arXiv subjects

Mikhail Garasev

Publications and source records attributed to Mikhail Garasev.

5 recordsLinked to original sources

arXiv2026

Ultra-long simulations of collisionless relativistic shocks in front-comoving frame: evidence for a steady state and its properties

We present a series of unprecedently long 2D3V PIC simulations of unmagnetized relativistic $e^{-}e^{+}$-pair shocks performed in a front-comoving frame. By implementing a moving-wall boundary condition in the downstream together with continuous injection at the upstream boundary, we maintain a fixed simulation domain size, opening the way to perform substantially longer simulations. Our longest runs extend beyond $100000\,\omega_p^{-1}$, exceeding the duration of the previously published simulations by a factor of several. Across a diverse set of simulations -- varying upstream/downstream lengths, transverse sizes, and particle-per-cell counts -- we find strong evidence that the shock approaches an asymptotic, time-independent state. In the downstream region, the steady state depends only on the upstream temperature at the injection boundary and does not depend on a particular numerical realization. The upstream precursor evolves slower and retains a dependence on the simulation's upstream length, that may be of minor observational consequence, since radiation from astrophysical shocks predominantly originates from the downstream region. We also find that Fermi-type acceleration is limited in energy and a true power-law tail never forms. Another important finding is that the downstream magnetic field has a soliton-like structure, where individual magnetic domains evolve independently, each comprising a compact, highly magnetized core embedded within an extended, weakly magnetized region. The magnetic-field distribution around the centers of these spots has approximately Lorentzian profile.

astro-ph.HE↗

arXiv2023

Turbulent multicomponent magnetopause: Analytical description and kinetic simulation of complex distributed current sheets

We carry out particle-in-cell simulations of complex current sheets of the family of analytically found Vlasov--Maxwell equilibria that model a collisionless magnetopause and allow for arbitrary energy distributions and countercurrents of various particle component. We find that, depending on the parameters, i)~a weak small-scale bending-type instability is always present at the sharp low-density plasma boundary of such a magnetopause, and ii)~the development of a small-scale Weibel-type instability inside a magnetopause is either excluded or possible. Both cases are studied and compared for two variants of the particle energy distribution~ -- Maxwellian and Kappa. The long-term stability of the whole current sheet is observed either without or with saturated Weibel-type turbulence, which can be generated in a low-magnetized region between neighboring countercurrents. We point out the applicability of the developed model of distributed current sheets and the results of analysis of their small-scale instability for the description of physical phenomena in the magnetopauses of planets and late-type stars.

physics.plasm-ph↗

arXiv2023

Electron Weibel instability and~quasi-magnetostatic structures in~an~expanding collisionless plasma

We outline transient quasi-magnetostatic phenomena associated with Weibel-type instabilities, mainly, the formation and decay of the current sheets and filaments. We consider a collisionless anisotropic plasma cloud with hot electrons expanding into an inhomogeneous background plasma taking into account an external magnetic field in different geometries entailing (i) a hot-electron spot of a circular or cylindrical form within an initial-value problem or a finite-time injection of electrons from a target surface, (ii) inhomogeneous layers of cold background plasma of different densities and spatial scales, (iii) an external magnetic field with three orientations: perpendicular to the target or along it, directed either across or parallel to a long axis of the hot-electron spot. We heed typical laser-plasma experiments. We outline development of the principle current structures linked with distinct forms of the anisotropic electron velocity distribution using particle-in-cell modeling of the instability process for diverse sets of the attributes (i)--(iii). Applications to the analysis of laboratory and space plasma problems involving an explosive development of the small-scale filamentation of the electric current and self-consistent magnetic turbulence are discussed.

physics.plasm-ph↗

arXiv2022

Multi-scale magnetic field structures in an expanding elongated plasma cloud with hot electrons subject to an external magnetic field

We carry out 3D and 2D PIC-simulations of the expansion of a magnetized plasma that initially uniformly fills a half-space and contains a semi-cylindrical region of heated electrons elongated along the surface of the plasma boundary. This geometry is related, for instance, to the ablation of a plane target by a femtosecond laser beam under quasi-cylindrical focusing. We find that the decay of the inhomogeneous plasma--vacuum discontinuity is strongly affected by an external magnetic field parallel to its boundary. We observe various transient phenomena, including the anisotropic scattering of electrons and the accompanying Weibel instability, and reveal various spatial structures of the arising magnetic field and current, including multiple flying apart filaments of a z-pinch type and slowly evolving current sheets with different orientations. The magnitude of the self-generated magnetic field can be of the order of or significantly exceed that of the external one. Such phenomena are expected in the laser and cosmic plasmas, including the explosive processes in the planetary magnetospheres and stellar coronal arches.

physics.plasm-ph↗

arXiv2016

Impact of continuous particle injection on generation and decay of the magnetic field in collisionless shocks

We present numerical simulations of the magnetic field turbulence in collisionless electron-positron plasma with continuous injection of new pairs, which maintains anisotropy in the particle distribution over long time. {With these simulations we follow evolution of a small (and therefore uniform) region in the fluid comoving frame modelling} generation and decay of the magnetic field in shocks, where the upstream is modified by two-photon pair production due to self-absorption of the shock's high-energy radiation. We find that the overall picture of magnetic field build-up is consistent with development of Weibel instability. However, the long-term injection of anisotropic pairs in the upstream leads to formation of large-scale structures in the magnetic field, while the small-scale structures are almost absent. We find that being amplified at the shock front this magnetic field mostly preserves its large spatial scale and then slowly decays in the downstream on a timescale approximately equal to duration of the injection phase. The observed decay of the magnetic field is in exceptionally good agreement with predictions of the so-called phase mixing model. Generation of the long-lived magnetic field in relativistic collisionless shocks with injection-modified upstream explains how they can efficiently produce the synchrotron radiation in Gamma-Ray Bursts.

astro-ph.HE↗