Sarov Institute of Physics and Technology

The concept of a new form of solitary waves-super solitary waves-is proposed, specific for embracing one or several interior separatrices on their wave phase portraits. The super solitary waves of an ion-acoustic type exist, for example, in nonmagnetized plasma containing five species of charged particles. For such plasma, electrostatic potential for ion-acoustic super solitary waves is calculated. The super solitary waves can be easily identified among usual solitons, e.g., in differential circuits installed into the measuring channel.

A molecular dynamics model is used to understand the layer-by-layer etching of Si and SiO2 using fluorocarbon and Ar+ ions. In these two-step etch processes, a nanometer-scale fluorocarbon passivation layer is grown on the material’s surface using low energy CFx+ ions or radicals. The top layers of the material are then reactive ion etched by Ar+ ions utilizing the fluorocarbon already present on the material surface. By repeating these two steps, Si or SiO2 can be etched with nanometer-scale precision and the etch rate is considerably faster than what traditional atomic layer etching techniques provide. The modeling results show that fluorocarbon passivation films can be grown in a self-limiting manner on both Si and SiO2 using low energy CF2+ and CF3+ ions. The fluorocarbon passivation layer is a few angstroms thick, and its thickness increases with the fluorocarbon ion’s energy. Increasing the ion energy, however, amorphizes the top atomic layers of the material. In addition, the fluorocarbon film becomes F rich with increasing ion energy. Simulations of fluorocarbon passivated SiO2 surface show that Ar+ ions with energy below 50eV etch Si (within SiO2) in a self-limiting manner. Si etching stops once F in the fluorocarbon passivation layer is exhausted or is pushed too deep into the substrate. Oxygen within SiO2 is more easily sputtered from the material surface than Si, and the top layers of SiO2 are expected to become O deficient during Ar+ ion bombardment. Ar+ ion etching of fluorocarbon passivated Si also appears to be self-limiting below 30eV ion energy, and etching stops once F on the material surface is either consumed or becomes inaccessible.

The creation and training of a smart conversational agent has long been a dream of the human race and a huge challenge for scientists and programmers. Until recently, it was still just a dream. However, state-of-the-art technologies, like deep learning and neural networks, give language teachers a new hope, providing them with smart chatbots able to learn through communication just like humans. Today, the availability of chatbots helps to create a new educational scenario for foreign language learners. It suits well their fast-paced lives, allowing multitasking and making the work of ESL educators a lot easier and a way more effective. In that way chatbots could represent one of the basic components of microlearning.

A new class of solitary waves—supernonlinear solitons (supersolitons)—the phase trajectories of which envelop one or several inner separatrices on the wave phase portrait has been revealed. It is shown that supersolitons of the ion-acoustic type can exist in an unmagnetized plasma that contains no less than four kinds of charged particles. The conditions for the existence of supersolitons are specified. The profile of the electrostatic potential in an ion-acoustic supersoliton is determined. It is shown that a supersoliton can be easily recognized experimentally among conventional solitons by using a differentiating circuit in the measuring channel.

A new class of nonlinear waves in plasma—supernonlinear waves (SNWs) characterized by the nontrivial topology of their phase portraits—has been revealed. The topological classification of such waves is given, and suitable notation for them is proposed. It is demonstrated using several examples that SNWs can exist in the form of plasma waves of different physical nature, e.g., electrostatic (ion-acoustic) and MHD (Alfvén) waves. It is shown that a necessary condition for the existence of SNWs is the presence of at least three different charged plasma components (electrons, positrons, ions, dust grains, etc.). As the number of plasma components increases, the topology of the SNW phase portrait becomes more complicated. Typical indications of SNWs are given, which make is possible to easily reveal such waves experimentally.

Past decades significantly advanced our understanding of Rayleigh-Taylor (RT) mixing. We briefly review recent theoretical results and numerical modelling approaches and compare them with state-of-the-art experiments focusing the reader's attention on qualitative properties of RT mixing.

The emergence in SF6-Ar plasma of ion-acoustic solitons with angular profiles or profiles with several maxima has been explained. It has been shown that the cause of these profiles is that the phase trajectories of the solitons of this type in the phase portrait cover one or more separatrices, which in turn can appear only in plasma with a complex chemical composition.

A possibility of stationary solitary electrostatic waves with large amplitude in symmetric unmagnetized symmetric pair plasmas (e−e+ plasma, C60−C60+ plasma or e−h+ plasma) is proven. The main idea of the work is a thermodynamic unequilibrium of plasma species which may be created in low-density ideal pair plasmas. Ranges of parameters (Mach number M and a nonequilibrium degree τ=T+∕T−) which lead to the possibility of solitary waves are found.

A theory of ion-sound waves in a dusty electron-positron-ion plasma is developed. It is shown in the linear approximation that periodic waves exist in a bounded range of parameters. The expression for the sound velocity is derived and the dependence of the velocity on the space charge of dust particles is analyzed. In the nonlinear theory, the general exact solution is obtained, which is then analyzed using the Bernoulli pseudopotential method. Particular solutions are obtained in the form of nonlinear periodic waves, large-amplitude periodic waves (superlinear waves), and solitary compression and rarefaction waves (solitons).

The magnetization dynamics of the triangular lattice of Ising spin chains is investigated in the framework of a two-dimensional model. The rigid chains are assumed to interact with the nearest neighboring chains, an external magnetic field, and a heat reservoir that causes the chains to change their states randomly with time. A probability of a single spin-flip process is assumed in a Glauber-like form. This technique allows describing properly the steps in the magnetization curves observed in Ca$_3$Co$_2$O$_6$ and their dependence on a magnetic field sweep rate and temperature. A transition from a low-temperature to high-temperature phase is also observed.

Using experimental data on compression and heating of dense metallic plasma by powerful shock waves, we have analyzed the effect of strong Coulomb interaction on both discrete and continuum bands of energy spectrum, the role of short-range repulsion, and the effect of degeneracy on the equation of state for a dense, nonideal metallic plasma. Explosive devices have been used to produce plasma for which the degree of ionization, nonideal parameter, and degeneracy varied over wide ranges. In order to increase effects of irreversible energy dissipation, metal targets of low densities have been used. Thermodynamic measurements have been compared to theoretical models taking into account Coulomb interaction, short-range repulsion, and degeneracy of electrons. The plasma models have been shown to be applicable to the equilibrium properties of multiply ionized plasma in a wide region of the phase diagram characterized by extremely high parameters [T⩾104 K, P⩾10 GPa, and ρ=(0.1–1)ρ 0], which is beyond the traditional domain of plasma physics.

An analytical nonlinear gasdynamic theory of ion-acoustic waves in an e-p-i plasma is developed for the case in which all the plasma components in the wave undergo polytropic compression and rarefaction. An exact solution to the basic equations is found and analyzed by the Bernoulli pseudopotential method. The parameter range in which periodic waves can propagate and the range in which solitary waves (solitons) exist are determined. It is shown that the propagation velocity of a solitary is always higher than the linear ion sound velocity. The profiles of all the physical quantities in both subsonic and supersonic waves are calculated. The results obtained agree well with both the data from other papers and particular limiting cases.

This paper reviews the results of experimental research on shock-compressed porous metals conducted in laboratory conditions and underground nuclear explosion environments. The general properties of shock adiabats are discussed. A rather simple wide-range equation of state is applied to describe the totality of test data. Porous metals and silicates are comparatively studied for, and found to qualitatively differ in, their behavior over a wide pressure range (tens of GPa). A possible explanation for the nonstandard behavior of silicates is that the Gruneisen coefficient in these states of matter can assume negative values at elevated pressures and temperatures. A similar anomaly is hypothesized to account for the superadiabatic density growth in the upper mantle of Earth.

The hip bone is considered to be one of the most reliable indicators in sex determination. The aim of this study was to test the reliability of the DSP method for the hip bone proposed by Murail et al. (Bull Mem Soc Anthropol Paris, 17, 2005, 167) on a sample from a present-day population in France (52 males and 54 females). Ten linear measurements were collected from three-dimensional models derived from computed tomography images (CTI). To quantify the proportions of correct sex determinations, a more rigorous posterior probability threshold of 0.95 was applied. Using all 10 measurements, 92.3% of males and 97.2% of females were sexed correctly. The percentage of undetermined specimens varied depending on the used combination of measurements; however, all sexes were assigned with a 100% accuracy. This study proves that DSP is an appropriate and reliable tool for sex determination, based on dimensions obtained from CTI.

At present, a technique potentially capable of measuring values of Young's modulus at the nanoscale is atomic force microscopy (AFM) working in the indentation mode. However, the question if AFM indentation data can be translated into absolute values of the modulus is not well-studied as yet, in particular, for the most interesting case of stiff nanocomposite materials. Here we investigate this question. A special sample of nanocomposite material, shale rock, was used, which is relatively homogeneous at the multi-micron scale. Two AFM modes, force-volume and PeakForce QNM were used in this study. The nanoindentation technique was used as a control benchmark for the measurement of effective Young's modulus of the shale sample. The indentation rate was carefully controlled. To ensure the self-consistency of the mechanical model used to analyze AFM data, the model was modified to take into account the presence of the surface roughness. We found excellent agreement between the average values of effective Young's modulus calculated within AFM and the nanoindenter benchmark method. At the same time, the softest and hardest areas of the sample were seen only with AFM.

A new code, CFD-BWR [1], is being developed for the simulation of two-phase flow phenomena inside a BWR fuel bundle. These phenomena include coolant phase changes and multiple flow regimes which directly influence the coolant interaction with fuel assembly and, ultimately, the reactor performance. CFD-BWR is a specialized module built on the foundation of the commercial CFD code STAR-CD [2] which provides general two-phase flow modeling capabilities. New models describing the inter-phase mass, momentum, and energy transfer phenomena specific for BWRs have been developed and implemented in the CFD-BWR module. A set of experiments focused on two-phase flow and phase-change phenomena has been identified for the validation of the CFD-BWR code and results of two experiment analyses focused on the radial void distribution are presented. The close agreement between the computed results, the measured data and the correlation results provides confidence in the accuracy of the models.

Abstract Rayleigh–Taylor (RT) instability develops at the interface between two fluids of different densities accelerated against their density gradients. Intense interfacial fluid mixing ensues with time. RT mixing controls a broad range of processes in fluids, plasmas, materials, at astrophysical and at molecular scales. In this work we focus on the physics of RT mixing, which we have identified through our theoretical and experimental studies. The theory analyzes symmetries and invariants of RT dynamics and finds that RT mixing has strong correlations, weak fluctuations, and is sensitive to deterministic conditions. The experiment unambiguously observes heterogeneity, anisotropy and sensitivity to deterministic conditions of RT mixing in a broad range of setups. The theory and the experiment agree with one another, reveal that RT mixing may exhibit order and suggest new avenue for studies interfacial mixing in nature and technology.

A nonlinear differential equation describing oscillations of the chemical potential in a one-dimensional steady-state wave propagating in a degenerate electron gas against an immobile neutralizing ion background is derived, investigated, and solved exactly. It is found that the wave phase velocity is bounded below by a critical velocity, whose exact value is obtained.

We report on the possibility of the beam-plasma instability development in the system with electron beam interacting with the single-component hot electron plasma without ions. As considered system, we analyse the interaction of the low-current relativistic electron beam (REB) with squeezed state in the high-current REB formed in the relativistic magnetically insulated two-section vircator drift space. The numerical analysis is provided by means of 3D electromagnetic simulation in CST Particle Studio. We have conducted an extensive study of characteristic regimes of REB dynamics determined by the beam-plasma instability development in the absence of ions. As a result, the dependencies of instability increment and wavelength on the REB current value have been obtained. The considered process brings the new mechanism of controlled microwave amplification and generation to the device with a virtual cathode. This mechanism is similar to the action of the beam-plasma amplifiers and oscillators.

A molecular-dynamics-based model has been developed to understand etching of amorphous SiO2, with and without a fluorocarbon reactive layer, by energetic fluorocarbon (CFx+) ions. The model includes a representation of the solid and a set of interatomic potentials required for the SiO2–CFx interaction system. Two- and three-body pseudopotentials have either been obtained from published literature or computed using ab initio techniques. The Stillinger–Weber potential construct is used to represent potentials in our model and particle trajectories are advanced using the velocity-Verlet algorithm. The model is validated by comparing computed bond lengths and energies with published experimental results. Computed yield for Ar+ ion sputtering of SiO2 is also compared with published data. In the computational results described in this article, the model SiO2 test structure (with a thin fluorocarbon reactive layer) is prepared by starting with α-quartz ([001] orientation) and bombarding it with 50-eV CF2+ ions. Energetic CF2+ ions with different energies and angles of impact are then bombarded on this test structure to determine ion etch characteristics. Results show that etch yield increases with ion energy for all angles of impact. Etch yield, however, exhibits a nonlinear dependence on angle of impact with a peak around 60°. This nonlinear behavior is attributed to the balance among fraction of incident ion energy deposited in the material, ion energy deposition depth, and direction of scattering during secondary interaction events. Si in the lattice is primarily etched by F atoms and the primary Si-containing etch by-products are SiFx and SiOxFy radicals. However, oxygen either leaves the test structure as atomic O or in combination with C. While fragments of the energetic incident ion retain a substantial fraction of incident ion energy on ejection from the surface, etch by-products that have their origin in test structure atoms only have a few eV of energy on exit. Etch results are sensitive to fluorocarbon layer characteristics and etch yields decrease as the fluorocarbon reactive layer thickens.