Z. Henis

An increase of the spall strength in aluminum, copper, and Metglas at strain rates larger than 107 s−1

Measurements of the dynamic spall strength in aluminum, copper, and Metglas shocked by a high-power laser to hundreds of kilobars pressure are reported. The strain rates in these experiments are of the order of 107 s−1, which cannot be reached in impact experiments. The free-surface velocity behavior associated with spallation is characterized by oscillations caused by the reverberations of the spall layer. An optically recording velocity interferometer system was developed to measure the free-surface velocity time history. This diagnostic method has the advantages of being a noninterfering system and produces a highly accurate continuous measurement in time. The spall strength was calculated from the free-surface velocity as a function of the strain rate. The results show a rapid increase in the spall strength, suggesting that a critical phenomenon occurs at strain rates ∼107 s−1, expressed by the sudden approach to the theoretical value of the spall strength.

Spallation model for the high strain rates range

Measurements of the dynamic spall strength in aluminum and copper shocked by a high power laser to pressures of hundreds of kbars show a rapid increase in the spall strength with the strain rate at values of about 107 s−1. We suggest that this behavior is a result of a change in the spall mechanism. At low strain rates the spall is caused by the motion and coalescence of material’s initial flaws. At high strain rates there is not enough time for the flaws to move and the spall is produced by the formation and coalescence of additional cavities where the interatomic forces become dominant. Material under tensile stress is in a metastable condition and cavities of a critical radius are formed in it due to thermal fluctuations. These cavities grow due to the tension. The total volume of the voids grow until the material disintegrates at the spall plane. Simplified calculations based on this model, describing the metal as a viscous liquid, give results in fairly good agreement with the experimental data and p...

Experimental measurements of the strength of metals approaching the theoretical limit predicted by the equation of state

The approach to the ultimate strength of metals is determined experimentally. The ultimate strength of metals was calculated using a realistic wide-range equation of state. The strength of metals was measured using shock waves created in aluminum and copper foils with a short- (20–100 ps) pulse high-power laser. The strength of the materials was determined from the free-surface-velocity time history, which was measured with an optically recording velocity interferometer system. The strain rates in these experiments were in the range (1.5–5)×108 s−1.

Self-focusing Distance of Very High Power Laser Pulses.

We show numerically for continuous-wave beams and experimentally for femtosecond pulses propagating in air, that the collapse distance of intense laser beams in a bulk Kerr medium scales as 1/P;1/2 for input powers P that are moderately above the critical power for self focusing, but that at higher powers the collapse distance scales as 1/P.

Nanoparticles and nanotubes induced by femtosecond lasers

In this paper, we suggest the creation of a nanoparticles and nanotubes by using the interaction of a femtosecond laser with a solid target in a vacuum. A simple model is used to predict the optimal target and the laser parameters for the production of efficient nanoparticles. At the Soreq laboratory, experiments are performed with aluminium and carbon targets using a femtosecond laser. The irradiated targets are composed of either a thin layer of aluminium or of carbon, deposited on a transparent heat-insulating glass substrate. The nanoparticle debris is collected on a silicone wafer for X-ray diffraction (XRD), for scanning electron microscopy (SEM), and for atomic force microscopy (AFM). For transmission electron microscopy (TEM), the debris is caught on a copper grid covered on one side with a carbon membrane. Our experiments confirm the creation of crystal nanoparticles for aluminium and nanotubes for carbon experiments.

5.5-7.5 MeV Proton Generation by a Moderate-Intensity Ultrashort-Pulse Laser Interaction with H{sub 2}O Nanowire Targets

We report on the first generation of 5.5-7.5 MeV protons by a moderate-intensity short-pulse laser (∼5×10(17)  W/cm(2), 40 fsec) interacting with frozen H(2)O nanometer-size structure droplets (snow nanowires) deposited on a sapphire substrate. In this setup, the laser intensity is locally enhanced by the snow nanowire, leading to high spatial gradients. Accordingly, the nanoplasma is subject to enhanced ponderomotive potential, and confined charge separation is obtained. Electrostatic fields of extremely high intensities are produced over the short scale length, and protons are accelerated to MeV-level energies.

Efficient coupling of high intensity short laser pulses into snow clusters

Measurements of energy absorption of high intensity laser pulses in snow clusters are reported. Targets consisting of sapphire coated with snow nanoparticles were found to absorb more than 95% of the incident light compared to 50% absorption in flat sapphire targets.

Reduction of damage threshold in dielectric materials induced by negatively chirped laser pulses

The threshold fluence for laser induced damage in wide band gap dielectric materials, fused silica and MgF2, is observed to be lower by up to 20% for negatively (down) chirped pulses than for positively (up) chirped, at pulse durations ranging from 60 fs to 1 ps. This behavior of the threshold fluence for damage on the chirp direction was not observed in semiconductors (silicon and GaAs). Based on a model including electron generation in the conduction band and Joule heating, it is suggested that the decrease in the damage threshold for negatively chirped pulse is related to the dominant role of multiphoton ionization in wide gap materials.

Measurements of laser driven spallation in tin and zinc using an optical recording velocity interferometer system

Measurements of the dynamic strength, in tin and zinc, shocked by a high power pulsed laser to tens of kilobars pressures are reported. The strain rates in these experiments are of the order of 107 s−1, higher by two-to-three orders of magnitude than those reached with conventional shock generators like plane impacts or explosives. The free surface velocity time history, which is related to the spallation process, was measured with an optical recording velocity interferometer system. This diagnostic technique is noninterfering and provides a highly accurate continuous measurement in time. The spall strength estimated from the free surface velocity profile was compared with the theoretical upper limit for the spall strength, calculated from a wide range equation of state for metals.

Extended lifetime of high density plasma filament generated by a dual femtosecond?nanosecond laser pulse in air

A substantially extended lifetime of a high-density plasma channel generated in the wake of an intense femtosecond pulse propagating in air is experimentally demonstrated. Free electron density above 1015 cm−3 in the formed plasma filament is measured to be sustained for more than 30 ns. This high-density plasma lifetime prolongation of more than one order of magnitude is achieved by properly timed irradiation of the filament with a relatively low-intensity nanosecond laser pulse, in comparison with a filament without such irradiation. The experimental results are in good agreement with our theoretical model that follows the evolution of the temperature and density of various molecules, atoms, and ion species. The results point to the possibility of generating extremely long time duration, stable high-density plasma filaments in air.