K. V. Khishchenko

Determination of the Transport and Optical Properties of a Nonideal Solid-Density Plasma Produced by Femtosecond Laser Pulses

Experimental data on the amplitude and phase of the complex reflection coefficient of a laser pulse from a non-ideal solid-density plasma, which is produced on the surface of a metallic target by intense femtosecond laser radiation, have been obtained using femtosecond interference microscopy. A theoretical model developed for the interaction of intense femtosecond laser pulses with solid targets on the basis of the two-temperature equation of state for an irradiated substance allows the description of the dynamics of the formation and scattering of the plasma. Comparison of the experimental data with the simulation results provides new information on the transport coefficients and absorption capacity of the nonideal plasma.

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.

The Effects of Preheating of a Fine Tungsten Wire and the Polarity of a High-Voltage Electrode on the Energy Characteristics of an Electrically Exploded Wire in Vacuum

Results obtained from experimental and numerical studies of tungsten wires electrical explosion in vacuum are presented. The experiments were performed both with and without preheating of the wires using positive or negative polarity of a high-voltage electrode. Preheating is shown to increase energy deposition in the wire core due to a longer resistive heating stage. The effect was observed both in single wire and wire array experiments. The evolution of the phase state of the wire material during explosion was examined by means of a one-dimensional numerical simulation using a semiempirical wide-range equation of state describing the properties of tungsten, with allowance made for melting and vaporization

Metastable States of Liquid Tungsten Under Subsecond Wire Explosion

A numerical simulation of the initial stage of tungsten wire self-heating by a high-power microsecond current pulse was carried out. A wide-range semiempirical equation of state to account for the effects of melting and evaporation of tungsten at high temperatures was used. The metastable states were included in the process model, and the results of the simulation are in good agreement with experimental data.

Metastable States of Liquid Metal under Conditions of Electric Explosion

The paper deals with the simulation of the initial stage of tungsten wire explosion under the effect of a high-power nanosecond current pulse. The calculations involve the use of a semiempirical equation of state for tungsten, which allows for the effects of melting and evaporation at high temperatures. The laws of conservation that take into account the presence of a magnetic field and relative motion of the media are written at the liquid–vapor interface. It is demonstrated that a description fitting adequately the available experimental data is possible if one takes into account the possibility of realizing metastable states of liquid metal in the process of evolution of the system.

Two-temperature thermodynamic and kinetic properties of transition metals irradiated by femtosecond lasers

We consider the thermodynamic and kinetic properties of Nickel as an example of transition metal in two-temperature state (Te≫Ti,) produced by femtosecond laser heating. Our physical model includes essential processes induced in metals by ultrafast laser energy absorption. Specifically, the electron-ion collision frequency was obtained from recent high-temperature measurements of electrical conductivity and electron-electron screened Coulomb scattering was calculated by taking into account s-s and s-d collisions. In addition, chemical potential, energy, heat capacity, and pressure were obtained from first-principles density functional theory calculations. This model was implemented in two-temperature hydrodynamic code (2T-HD) and combined with molecular dynamics (MD) to determine strength of molten Ni at high strain rates ∼108-109s−1 under conditions of femtosecond laser ablation experiments. The simulated ablation threshold, which depends on material strength, was found to be in good agreement with our e...

Database on Shock‐Wave Experiments and Equations of State Available via Internet

The information on thermodynamic properties of matter at extremely high pressures and temperatures is very important both for fundamental researches and applications. We have collected about 14000 experimental points on shock compression, adiabatic and isobaric expansion and measurements of sound velocities behind the shock front for more than 400 substances. The database with graphical user interface containing experimental data, approximation modules, and caloric equations of state models has been worked out. One can search the information in the database in two different ways and obtain the experimental points in tabular or plain text formats directly via the Internet using common browsers. Registered users can remotely add new data into the database. It is also possible to draw the experimental points on graphs in comparison with different approximations and results of equation‐of‐state calculations. Recently we have added an ability to make calculations of shock Hugoniots, isentropes, isobars, and ot...

Strata formation at fast electrical explosion of cylindrical conductors

The process of electrical explosion of aluminum and tungsten wires at current densities above 108 A/cm2 (the fast electrical explosion regime) is investigated. Within the frame of 2D magnetohydrodynamic calculations based on the Particle-in-Cell technique with realistic equations of state of the metals, the processes of strata formation in the plasma are considered. In the fast electrical explosion regime, strata formation is shown to take place due to the overheat instability. The strata occurrence is caused by the character of the conductivity change near the critical point of the liquid-vapor phase transition, that is, by metal conductivity decrease with a temperature increase and a density decrease. To provoke strata formation, the energy deposited into the wire substance should be of about the sublimation energy.

Thermal contribution to thermodynamic functions in the Thomas-Fermi model

We propose a method of calculation of thermodynamic functions in the Thomas–Fermi model at finite temperature θ. Expressions for first and second derivatives of the free energy are analytically obtained in the framework of the model. A special treatment of thermodynamic functions at low temperatures is provided by asymptotic series expansion at θ → 0. A special algorithm is used to ensure required accuracy for all values in a wide range of volumes and temperatures. We compare the results of our computations with ideal Boltzmann and Fermi gas models.

Femtosecond optical diagnostics and hydrodynamic simulation of Ag plasma created by laser irradiation of a solid target

A combined experimental‐theoretical method of diagnostics of the plasma created on a surface of Ag target irradiated by intense femtosecond laser pulses is proposed. The method is based on semiempirical wide-range models of optical, transport and thermodynamic properties of Ag plasma. Numerical coefficients in these models are chosen so as to ensure the best accordance of simulations to measurements of a complex absorption coefficient of Ag plasma by means of femtosecond interference microscopy. A two-temperature hydrodynamic modelling of non-stationary laser-produced Ag plasma is carried out; calculated results are presented in comparison with experiments. Unexpectedly high values of the phase of the complex reflection coefficient at short (200 fs) time delay between pump and probe laser pulses are obtained experimentally; possible explanations of this phenomenon are discussed. (Some figures in this article are in colour only in the electronic version)