P. R. Levashov

Material decomposition mechanisms in femtosecond laser interactions with metals

A numerical hydrodynamic study of femtosecond laser ablation is presented. A detailed analysis of material decomposition is performed using a thermodynamically complete equation of state with separate stable and metastable phase states and phase boundaries. The lifetime of the metastable liquid state is estimated based on the classical theory of homogeneous nucleation. In addition, mechanical fragmentation of the target material is controlled based on available criteria. 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 and chunks. The calculation results explain available experimental findings.

Suppression of ablation in femtosecond double pulse experiments

We report the physical reasons of a curious decrease in the crater depth observed for long delays in experiments with two successive femtosecond pulses. Detailed hydrodynamic modeling demonstrates that the ablation mechanism is dumped when the delay between the pulses exceeds the electron-ion relaxation time. In this case, the interaction of the second laser pulse with the expanding target material leads to the formation of the second shock wave suppressing the rarefaction wave created by the first pulse. The evidence of this effect follows from the pressure and density profiles obtained at different delays after the first laser pulse.

Wide-range multi-phase equations of state for metals

Abstract The generalization of available experimental and theoretical information is given in the form of a multi-phase wide-range equation of state (EOS). The semiempirical EOS model accounts for solid, liquid, gas and plasma states as well as two-phase regions of melting and evaporation. Results obtained on construction, calculation of the phase diagrams and thermodynamic properties of 30 simple and refractory metals are discussed. Major attention is given to the region of the phase diagram occupied by the hot dense liquid.

Crystallization in two-component Coulomb systems

The analysis of Coulomb crystallization is extended from one-component to two-component plasmas. Critical parameters for the existence of Coulomb crystals are derived for both classical and quantum crystals. In the latter case, a critical mass ratio of the two charged components is found, which is of the order of 80. Thus, holes in semiconductors with sufficiently flat valence bands are predicted to spontaneously order into a regular lattice. Such hole crystals are intimately related to ion Coulomb crystals in white dwarf and neutron stars as well as to ion crystals produced in the laboratory. A unified phase diagram of two-component Coulomb crystals is presented and is verified by first-principles computer simulations.

Wire explosion in vacuum: Simulation of a striation appearance

This paper presents the simulation results of electrical explosion of thin Al wires at a current rise time of several tens of nanoseconds and at a current density of ∼108 A/cm2. Studies include the matter phase transfers and magnetohydrodynamic (MHD) model. A two-dimensional MHD model based on the particle-in-cell method is used to consider the formation of striations and a low-density plasma corona surrounding the wire. The striations are shown to occur through evolving overheat instabilities early in the explosion, when the conductor material is in the liquid or two-phase states. The process results from the decrease in liquid metal conductivity with increasing temperature and decreasing density.

Study of metal conductivity near the critical point using a microwire electrical explosion in water

Electrical explosion of aluminum and tungsten microwires in water was studied both experimentally and numerically. The experimental range of currents through the wire was 0.1–1 kA for explosion times of 40–300 ns and current densities up to 1.5×108 A/cm2. The experimental results were interpreted on the basis of magnetohydrodynamical simulation with various metal conductivity models. A comparison of the experimental and numerical results allows the conclusion to be drawn that the metal conductivity models used in this work are adequate.

Dynamics of thin metal foils irradiated by moderate-contrast high-intensity laser beams

Laser contrast is a crucial parameter in experiments with high-intensity high-energy pulses. For relativistic intensities of the main pulse ≳1019W/cm2, even high-contrast beams can produce plasma on the target surface due to a long nanosecond prepulse action which results in an undesirable early smearing of the target. In particular, dynamics of thin foils under the prepulse action is especially important for the laser ion acceleration technique and x-rays generation. To avoid the influence of the long laser prepulse, a thin foil can be arranged in front of the target. The analysis of the multi-stage foil dynamics is performed using a wide-range two-temperature hydrodynamic model, which correctly describes the foil expansion starting from the normal solid density at room temperature. Simulations show that varying the foil thickness, one can diminish the prepulse transmission through the foil material in many orders of magnitude and at the same time provide the total transparency of the foil plasma by the ...

Monte Carlo results for the hydrogen Hugoniot

We propose a theoretical Hugoniot relation obtained by combining results for the equation of state from the direct path integral Monte Carlo technique (DPIMC) and those from reaction ensemble Monte Carlo (REMC) simulations. The main idea of this proposal is based on the fact that the DPMIC technique provides first-principle results for a wide range of densities and temperatures including the region of partially ionized plasmas. On the other hand, for lower temperatures where the formation of molecules becomes dominant, DPIMC simulations become cumbersome and inefficient. For this region it is possible to use accurate REMC simulations where bound states (molecules) are treated on the Born-Oppenheimer level. The remaining interaction is then reduced to the scattering between neutral particles which is reliably treated classically by applying effective potentials. The resulting Hugoniot is located between the experimental values of Knudson et al. [Phys. Rev. Lett. 87, 225501 (2001)] and Collins et al. [Science 281, 1178 (1998)].

Phase transition in strongly degenerate hydrogen plasma

Direct fermionic path-integral Monte Carlo simulations of strongly coupled hydrogen are presented. Our results show evidence for the hypothetical plasma phase transition. Its most remarkable manifestation is the appearance of metallic droplets, which are predicted to be crucial for the electrical conductivity and allow one to explain the rapid increase found in recent shock compression measurements.

Correlation effects in partially ionized mass asymmetric electron-hole plasmas.

The effects of strong Coulomb correlations in dense three-dimensional electron-hole plasmas are studied by means of unbiased direct path integral Monte Carlo simulations. The formation and dissociation of bound states, such as excitons and biexcitons, is analyzed and the density-temperature region of their appearance is identified. At high density, the Mott transition to the fully ionized metallic state (electron-hole liquid) is detected. Particular attention is paid to the influence of the hole to electron mass ratio M on the properties of the plasma. Above a critical value of about M=80 formation of a hole Coulomb crystal was recently verified [Bonitz, Phys. Rev. Lett. 95, 235006 (2005)] which is supported by additional results. Results are related to the excitonic phase diagram of intermediate valent Tm[Se,Te], where large values of M have been observed experimentally.