical, are used together with the blast wave scaling laws and the International Standard Atmosphere (ISA) to compute the dynamic response of Single-Degree-of-Freedom (SDOF) systems subject to blast loading. The underlying simplifications in the analysis of the structural response follow the directives established by UFC 3-340-02 and the Protective Design Center Technical Reports of the US Army Corps of Engineers. This offers useful estimates with a low computational cost that enable in particular the computation of damage diagrams in the Charge Weight–Standoff distance (CW–S) space for the rapid screening of component (or building) damage levels. SimEx is a computer application based on Matlab and developed by the Fluid Mechanics Research Group at University Carlos III of Madrid (UC3M). It has been successfully used for both teaching and research purposes in the Degree in Security Engineering, taught to the future Guardia Civil officers at the Spanish University Center of the Civil Guard (CUGC). This dual use has allowed the development of the application well beyond its initial objective, testing on one hand the implemented capacities by undergraduate cadets with the end-user profile, and implementing new functionalities and utilities by Masters and PhD students. With this experience, the application has been continuously growing since its initial inception in 2014 both at a visual and a functional level, including new effects in the propagation of the blast waves, such as clearing and confinement, and incorporating new calculation assistants, such as those for the thermochemical analysis of explosive mixtures; crater formation; fragment mass distributions, ejection speeds and ballistic trajectories; and the statistical evaluation of damage to people due to overpressure, body projection, and fragment injuries.

Misaligned Circumbinary Disks as Efficient Progenitors of Interstellar Asteroids

Gaseous circumbinary disks (CBDs) that are highly inclined to the binary orbit are commonly observed in nature. These disks harbor particles that can reach large mutual inclinations as a result of nodal precession once the gas disk has dissipated. With n-body simulations that include fragmentation we demonstrate that misaligned disks of particles can be efficient progenitors of interstellar asteroids (ISAs). Collisions that take place between particles with large mutual inclinations have large impact velocities, which can result in mass ejection, with a wide range of fragment sizes and ejection velocities. We explore the binary parameters for which the majority of the terrestrial planet-forming material is ejected rather than accreted into planets. The misalignment required to eject significant material decreases with binary eccentricity. If the distribution of binary eccentricity is uniform and the initial particle CBD orientation relative to the binary orbit is isotropic, about 59% of binaries are more likely to eject the majority of their CBD terrestrial planet disk mass through high-velocity body–body collisions than to retain this material and build terrestrial planets. However, binary–disk interactions during the gas disk phase with nonzero disk viscosity will reduce this fraction. The composition, small size, highly elongated shape, and tumbling motion of ‘Oumuamua are consistent with ISAs generated by misaligned CBDs.

Ejecta Formation, Early Collisional Processes, and Dynamical Evolution after the DART Impact on Dimorphos

NASA’s DART spacecraft is planned to reach and impact asteroid Dimorphos, the small moon of binary asteroid (65803) Didymos, at a velocity of 6 km s−1 in late 2022 September. DART will be the first mission to test the “kinetic impactor” technique, aimed at deflecting the orbital path of a potentially hazardous asteroid. The success and effectiveness of this technique resides in the efficiency of momentum exchange between the spacecraft and the impacted target. This depends on many factors, including the cratering process, the formation of ejecta, and their fate, as they remain in the system or escape from it, carrying momentum away. Here we provide an overview of the cratering process, including ejecta formation and their subsequent dynamical evolution. We use different methodologies to model the physics of the problem, including smoothed particle hydrodynamics to model the cratering and ejecta formation process after the hypervelocity impact, N-body granular simulations to model early collisional processes between ejecta fragments right after cratering, and high-fidelity planetary propagation to model the dynamical evolution of ejecta during their purely ballistic phase. We highlight the key features of each phase and their role in defining the dynamical fate of ejecta. We investigate the effect of surface cohesion in the impacted target and identify the qualitative behavior of ejecta particles as a function of the key parameters of the problem. We provide quantitative estimates for the specific case study related to the DART–Dimorphos scenario and a selected range of target properties.

Failure and fragmentation of ceramic target with varying geometric configuration under ballistic impact

The failure and fragmentation of monolithic bare alumina 99.5% ceramic target and energy dissipation of steel 4340 projectile have been studied in a series of ballistic experiments carried out, with the incidence velocities in a range, 122–290 m/s. The velocity drop and energy dissipation increased with incidence velocity for 10 mm thick target with damage zone extended upon the whole area of rear face at higher velocities. The ballistic results obtained with the 10 mm thick target have been compared with the ballistic performance of the 5 mm thick target used in a previous study to explore the effects of target thickness on the failure mechanism. A model for the residual velocity of projectile after perforation of the single layered ceramic target has been developed based on the Lambert Jonas model by using the experimental data available for 5 mm and 10 mm thick alumina 99.5% target against 10.9 mm projectile. The residual velocities and damage patterns were reproduced with a reasonable amount of accuracy by a three-dimensional finite element model developed on commercial ABAQUS/CAE. The effect of obliquity and projectile diameter to target thickness ratio (D/T) on ballistic performance has been determined by the numerical simulation model with impact velocity in a range of 300–500 m/s. A spatial variation of ejected fragments velocity at different time steps was plotted to develop a velocity profile for the ceramic fragments coming out of the target. A semi-empirical model has been proposed for residual velocity after perforation of a monolithic ceramic target, relating to the incidence velocity and projectile diameter to target thickness ratio. The monolithic ceramic targets have been investigated for a comparative assessment of energy dissipation by the ceramic layer to eventually design an efficient front layer of a ceramic based composite armour in future studies.

Asymmetric Dissociative Tunneling Ionization of Tetrafluoromethane in ω - 2ω Intense Laser Fields.

Dissociative ionization of tetrafluoromethane (CF4) in linearly polarized ω-2ω ultrashort intense laser fields (1.4 × 1014 W/cm2, 800 and 400 nm) has been investigated by three-dimensional momentum ion imaging. The spatial distribution of produced by CF4 → + F + e− exhibited a clear asymmetry with respect to the laser polarization direction. The degree of the asymmetry varies by the relative phase of the ω and 2ω laser fields, showing that 1) the breaking of the four equivalent C-F bonds can be manipulated by the laser pulse shape and 2) the C-F bond directed along the larger amplitude side of the ω-2ω electric fields tends to be broken. Weak-field asymptotic theory (WFAT) shows that the tunneling ionization from the 4t 2 second highest-occupied molecular orbital (HOMO-1) surpasses that from the 1t 1 HOMO. This predicts the enhancement of the tunneling ionization with electric fields pointing from F to C, in the direction opposite to that observed for the asymmetric fragment ejection. Possible mechanisms involved in the asymmetric dissociative ionization, such as post-ionization interactions, are discussed.