| 1. |
- Bret, Antoine, et al.
(författare)
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How large can the electron to proton mass ratio be in particle-in-cell simulations of unstable systems?
- 2010
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Ingår i: Physics of Plasmas. - : American Institute of Physics. - 1070-664X .- 1089-7674. ; 17:3, s. 032109-
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Tidskriftsartikel (refereegranskat)abstract
- Particle-in-cell simulations are widely used as a tool to investigate instabilities that develop between a collisionless plasma and beams of charged particles. However, even on contemporary supercomputers, it is not always possible to resolve the ion dynamics in more than one spatial dimension with such simulations. The ion mass is thus reduced below 1836 electron masses, which can affect the plasma dynamics during the initial exponential growth phase of the instability and during the subsequent nonlinear saturation. The goal of this article is to assess how far the electron to ion mass ratio can be increased, without changing qualitatively the physics. It is first demonstrated that there can be no exact similarity law, which balances a change in the mass ratio with that of another plasma parameter, leaving the physics unchanged. Restricting then the analysis to the linear phase, a criterion allowing to define a maximum ratio is explicated in terms of the hierarchy of the linear unstable modes. The criterion is applied to the case of a relativistic electron beam crossing an unmagnetized electron-ion plasma.
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| 2. |
- Bret, Antoine, et al.
(författare)
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Recent progresses in relativistic beam-plasma instability theory
- 2010
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Ingår i: Annales Geophysicae. - : Copernicus GmbH. - 0992-7689 .- 1432-0576. ; 28:11, s. 2127-2132
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Tidskriftsartikel (refereegranskat)abstract
- Beam-plasma instabilities are a key physical process in many astrophysical phenomena. Within the fireball model of Gamma ray bursts, they first mediate a relativistic collisionless shock before they produce upstream the turbulence needed for the Fermi acceleration process. While non-relativistic systems are usually governed by flow-aligned unstable modes, relativistic ones are likely to be dominated by normally or even obliquely propagating waves. After reviewing the basis of the theory, results related to the relativistic kinetic regime of the poorly-known oblique unstable modes will be presented. Relevant systems besides the well-known electron beam-plasma interaction are presented, and it is shown how the concept of modes hierarchy yields a criterion to assess the proton to electron mass ratio in Particle in cell simulations.
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| 3. |
- Dieckmann, Mark Eric, 1969-, et al.
(författare)
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Electric field generation by the electron beam filamentation instability: filament size effects
- 2010
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Ingår i: Physica Scripta. - BRISTOL, ENGLAND : IOP PUBLISHING LTD. - 0031-8949 .- 1402-4896. ; 81:1, s. 015502-
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Tidskriftsartikel (refereegranskat)abstract
- The filamentation instability (FI) of counter-propagating beams of electrons is modelled with a particle-in-cell simulation in one spatial dimension and with a high statistical plasma representation. The simulation direction is orthogonal to the beam velocity vector. Both electron beams have initially equal densities, temperatures and moduli of their non-relativistic mean velocities. The FI is electromagnetic in this case. A previous study of a small filament demonstrated that the magnetic pressure gradient force (MPGF) results in a nonlinearly driven electrostatic field. The probably small contribution of the thermal pressure gradient to the force balance implied that the electrostatic field performed undamped oscillations around a background electric field. Here, we consider larger filaments, which reach a stronger electrostatic potential when they saturate. The electron heating is enhanced and electrostatic electron phase space holes form. The competition of several smaller filaments, which grow simultaneously with the large filament, also perturbs the balance between the electrostatic and magnetic fields. The oscillations are damped but the final electric field amplitude is still determined by the MPGF.
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| 4. |
- Dieckmann, Mark Eric, 1969-, et al.
(författare)
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Expansion of a mildly relativistic hot pair cloud into an electron-proton plasma
- 2018
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Ingår i: Physics of Plasmas. - : American Institute of Physics (AIP). - 1070-664X .- 1089-7674. ; 25:6
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Tidskriftsartikel (refereegranskat)abstract
- The expansion of a charge-neutral cloud of electrons and positrons with the temperature 1 MeV into an unmagnetized ambient plasma is examined with a 2D particle-in-cell simulation. The pair outflow drives solitary waves in the ambient protons. Their bipolar electric fields attract electrons of the outflowing pair cloud and repel positrons. These fields can reflect some of the protons, thereby accelerating them to almost an MeV. Ion acoustic solitary waves are thus an efficient means to couple energy from the pair cloud to protons. The scattering of the electrons and positrons by the electric field slows down their expansion to a nonrelativistic speed. Only a dilute pair outflow reaches the expansion speed expected from the cloud's thermal speed. Its positrons are more energetic than its electrons. In time, an instability grows at the front of the dense slow-moving part of the pair cloud, which magnetizes the plasma. The instability is driven by the interaction of the outflowing positrons with the protons. These results shed light on how magnetic fields are created and ions are accelerated in pair-loaded astrophysical jets and winds.
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| 5. |
- Dieckmann, Mark Eric, 1969-, et al.
(författare)
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One-dimensional particle simulation of the filamentation instability: Electrostatic field driven by the magnetic pressure gradient force
- 2009
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Ingår i: Physics of Plasmas. - College Park, Maryland : American Institute of Physics. - 1070-664X .- 1089-7674. ; 16:7, s. 074502-1-074502-4
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Tidskriftsartikel (refereegranskat)abstract
- Two counterpropagating cool and equally dense electron beams are modeled with particle-in-cell simulations. The electron beam filamentation instability is examined in one spatial dimension, which is an approximation for a quasiplanar filament boundary. It is confirmed that the force on the electrons imposed by the electrostatic field, which develops during the nonlinear stage of the instability, oscillates around a mean value that equals the magnetic pressure gradient force. The forces acting on the electrons due to the electrostatic and the magnetic field have a similar strength. The electrostatic field reduces the confining force close to the stable equilibrium of each filament and increases it farther away, limiting the peak density. The confining time-averaged total potential permits an overlap of current filaments with an opposite flow direction.
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| 6. |
- Dieckmann, Mark Eric, 1969-, et al.
(författare)
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Particle-in-cell simulation of a fast nonrelativistic oblique shock: Extreme electron acceleration and magnetic field amplification
- 2010
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Ingår i: EUROPEAN CONFERENCE ABSTRACTS ECA. - : European Physical Society. - 2914771622 ; , s. P2.402-
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Konferensbidrag (refereegranskat)abstract
- Plasma processes close to astrophysical shocks result in the amplification of magnetic fields and in the acceleration of electrons.We examine with PIC simulations the magnetic field amplification by the collision of two plasma clouds at a speed 0.5c, each consisting of electrons and ions. A quasi-parallel guiding magnetic field, a cloud density ratio of 10 and a plasma temperature of 25 keV are considered.We demonstrate that the magnetic energy density reaches that of the ions and that electrons are accelerated to highly relativistic speeds.
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| 7. |
- Dieckmann, Mark Eric, 1969-, et al.
(författare)
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Particle-in-cell simulation of a mildly relativistic collision of an electron-ion plasma carrying a quasi-parallel magnetic field : Electron acceleration and magnetic field amplification at supernova shocks
- 2010
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Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 509:1, s. A89-
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Tidskriftsartikel (refereegranskat)abstract
- Context. Plasma processes close to supernova remnant shocks result in the amplification of magnetic fields and in the acceleration of electrons, injecting them into the diffusive acceleration mechanism. Aims. The acceleration of electrons and the magnetic field amplification by the collision of two plasma clouds, each consisting of electrons and ions, at a speed of 0.5c is investigated. A quasi-parallel guiding magnetic field, a cloud density ratio of 10 and a plasma temperature of 25 keV are considered. Methods. A relativistic and electromagnetic particle-in-cell simulation models the plasma in two spatial dimensions employing an ion-to-electron mass ratio of 400. Results. A quasi-planar shock forms at the front of the dense plasma cloud. It is mediated by a circularly left-hand polarized electromagnetic wave with an electric field component along the guiding magnetic field. Its propagation direction is close to that of the guiding field and orthogonal to the collision boundary. It has a frequency too low to be determined during the simulation time and a wavelength that equals several times the ion inertial length. These properties would be indicative of a dispersive Alfvén wave close to the ion cyclotron resonance frequency of the left-handed mode, known as the ion whistler, provided that the frequency is appropriate. However, it moves with the super-alfvénic plasma collision speed, suggesting that it is an Alfvén precursor or a nonlinear MHD wave such as a Short Large-Amplitude Magnetic Structure (SLAMS). The growth of the magnetic amplitude of this wave to values well in excess of those of the quasi-parallel guiding field and of the filamentation modes results in a quasi-perpendicular shock. We present evidence for the instability of this mode to a four wave interaction. The waves developing upstream of the dense cloud give rise to electron acceleration ahead of the collision boundary. Energy equipartition between the ions and the electrons is established at the shock and the electrons are accelerated to relativistic speeds. Conclusions. The magnetic fields in the foreshock of supernova remnant shocks can be amplified substantially and electrons can be injected into the diffusive acceleration, if strongly magnetised plasma subshells are present in the foreshock, with velocities an order of magnitude faster than the main shell.
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| 8. |
- Dieckmann, Mark Eric, 1969-, et al.
(författare)
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Simulation of a collisionless planar electrostatic shock in a proton–electron plasma with a strong initial thermal pressure change
- 2010
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Ingår i: Plasma Physics and Controlled Fusion. - Bristol : Institute of Physics and IOP Publishing Limited. - 0741-3335 .- 1361-6587. ; 52:2, s. 025001-
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Tidskriftsartikel (refereegranskat)abstract
- The localized deposition of the energy of a laser pulse, as it ablates a solid target, introduces high thermal pressure gradients in the plasma. The thermal expansion of this laser-heated plasma into the ambient medium (ionized residual gas) triggers the formation of non-linear structures in the collisionless plasma. Here an electron–proton plasma is modelled with a particle-in-cell simulation to reproduce aspects of this plasma expansion. A jump is introduced in the thermal pressure of the plasma, across which the otherwise spatially uniform temperature and density change by a factor of 100. The electrons from the hot plasma expand into the cold one and the charge imbalance drags a beam of cold electrons into the hot plasma. This double layer reduces the electron temperature gradient. The presence of the low-pressure plasma modifies the proton dynamics compared with the plasma expansion into a vacuum. The jump in the thermal pressure develops into a primary shock. The fast protons, which move from the hot into the cold plasma in the form of a beam, give rise to the formation of phase space holes in the electron and proton distributions. The proton phase space holes develop into a secondary shock that thermalizes the beam.
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| 9. |
- Dieckmann, Mark Eric, 1969-
(författare)
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The filamentation instability driven by warm electron beams: statistics and electric field generation
- 2009
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Ingår i: Plasma Physics and Controlled Fusion. - BRISTOL, ENGLAND : IOP PUBLISHING LTD. - 0741-3335 .- 1361-6587. ; 51:12, s. 124042-
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Tidskriftsartikel (refereegranskat)abstract
- The filamentation instability of counterpropagating symmetric beams of electrons is examined with 1D and 2D particle-in-cell simulations, which are oriented orthogonally to the beam velocity vector. The beams are uniform, warm and their relative speed is mildly relativistic. The dynamics of the filaments is examined in 2D and it is confirmed that their characteristic size increases linearly in time. Currents orthogonal to the beam velocity vector are driven through the magnetic and electric fields in the simulation plane. The fields are tied to the filament boundaries and the scale size of the flow aligned and the perpendicular currents are thus equal. It is confirmed that the electrostatic and the magnetic forces are equally important, when the filamentation instability saturates in 1D. Their balance is apparently the saturation mechanism of the filamentation instability for our initial conditions. The electric force is relatively weaker but not negligible in the 2D simulation, where the electron temperature is set higher to reduce the computational cost. The magnetic pressure gradient is the principal source of the electrostatic field, when and after the instability saturates in the 1D simulation and in the 2D simulation.
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| 10. |
- Lazar, Marian, et al.
(författare)
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Resonant Weibel instability in counterstreaming plasmas with temperature anisotropies
- 2010
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Ingår i: Journal of Plasma Physics. - 0022-3778 .- 1469-7807. ; 76:1, s. 49-56
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Tidskriftsartikel (refereegranskat)abstract
- The Weibel instability, driven by a plasma temperature anisotropy, is non-resonant with plasma particles: it is purely growing in time, and does not oscillate. The effect of a counterstreaming plasma is examined. In a counterstreaming plasma with an excess of transverse temperature, the Weibel instability arises along the streaming direction. Here it is proved that for large wave-numbers the instability becomes resonant with a finite real (oscillation) frequency, ωr ≠ 0. When the plasma flows faster, with a bulk velocity larger than the parallel thermal velocity, the instability becomes dominantly resonant. This new feature of the Weibel instability can be relevant for astrophysical sources of non-thermal emissions and the stability of counterflowing plasma experiments.
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