| 1. |
- Dieckmann, Mark E, 1969-, et al.
(författare)
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Aspects of self-similar current distributions resulting from the plasma filamentation instability
- 2007
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Ingår i: New Journal of Physics. - : IOP Publishing. - 1367-2630. ; 9, s. 10-1-10-22
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Tidskriftsartikel (refereegranskat)abstract
- Colliding plasmas can form current filaments that are magnetically confined and interact through electromagnetic fields during the nonlinear evolution of this filamentation instability. A nonrelativistic and a relativistic electron flow are examined. Two-dimensional (2D) particle-in-cell (PIC) simulations evolve the instability in a plane orthogonal to the flow vector and confirm that the current filaments move, merge through magnetic reconnection and evolve into current sheets and large flux tubes. The current filaments overlap over limited spatial intervals. Electrons accelerate in the overlap region and their final energy distribution decreases faster than exponential. The spatial power spectrum of the filaments in the flow-aligned current component can be approximated by a power-law during the linear growth phase. This may reflect a phase transition. The power spectrum of the current component perpendicular to the flow direction shows a power-law also during the nonlinear phase, possibly due to preferential attachment. The power-law distributed power spectra evidence self-similarity over a limited scale size and the wavenumber of the maximum of the spatial power spectrum of the filament distribution decreases linearly in time. Power-law correlations of velocity fields, which could be connected to the current filaments, may imply super-diffusion.
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| 2. |
- Dieckmann, Mark E, 1969-, et al.
(författare)
-
Aspects of self-similarity of the filamentation instability
- 2007
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Ingår i: 34th European Physical Society Conference on Plasma Physics,2007. - Warsaw : European Physical Society. ; , s. P2.080-
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Konferensbidrag (refereegranskat)abstract
- The filamentation instability (FI) is an aperiodically growing instability driven by counterpropagating electron beams. Its ability to generate magnetic fields is important for the energetic plasmas in gamma ray burst jets and inertial confinement fusion plasmas. The FI has been examined both analytically and with particle-in-cell (PIC) simulations. We perform PIC simulations and follow the FI through its nonlinear saturation. The power spectrum of the flow-aligned current component is self-similar during the linear phase. We show that the perpendicular current distribution is self-similar during the nonlinear evolution and that the filament size increases linearly with time. We demonstrate that, at least for warm plasmas, the current filaments can't be described by simple flux tubes. Instead, the filaments merge by magnetic reconnection to form larger, partially overlapping current sheets. In the filament overlap region the electrons are accelerated.
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| 3. |
- Dieckmann, Mark E, 1969-, et al.
(författare)
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Particle-in-cell simulation studies of the non-linear evolution of ultrarelativistic two-stream instabilities
- 2006
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Ingår i: Monthly notices of the Royal Astronomical Society. - : Oxford University Press (OUP). - 0035-8711 .- 1365-2966. ; 367:3, s. 1072-1082
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Tidskriftsartikel (refereegranskat)abstract
- Gamma-ray bursts are associated with relativistic plasma flow and intense X-ray and soft gamma-ray emissions. We perform particle-in-cell simulations to explore the growth and saturation of waves driven by the electrostatic two-stream instability that may contribute to the thermalization of the relativistic plasma flows and to the electromagnetic emissions. We evolve self-consistently the instability driven by two charge-neutral and cool interpenetrating beams of electrons and protons that move at a relative Lorentz factor of 100. We perform three simulations with the beam density ratios of 1, 2 and 10. The simulations show that the electrostatic waves saturate by trapping the electrons of only one beam and that the saturated electrostatic wave fields spatially modulate the mean momentum of the second beam, while retaining its temperature. Cavities form in the charge density of the latter beam which, in turn, compress the electrostatic waves to higher intensities. A runaway process develops that terminates with the collapse of the waves and the development of an exponential electron high-energy tail. We bring forward evidence that this energetic tail interacts stochastically with the charge density fluctuations of the relativistic proton beam. In response, an electron momentum distribution develops that follows an inverse power law up to a spectral break at four times the beam Lorentz factor.
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| 4. |
- Dieckmann, Mark E, 1969-, et al.
(författare)
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The formation of a relativistic partially electromagnetic planar plasma shock
- 2008
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Ingår i: Astrophysical Journal. - : American Astronomical Society. - 0004-637X .- 1538-4357. ; 675:1, s. 586-595
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Tidskriftsartikel (refereegranskat)abstract
- Relativistically colliding plasma is modeled by particle-in-cell simulations in one and two spatial dimensions, with an ion-to-electron mass ratio of 400 and a temperature of 100 keV. The energy of an initial quasi-parallel magnetic field is 1% of the plasma kinetic energy. Energy dissipation by a growing wave pulse of mixed polarity, probably an oblique whistler wave, and different densities of the colliding plasma slabs result in the formation of an energetic electromagnetic structure within milliseconds. The structure, which develops for an initial collision speed of 0.9c, accelerates electrons to Lorentz factors of several hundred. A downstream region forms, separating the forward and reverse shocks. In this region, the plasma approaches an energy equipartition between electrons, ions, and the magnetic field. The electron energy spectrum resembles a power law at high energies, with an exponent close to −2.7, or . The magnetic field reflects upstream ions, which form a beam and drag the electrons along to preserve the plasma quasineutrality. The forward and reverse shocks are asymmetric due to the unequal slab densities. The forward shock may be representative for the internal shocks of gamma-ray bursts.
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| 5. |
- Dieckmann, Mark E, 1969-, et al.
(författare)
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The formation of a relativistic planar plasma shock
- 2008
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Ingår i: 35th EPS Conference on Plasma Physics,2008. - Switzerland : Europhysics Conference Abstracts. ; , s. O5.067-
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Konferensbidrag (refereegranskat)abstract
- The shock is considered that develops, when two plasma clouds collide at the speed 0.9 c. Initially, an almost flow-aligned magnetic field is introduced, which decreases the growth rates of the oblique mixed mode instability and of the filamentation instability. A 2D PIC simulation demonstrates that a planar, electromagnetic wave structure is growing that amplifies the magnetic field component orthogonal to the flow velocity vector. The formation of the forward and reverse shocks is followed with a 1D PIC simulation and it is shown that an energy equi-partition is established downstream between the ions, the electrons and the magnetic field.
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| 6. |
- Dieckmann, Mark E, 1969-, et al.
(författare)
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Two-dimensional PIC simulations of ion beam instabilities in Supernova-driven plasma flows
- 2008
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Ingår i: Plasma Physics and Controlled Fusion. - : IOP Publishing. - 0741-3335 .- 1361-6587. ; 50, s. 065020-1-14
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Tidskriftsartikel (refereegranskat)abstract
- Supernova remnant blast shells can reach the flow speed vs = 0.1c and shocks form at its front. Instabilities driven by shock-reflected ion beams heat the plasma in the foreshock, which may inject particles into diffusive acceleration. The ion beams can have the speed vb vs. For vb vs the Buneman or upper-hybrid instabilities dominate, while for vb vs the filamentation and mixed modes grow faster. Here the relevant waves for vb vs are examined and how they interact nonlinearly with the particles. The collision of two plasma clouds at the speed vs is modelled with particle-in-cell simulations, which convect with them magnetic fields oriented perpendicular to their flow velocity vector. One simulation models equally dense clouds and the other one uses a density ratio of 2. Both simulations show upper-hybrid waves that are planar over large spatial intervals and that accelerate electrons to ~10 keV. The symmetric collision yields only short oscillatory wave pulses, while the asymmetric collision also produces large-scale electric fields, probably through a magnetic pressure gradient. The large-scale fields destroy the electron phase space holes and they accelerate the ions, which facilitates the formation of a precursor shock.
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