Phase change and stress wave in picosecond laser–material interaction with shock wave formation

When background gas is present in pulsed laser–material interaction, a shock wave down to the nanoscale will emerge. The background gas will affect the phase change and explosion in the target. This study is focused on the void dynamics and stress wave in a model material (argon crystal) under picosecond pulsed laser irradiation. Our results show that existence of ambient gas and the shock wave significantly suppresses the void formation and their lifetime. Void dynamics, including their growing rate, lifetime, and size under the influence of ambient gas are studied in detail. All the voids undergo an accelerating and decelerating process in the growth. The collapsing process is almost symmetrical to the growing process. Higher laser fluence is found to induce an obvious foamy structure. Stress wave formation and propagation, temperature contour, and target and gas atom number densities are studied to reveal the underlying physical processes. Although the interaction of the plume with ambient gas significantly suppresses the void formation and phase explosion, no obvious effect is found on the stress wave within the target. Very interestingly, secondary stress waves resulting from re-deposition of ablated atoms and void collapse are observed, although their magnitude is much smaller than the directly laser-induced stress wave.     

Dynamics of nonautonomous rogue waves in Bose–Einstein condensate

We study rogue waves of Bose–Einstein condensate (BEC) analytically in a time-dependent harmonic trap with a complex potential. Properties of the nonautonomous rogue waves are investigated analytically. It is reported that there are possibilities to ‘catch’ rogue waves through manipulating nonlinear interaction properly. The results provide many possibilities to manipulate rogue waves experimentally in a BEC system.     

Studies on laser–plasma interaction physics for shock ignition

We realized a series of experiments to study the physics of laser–plasma interaction in an intensity regime of interest for the novel “Shock Ignition” approach to Inertial Fusion. Experiments were performed at the Prague Asterix Laser System laser in Prague using two laser beams: an “auxiliary” beam, for pre-plasma creation, with intensity around 7?×?10¹³?W/cm² (250?ps, 1ω, λ?=?1315?nm) and the “main” beam, up to 10¹⁶?W/cm (250?ps, 3ω, λ?=?438?nm), to launch a shock. The main goal of these experiments is to study the process of the formation of a very strong shock and the influence of hot electrons in the generation of very high pressures. The shock produced by the ablation of the plastic layer is studied by shock breakout chronometry. The generation of hot electrons is analyzed by imaging Kα emission.     

Gene–culture shock waves

A hyperbolic model is presented which generalises Aoki?s parabolic system for the combined propagation of a mutant gene together with a cultural innovation. It is shown that this model allows for the propagation of a shock wave and the shock amplitude is calculated numerically. Particular attention is paid to the case where the shock moves into a region where the frequencies of the mutant gene and of the individuals adopting the innovation are zero.     

Combustion waves in composite solid material of shell–core type

In this article, an asymptotic and numerical analysis of combustion wave propagation in shell–core composite solid energetic material is undertaken based on the diffusional–thermal model with an overall Arrhenius reaction step. Flame speed and structure are found for a broad range of parameter values. Two different regimes of flame propagation are identified. In the weak recuperation regime, the temperatures of the shell and core are monotonic functions of the coordinates, and they differ only slightly in the reaction zone of the flame. In the strong recuperation regime, the temperature of the shell is significantly higher than that of the core and has a sharp peak in the reaction zone with the maximum value exceeding the adiabatic flame temperature for pure energetic material. It is found that the highest level of flame acceleration in the composite material can be attained in the strong recuperation regime. The competition of these flame propagation regimes may lead to the coexistence of multiple combustion waves travelling with different velocities. The stability is investigated of combustion waves in the practically important strong recuperation regime.     

Dynamics of flow–soot interaction in wrinkled non-premixed ethylene–air flames

A two-dimensional simulation of a non-premixed ethylene–air flame was conducted by employing a detailed gas-phase reaction mechanism considering polycyclic aromatic hydrocarbons, an aerosol-dynamics-based soot model using a method of moments with interpolative closure, and a grey gas and soot radiation model using the discrete transfer method. Interaction of the sooting flame with a prescribed decaying random velocity field was investigated, with a primary interest in the effects of velocity fluctuations on the flame structure and the associated soot formation process for a fuel-strip configuration and a composition with mature soot growth. The temporally evolving simulation revealed a multi-layered soot formation process within the flame, at a level of detail not properly described by previous studies based on simplified soot models utilizing acetylene or naphthalene precursors for initial soot inception. The overall effect of the flame topology on the soot formation was found to be consistent with previous experimental studies, while a unique behaviour of localised strong oxidation was also noted. The imposed velocity fluctuations led to an increase of the scalar dissipation rate in the sooting zone, causing a net suppression in the soot production rate. Considering the complex structure of the soot formation layer, the effects of the imposed fluctuations vary depending on the individual soot reactions. For the conditions under study, the soot oxidation reaction was identified as the most sensitive to the fluctuations and was mainly responsible for the local suppression of the net soot production.     

Magnetosonic shock waves in dense astrophysical electron–positron–ion plasmas

Multifluid quantum magnetohydrodynamic model (QMHD) is used to investigate small but finite amplitude magnetosonic shock waves in dense) electron–positron–ion (e–p–i) plasmas. The Korteweg–de Vries–Burgers (KdVB) equation is derived by using reductive perturbation method. It is noticed that variations in the positron density modify the profile of magnetosonic shocks in dense e–p–i plasmas significantly. The numerical results are also presented by taking into account the dense plasma parameters from published literature of astrophysical conditions, in compact stars.     

Numerical modeling of laser–matter interaction in the region of “low” laser parameters

The interaction of laser radiation with matter leads to the certain kinds of modelling of its surface or volume. These effects have been demonstrated for a lot of materials, even causing the formation of new scientific and industrial domain, which is undoubtedly laser material processing and as well as laser cleaning of artworks. Those applications lie in the so-called “low' region of laser energy densities, represented for short laser pulses by power densities below 10⁹ W/cm². Paper presents set of equations describing in one-dimensional (1D) model phenomena accompanying to laser–matter interaction. Target geometry includes two and four layers of different materials, irradiated by ns laser pulses. Effects of radiation absorption and transport, heat conductivity, target transit to plastic state, melting and evaporation are taken into consideration. The part of the paper is devoted to the discussion of numerical results, selected in such a way to illustrate the phenomenon of radiation interaction with materials as well as to show, in whole, possibilities of computer simulation methods.     

Formation of ring-patterned nanoclusters by laser–plume interaction

Journal of Nanoparticle Research - Nanocrystalline SnS powder has been prepared using tin chloride (SnCl2) as a tin ion source and sodium sulfide (Na2S) as a sulfur ion source with the help of...     

Surface and volume waves in ferrodielectric–magnetically active material structures

The propagation of surface waves along the interface between a ferrodielectric and a material that represents an array of amorphous ferromagnetic microwires has been studied. The dispersion characteristic of a magnetically controlled TE wave has been obtained, and frequency intervals have been found in which surface or localized, partially localized, and volume waves may exist. It has been shown that a TM wave in the given structure cannot be a surface wave.     

Spatial and temporal study of shock waves generated by laser ablation for Ti target

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Shape of ion energy spectra in ultra-short and intense laser–matter interaction

Structured proton spectra and a two-temperature electron distribution were simulated using a 1D3V particle-in-cell code for a plasma created by 35-fs laser pulses with an intensity of I10¹⁹ W/cm². Such a two-temperature electron distribution was used as a prerequisite for a fluid dynamical model allowing us to describe the experimentally measured deep dips in the proton energy spectra of laser-produced plasmas from water-droplet targets. PACS 52.50.Jm; 52.38.Kd; 41.75.Jv     

Formation of linear momentum in a rod during a laser pulse–matter interaction

Recently, we developed an optodynamic experimental technique that makes it possible to measure the linear momentum obtained by a metal target sample in the shape of a rod during a nanosecond laser pulse interaction in the ablative regime. The height of the rod’s rear end axial step-like displacement, caused by the first reflection of the laser-generated ultrasonic wave, is proportional to the linear momentum acquired by the rod. In comparison with commonly used ballistic methods, we can determine the acquired momentum on a much shorter time scale corresponding to the wave transition time, from the front to the rear end of the rod. Using this method we investigated the ambient air pressure dependence on the formation of linear momentum over a laser intensity range, from the ablation threshold to values about ten times higher. Steel rods of various diameters were used to demonstrate the effect of an expanding blast wave which delivers additional momentum to the target, when the laser beam on the target surface is smaller than the target itself. The typical value of the acquired target momentum is on the order of μN s and 10 μN s/J for the momentum coupling coefficient.     

Generation of femtosecond X-ray pulses via laser–electron beam interaction

The generation of femtosecond X-ray pulses will have important scientific applications by enabling the direct measurement of atomic motion and structural dynamics in condensed matter on the fundamental time scale of a vibrational period. Interaction of femtosecond laser pulses with relativistic electron beams is an effective approach to generating femtosecond pulses of X-rays. In this paper we present recent results from proof-of-principle experiments in which 300 fs pulses are generated from a synchrotron storage ring by using an ultrashort optical pulse to create femtosecond time structure on the stored electron bunch. A previously demonstrated approach for generating femtosecond X-rays via Thomson scattering between terawatt laser pulses and relativistic electrons is reviewed and compared with storage-ring based schemes.     

Interactions and asymptotics of dispersive shock waves – Korteweg–de Vries equation

The long-time asymptotic solution of the Korteweg–de Vries equation for general, step-like initial data is analyzed. Each sub-step in well-separated, multi-step data forms its own single dispersive shock wave (DSW); at intermediate times these DSWs interact and develop multiphase dynamics. Using the inverse scattering transform and matched-asymptotic analysis it is shown that the DSWs merge to form a single-phase DSW, which is the ‘largest’ one possible for the boundary data. This is similar to interacting viscous shock waves (VSW) that are modeled with Burgers? equation, where only the single, largest-possible VSW remains after a long time.     

Role of finite Larmor radius in chaotic regime of waves—particle interaction

The paper presents the discussion of the interaction of two electrostatic waves, propagating in a homogeneous magnetized plasma, with ions. The same case for two and three waves was already discussed in the paper of Misguich and Nakach [Phys. Rev. A, Vol. 44 (1991) 3869]. Contrary to their paper, we solve the dynamics not in the drift approximation, but in a full solution, using the Hamiltonian, which includes also a cyclotron motion of particles. We show that in a realistic regime, the full solution can cause stochastization of the dynamics in cases, when the drift approximation permits only regular dynamics. This work was supported by the Grant Agency of the Academy of Sciences of the Czech Republic, project No. A1043201.     

Dynamics of Coulomb explosion of hydrogen clusters in a high-intensity femtosecond laser pulse

Using Bethe model, the dynamics of the ionization and Coulomb explosion of hydrogen clusters (0.5-5 nm) in high-intensity (1015 -1017 W/cm2) femtosecond laser pulses have been studied theoretically, and the dependence of energy of protons emitted from exploding clusters on cluster size and laser intensity has been investigated. It is found that the maximum proton energy increases exponentially with the cluster size, and the exponent is mainly determined by the laser intensity. For a given cluster size, the maximum proton energy increases with increasing laser intensity and gets saturation gradually. The calculation results are in agreement with the recent experimental observation.     

Study on the effect of laser parameters on wakefield excitation in femtosecond pulsed laser–plasma interaction using PIC method

Propagation of an intense short laser pulse through under-dense plasma can produce huge amplitude plasma wake field. A 3D particle in cell (PIC) method was used to simulate the wakefield generation for different laser parameters such as intensity, pulse duration, spot size and temporal pulse shape. Our study shows that the amplitude of wakefield is increased with laser intensity, but it is decreased with spot size. The results for pulse shape and pulse duration depend on their optimum values.