Yu. S. Sigov

Computer Simulation of Plasma Turbulence in Open Systems

A short review of the results of kinetic simulation of collective phenomena in open plasma systems with the variable total energy and number of particles, i.e., the particle and energy fluxes on boundary surfaces and/or their internal sources and channels is given. Three specific problems are considered in different detail for such systems in one-dimensional geometry: the generation and evolution of double layers in a currently unstable plasma: the collisionless relaxation of strongly non-equilibrium electron distributions; the Langmuir collapse and strong electrostatic turbulence in systems with parametric excitation of a plasma by an external pumping wave and with cooling the fast non-Maxwell electrons. In all these cases the non-linearity and a collective character of processes give examples of new dissipative plasma structures that essentially widen our idea about the nature of the plasma turbulence in non-homogeneous open systems.

“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.

Kinetic effects and reversal flows in the tokamak edge plasma

Preliminary results of the edge tokamak plasma simulation in the frame of 3D kinetic Vlasov-Fokker-Planck equations are presented. Currently two versions of the code are developed: one of them is based on a stochastic modeling method, another uses a finite difference approach via split techniques. In both versions, the self-consistent electric field may be calculated from the quasi-neutrality condition, while the sheath potential is obtained from the ambipolarity of plasma flux towards the neutralizing plate. Simplified equations, describing the dense SOL plasma parameters distributions and allowing the analytical solutions are obtained. It is shown, that for the high particle recycling in SOL, the radial scales of the temperature Δ T and densities Δ n are close to each other. The reversal flow is formed within a narrow layer near the separatrix magnetic surface.

Kinetic simulation of fields excitation and particle acceleration by laser beat wave in non-homogeneous plasmas

Resonance excitation of longitudinal plasma electrostatic wave by double-frequency laser radiation is investigated numerically to study in detail conditions of particle beat wave acceleration. The computer simulation is based on the highly specialized code SUR, using splitting technique. Both the space uniform and slightly non-uniform cases are investigated. Maintaining of phase synchronism between accelerated particles and excited longitudinal wave is provided by a choice of density plasma profile.

2.5-dimensional numerical simulation of relativistic electron bunch self-modulation in a plasma

This paper presents the results of 2.5-dimensional numerical simulation of both the modulation of long high-density relativistic electron bunches (REB) in a plasma and the excitation of wake fields by these bunches. The previous one-dimensional study has shown that the density profile modulation of a long bunch moving in plasma results in the growth of the wake wave amplitude. This is explained by the fact that the wake fields generated by microbunches being due to the evolution of the initially uniform bunch during the modulation, are coherent. The bunch modulation occurs at the plasma frequency. The present study is concerned with the REB motion, taking into account the plasma and REB nonlinearities. It is demonstrated that the radial REB dynamics exerts primary effect on both the REB self-modulation and the wake field excitation by the bunches formed.

Acceleration wake-field enhancement of excited by long relativistic electron bunch owing to self-modulation

Charged particle acceleration by charge density waves in a plasma and in uncompensated charged beams appears to be a most promising trend in the collective methods of acceleration. Here, we consider two different regimes with high amplitudes of plasma wake fields that are employed in accelerator physics. The first regime makes use of an extended short beam, then the high-amplitude waves excited by this beam and having high-gradient longitudinal electric fields can be used to accelerate other bunches. In the second case, a strong focusing can be attained with a long narrow beam, making use of its intrinsic magnetic field which is unbalanced because of space charge compensation by the plasma.

Formation by ions the plasma channel at propagation of the finite sequence of relativistic electron bunches (REB) in underdense plasmas

2.5-dimensional particle simulation results on the wake-field excitation by the programmed finite sequence of REB in underdense plasmas are presented. The REB/background plasma configuration is described by a full set of 2D3V relativistic Vlasov equations for each plasma species and Maxwell equations for self-consistent electromagnetic field. The computer simulation showed that the transversal dimension of the bunch, propagating in plasma, to vary over a wide range at our conditions when transversal and longitudinal bunch sizes are smaller than the skin-depth. In these cases significant nonlinearities in both plasma and beam behavior have been observed. The ions form the plasma channel due to their transversal motion in self-consistent fields. This phenomenon plays a considerable role in the REB propagation in plasmas.

2,5-dimensional numerical simulation of propagation of the finite sequence of relativistic electron bunches (REB) in tenuous and dense plasmas

Particle simulation results on the wake-field excitation with the programmed finite sequence of REB in tenuous and dense plasmas are presented. The REB/background plasma configuration is described by full set of 2D3V relativistic Vlasov equations for each plasma species and nonlinear Maxwell equations for self-consistent electromagnetic field. The physical parameters in runs carried out are close to those used in laboratory experiment at Kharkov IPT (Ya. Fainberg et al., 1994). The simulation points to the fact that the reached value of electric field depends on charge distribution and choice of intervals between bunches in programmed sequence.