V. D. Levchenko

A kinetic stochastic model of blistering and nanofilm islands deposition: self-organization problem

First-order phase transition at a fluctuation stage into non-linear dissipative plasma-like media is considered. The clustering of new phase germs (or nucleation) is represented by stochastic Wiener processes. Brownian motion of clusters induced by a long-range potential of indirect (through acoustic phonons and Friedels oscillation of electron density) interaction between one another is taken into account. Kinetic models for blistering materials in a controlled thermonuclear reactor and for melted metal thin film islands deposition during surface CVD modification are both put forward. The non-steady-state distribution of clusters versus their size and position in space is calculated using Ito–Stratonovich stochastic differential equations. Formation of radiation stimulated porosity layers in a lattice as well as liquid island chains on the surface are to be discussed as characteristics of phase transition at fluctuation stages as well as a new kind of self-organization phenomenon.

Charged particle acceleration by an intense wake-field excited in plasmas by either laser pulse or relativistic electron bunch

The results from theoretical and experimental studies, as well as from 2.5-dimensional (2.5-D) numerical simulation of plasma wake field excitation, by either relativistic electron bunch, laser pulse, and the charged particle wake field acceleration are discussed. The results of these investigations make it possible to evaluate the potentialities of the wake field acceleration method and to analyze whether it can serve as a basis for creating a new generation of devices capable of charged particle accelerating at substantially higher (on the order of two to three magnitudes) rates in comparison with those achievable in classical linear high-frequency (resonant) accelerators.

Phase diagram of a thin ferromagnetic film on the surface of an antiferromagnet

Magnetic characteristics of a thin ferromagnetic film on the surface of an antiferromagnet are examined. Due to the roughness of the film-substrate interface, the system is frustrated, giving rise to domain walls of new type. The distributions of the order parameters in the domain walls are studied by mathematical modeling, and the phase diagram is obtained.

“Unusual” domain walls in multilayer systems: Ferromagnet + layered antiferromagnet

The structure and conditions for the onset of a new type of domain wall in multilayer systems comprising a ferromagnet and a layered antiferromagnet is investigated by numerical simulation. Domain walls occur as the result of frustrations produced by interface roughness, i.e., by the existence of atomic steps on them. The domain walls are investigated both in a ferromagnetic film on a layered antiferromagnetic substrate and in multilayer structures. It is shown that a domain wall broadens with increasing distance from the interface; this trend is attributed to the nontrivial dependence of the wall energy on the thickness of the layer. The structure of the domain walls in multilayer ferromagnet-layered antiferromagnet systems varies dramatically as a function of the energies of interlayer and in-layer exchange interactions between adjacent layers.

Interaction of microwave radiation undergoing stochastic phase jumps with plasmas or gases

New types of beam-plasma devices generating intense stochastic microwave radiation in the interaction of electron beams with hybrid plasma waveguides were developed and put into operation at the National Science Center Kharkov Institute of Physics and Technology (Ukraine). The objective of the paper is to discuss the results of theoretical and experimental studies and numerical simulations of the normal and oblique incidence of linearly polarized electromagnetic waves on an interface between a vacuum and an overcritical plasma. The main results of the reported investigations are as follows: (i) for the parameter values under analysis, the transmission coefficient for microwaves with a stochastically jumping phase is one order of magnitude greater than that for a broadband regular electromagnetic wave with the same spectral density; (ii) the electrons are heated most efficiently by obliquely incident waves with a stochastically jumping phase and, in addition, the electron distribution function has a high-energy tail; and (iii) necessary conditions for gas breakdown and for the initiation of a microwave discharge in stochastic fields in a light source are determined. The anomalously large transmission coefficient for microwaves, the anomalous character of the breakdown conditions, the anomalous behavior of microwave gas discharges, and the anomalous nature of collisionless electron heating, are attributed to stochastic jumps in the phase of microwave radiation.

Detailed numerical simulation of shock-body interaction in 3D multicomponent flow using the RKDG numerical method and ”DiamondTorre” GPU algorithm of implementation

Interaction between a shock wave and an inhomogeneity in fluid has complicated behavior, including vortex and turbulence generating, mixing, shock wave scattering and reflection. In the present paper we deal with the numerical simulation of the considered process. The Euler equations of unsteady inviscid compressible three-dimensional flow are used into the four-equation model of multicomponent flow. These equations are discretized using the RKDG numerical method. It is implemented with the help of the DiamondTorre algorithm, so the effective GPGPU solver is obtained having outstanding computing properties. With its use we carry out several sets of numerical experiments of shock-bubble interaction problem. The bubble deformation and mixture formation is observed.

Phase Diagram of Multilayer Magnetic Structures

Multilayer ferromagnet-layered antiferromagnet (Fe/Cr) structures frustrated because of roughness of interlayer boundaries were studied by mathematical modeling methods. The phase diagram of a three-layer system (plotted as film thickness versus the degree of roughness of the interfaces) was obtained, and the order parameter distributions in each phase were determined. The character of phase transitions in this system was studied. The applicability range of the Slonczewski magnetic proximity model was determined.

High performance FDTD algorithm for GPGPU supercomputers

An implementation of FDTD method for solution of optical and other electrodynamic problems of high computational cost is described. The implementation is based on the LRnLA algorithm DiamondTorre, which is developed specifically for GPGPU hardware. The specifics of the DiamondTorre algorithms for staggered grid (Yee cell) and many-GPU devices are shown. The algorithm is implemented in the software for real physics calculation. The software performance is estimated through algorithms parameters and computer model. The real performance is tested on one GPU device, as well as on the many-GPU cluster. The performance of up to 0.65 • 1012 cell updates per second for 3D domain with 0.3 • 1012 Yee cells total is achieved.

DiamondTorre Algorithm for High-Performance Wave Modeling

Effective algorithms of physical media numerical modeling problems’ solution are discussed. The computation rate of such problems is limited by memory bandwidth if implemented with traditional algorithms. The numerical solution of the wave equation is considered. A finite difference scheme with a cross stencil and a high order of approximation is used. The DiamondTorre algorithm is constructed, with regard to the specifics of the GPGPU’s (general purpose graphical processing unit) memory hierarchy and parallelism. The advantages of these algorithms are a high level of data localization, as well as the property of asynchrony, which allows one to effectively utilize all levels of GPGPU parallelism. The computational intensity of the algorithm is greater than the one for the best traditional algorithms with stepwise synchronization. As a consequence, it becomes possible to overcome the above-mentioned limitation. The algorithm is implemented with CUDA. For the scheme with the second order of approximation, the calculation performance of 50 billion cells per second is achieved. This exceeds the result of the best traditional algorithm by a factor of five.

Implementation of the Kinetic Plasma Code with Locally Recursive non-Locally Asynchronous Algorithms

Numerical simulation is presently considered impractical for several relevant plasma kinetics problems due to limitations of computer hardware even with the use of supercomputers. To overcome the existing limitations it is suggested to develop algorithms which would effectively utilize the computer memory subsystem hierarchy to optimize the dependency graph traversal rules. The ideas for general cases of numerical simulation and implementation of such algorithms to particle-in-cell code is discussed in the paper. This approach enables the simulation of previously unaccessible for modeling problems and the execution of series of numerical experiments in reasonable time. The latter is demonstrated on a multiscale problem of the development of filamentation instability in laser interaction with overdense plasma. One variant of the simulation with parameters typical for simulations on supercomputers is performed with the use of one cluster node. The series of such experiments revealed the dependency of energy loss on incoming laser pulse amplitude to be nonmonotonic and reach over 4%, an interesting result for research of fast ignition concept.