| 57001. |
- Dieckmann, Mark Eric, et al.
(författare)
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Magnetic instability in a dilute circular rarefaction wave
- 2012
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Ingår i: Physics of Plasmas. - : American Institute of Physics (AIP). - 1070-664X .- 1089-7674. ; 19:12, s. 122102-1-122102-7
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Tidskriftsartikel (refereegranskat)abstract
- The growth of magnetic fields in the density gradient of a rarefaction wave has been observed in simulations and in laboratory experiments. The thermal anisotropy of the electrons, which gives rise to the magnetic instability, is maintained by the ambipolar electric field. This simple mechanism could be important for the magnetic field amplification in astrophysical jets or in the interstellar medium ahead of supernova remnant shocks. The acceleration of protons and the generation of a magnetic field by the rarefaction wave, which is fed by an expanding circular plasma cloud, is examined here in form of a 2D particle-in-cell simulation. The core of the plasma cloud is modeled by immobile charges, and the mobile protons form a small ring close to the cloud's surface. The number density of mobile protons is thus less than that of the electrons. The protons of the rarefaction wave are accelerated to 1/10 of the electron thermal speed, and the acceleration results in a thermal anisotropy of the electron distribution in the entire plasma cloud. The instability in the rarefaction wave is outrun by a TM wave, which grows in the dense core distribution, and its magnetic field expands into the rarefaction wave. This expansion drives a secondary TE wave.
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| 57002. |
- Dieckmann, Mark Eric, et al.
(författare)
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Modification of the formation of high-Mach number electrostatic shock-like structures by the ion acoustic instability
- 2013
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Ingår i: Physics of Plasmas. - : American Institute of Physics (AIP). - 1070-664X .- 1089-7674. ; 20:10, s. 102112-1-102112-12
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Tidskriftsartikel (refereegranskat)abstract
- The formation of unmagnetized electrostatic shock-like structures with a high Mach number is examined with one-and two-dimensional particle-in-cell (PIC) simulations. The structures are generated through the collision of two identical plasma clouds, which consist of equally hot electrons and ions with a mass ratio of 250. The Mach number of the collision speed with respect to the initial ion acoustic speed of the plasma is set to 4.6. This high Mach number delays the formation of such structures by tens of inverse ion plasma frequencies. A pair of stable shock-like structures is observed after this time in the 1D simulation, which gradually evolves into electrostatic shocks. The ion acoustic instability, which can develop in the 2D simulation but not in the 1D one, competes with the nonlinear process that gives rise to these structures. The oblique ion acoustic waves fragment their electric field. The transition layer, across which the bulk of the ions change their speed, widens and their speed change is reduced. Double layer-shock hybrid structures develop.
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| 57003. |
- Dieckmann, Mark E, 1969-, et al.
(författare)
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On the ultrarelativistic two-stream instability, electrostatic turbulence and Brownian motion
- 2006
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Ingår i: New Journal of Physics. - Bristor : Institute of Physics (IOP). - 1367-2630. ; 8
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Tidskriftsartikel (refereegranskat)abstract
- Experimental evidence indicates that bulk plasma flow at ultrarelativistic speeds is common in astrophysical settings, e. g. the collimated jets of active galactic nuclei and gamma ray bursts. The low-plasma density of such flows implies their collisionless relaxation by means of wave-particle interactions. Such processes are not well understood in the ultrarelativistic regime. The thermalization of two interpenetrating equally dense electron-proton (e(-)p) beams in the absence of a magnetic field is examined here by means of 1.5D particle-in-cell simulations. The relative beam speeds correspond to Lorentz factors in the range 200-1000. The constraint to one spatial simulation dimension, which is aligned with the beam velocity vectors, implies that only the two-stream (TS) instability and the Weibel-type instability can grow, while filamentation instabilities are excluded. With this constraint and for our plasma parameters, the TS instability dominates. The electrostatic waves grow, saturate by the trapping of electrons, and collapse. The interaction of the electrons with the electric fields after the wave collapse represents a relativistic Wiener process. In response, the electrons are rapidly thermalized. The final electron distribution can be interpreted as a relativistic Maxwellian distribution with a high-energy tail arising from ultrarelativistic phase space holes.
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| 57004. |
- 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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| 57005. |
- Dieckmann, Mark E, 1969-, et al.
(författare)
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One-dimensional thermal pressure-driven expansion of a pair cloud into an electron-proton plasma
- 2018
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Ingår i: Physics of Plasmas. - Melville, NY, United States : A I P Publishing LLC. - 1070-664X .- 1089-7674. ; 25:5
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Tidskriftsartikel (refereegranskat)abstract
- Recently, a filamentation instability was observed when a laser-generated pair cloud interacted with an ambient plasma. The magnetic field it drove was strong enough to magnetize and accelerate the ambient electrons. It is of interest to determine if and how pair cloud-driven instabilities can accelerate ions in the laboratory or in astrophysical plasma. For this purpose, the expansion of a localized pair cloud with the temperature 400 keV into a cooler ambient electron-proton plasma is studied by means of one-dimensional particle-in-cell simulations. The cloud's expansion triggers the formation of electron phase space holes that accelerate some protons to MeV energies. Forthcoming lasers might provide the energy needed to create a cloud that can accelerate protons.
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| 57006. |
- 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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| 57007. |
- 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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| 57008. |
- Dieckmann, Mark E, et al.
(författare)
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Particle-in-cell simulation study of a lower-hybrid shock
- 2016
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Ingår i: Physics of Plasmas. - : AMER INST PHYSICS. - 1070-664X .- 1089-7674. ; 23:6, s. 062111-
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Tidskriftsartikel (refereegranskat)abstract
- The expansion of a magnetized high-pressure plasma into a low-pressure ambient medium is examined with particle-in-cell simulations. The magnetic field points perpendicular to the plasmas expansion direction and binary collisions between particles are absent. The expanding plasma steepens into a quasi-electrostatic shock that is sustained by the lower-hybrid (LH) wave. The ambipolar electric field points in the expansion direction and it induces together with the background magnetic field a fast E cross B drift of electrons. The drifting electrons modify the background magnetic field, resulting in its pile-up by the LH shock. The magnetic pressure gradient force accelerates the ambient ions ahead of the LH shock, reducing the relative velocity between the ambient plasma and the LH shock to about the phase speed of the shocked LH wave, transforming the LH shock into a nonlinear LH wave. The oscillations of the electrostatic potential have a larger amplitude and wavelength in the magnetized plasma than in an unmagnetized one with otherwise identical conditions. The energy loss to the drifting electrons leads to a noticeable slowdown of the LH shock compared to that in an unmagnetized plasma. Published by AIP Publishing.
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| 57009. |
- Dieckmann, Mark Eric, et al.
(författare)
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PIC simulations of stable surface waves on a subcritical fast magnetosonic shock front
- 2023
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Ingår i: Physica Scripta. - : IOP Publishing Ltd. - 0031-8949 .- 1402-4896. ; 98:9
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Tidskriftsartikel (refereegranskat)abstract
- We study with particle-in-cell (PIC) simulations the stability of fast magnetosonic shocks. They expand across a collisionless plasma and an orthogonal magnetic field that is aligned with one of the directions resolved by the 2D simulations. The shock speed is 1.6 times the fast magnetosonic speed when it enters a layer with a reduced density of mobile ions, which decreases the shock speed by up to 15% in 1D simulations. In the 2D simulations, the density of mobile ions in the layer varies sinusoidally perpendicularly to the shock normal. We resolve one sine period. This variation only leads to small changes in the shock speed evidencing a restoring force that opposes a shock deformation. As the shock propagates through the layer, the ion density becomes increasingly spatially modulated along the shock front and the magnetic field bulges out where the mobile ion density is lowest. The perturbed shock eventually reaches a steady state. Once it leaves the layer, the perturbations of the ion density and magnetic field oscillate along its front at a frequency close to the lower-hybrid frequency; the shock is mediated by a standing wave composed of obliquely propagating lower-hybrid waves. We perform three 2D simulations with different box lengths along the shock front. The shock front oscillations are aperiodically damped in the smallest box with the fastest variation of the ion density, strongly damped in the intermediate one, and weakly damped in the largest box. The shock front oscillations perturb the magnetic field in a spatial interval that extends by several electron skin depths upstream and downstream of the shock front and could give rise to Whistler waves that propagate along the shock's magnetic field overshoot. Similar waves were observed in hybrid and PIC simulations and by the MMS satellite mission.
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| 57010. |
- Dieckmann, Mark E, 1969-, et al.
(författare)
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Preferential acceleration of positrons by a filamentation instability between an electron–proton beam and a pair plasma beam
- 2020
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Ingår i: Physics of Plasmas. - : American Institute of Physics (AIP). - 1070-664X .- 1089-7674. ; 27:12
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Tidskriftsartikel (refereegranskat)abstract
- Particle-in-cell simulations of jets of electrons and positrons in an ambient electron–proton plasma have revealed an acceleration of positrons at the expense of electron kinetic energy. We show that a filamentation instability, between an unmagnetized ambient electron–proton plasma at rest and a beam of pair plasma that moves through it at a non-relativistic speed, indeed results in preferential positron acceleration. Filaments form that are filled predominantly with particles with the same direction of their electric current vector. Positron filaments are separated by electromagnetic fields from beam electron filaments. Some particles can cross the field boundary and enter the filament of the other species. Positron filaments can neutralize their net charge by collecting the electrons of the ambient plasma, while protons cannot easily follow the beam electron filaments. Positron filaments can thus be compressed to a higher density and temperature than the beam electron filaments. Filament mergers, which take place after the exponential growth phase of the instability has ended, lead to an expansion of the beam electron filaments, which amplifies the magnetic field they generate and induces an electric field in this filament. Beam electrons lose a substantial fraction of their kinetic energy to the electric field. Some positrons in the beam electron filament are accelerated by the induced electric field to almost twice their initial speed. The simulations show that a weaker electric field is induced in the positron filament and particles in this filament hardly change their speed.
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