| 1. |
- Ahmed, Hamad, et al.
(författare)
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Time-Resolved Characterization of the Formation of a Collisionless Shock
- 2013
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Ingår i: Physical Review Letters. - : American Physical Society. - 0031-9007 .- 1079-7114. ; 110:20
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Tidskriftsartikel (refereegranskat)abstract
- We report on the temporally and spatially resolved detection of the precursory stages that lead to the formation of an unmagnetized, supercritical collisionless shock in a laser-driven laboratory experiment. The measured evolution of the electrostatic potential associated with the shock unveils the transition from a current free double layer into a symmetric shock structure, stabilized by ion reflection at the shock front. Supported by a matching particle-in-cell simulation and theoretical considerations, we suggest that this process is analogous to ion reflection at supercritical collisionless shocks in supernova remnants.
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| 2. |
- 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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| 3. |
- Dieckmann, Mark Eric, et al.
(författare)
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Parametric study of non-relativistic electrostatic shocks and the structure of their transition layer
- 2013
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Ingår i: Physics of Plasmas. - : American Institute of Physics (AIP). - 1070-664X .- 1089-7674. ; 20:4, s. 042111-1-042111-10
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Tidskriftsartikel (refereegranskat)abstract
- Nonrelativistic electrostatic unmagnetized shocks are frequently observed in laboratory plasmas and they are likely to exist in astrophysical plasmas. Their maximum speed, expressed in units of the ion acoustic speed far upstream of the shock, depends only on the electron-to-ion temperature ratio if binary collisions are absent. The formation and evolution of such shocks is examined here for a wide range of shock speeds with particle-in-cell simulations. The initial temperatures of the electrons and the 400 times heavier ions are equal. Shocks form on electron time scales at Mach numbers between 1.7 and 2.2. Shocks with Mach numbers up to 2.5 form after tens of inverse ion plasma frequencies. The density of the shock-reflected ion beam increases and the number of ions crossing the shock thus decreases with an increasing Mach number, causing a slower expansion of the downstream region in its rest frame. The interval occupied by this ion beam is on a positive potential relative to the far upstream. This potential pre-heats the electrons ahead of the shock even in the absence of beam instabilities and decouples the electron temperature in the foreshock ahead of the shock from the one in the far upstream plasma. The effective Mach number of the shock is reduced by this electron heating. This effect can potentially stabilize nonrelativistic electrostatic shocks moving as fast as supernova remnant shocks.
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| 4. |
- Dieckmann, Mark Eric, et al.
(författare)
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Particle simulation study of electron heating by counter-streaming ion beams ahead of supernova remnant shocks
- 2012
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Ingår i: Plasma Physics and Controlled Fusion. - : Institute of Physics Publishing (IOPP). - 0741-3335 .- 1361-6587. ; 54:8, s. 085015-
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Tidskriftsartikel (refereegranskat)abstract
- The growth and saturation of Buneman-type instabilities is examined with a particle-in-cell (PIC) simulation for parameters that are representative for the foreshock region of fast supernova remnant shocks. A dense ion beam and the electrons correspond to the upstream plasma and a fast ion beam to the shock-reflected ions. The purpose of the 2D simulation is to identify the nonlinear saturation mechanisms, the electron heating and potential secondary instabilities that arise from anisotropic electron heating and result in the growth of magnetic fields. We confirm that the instabilities between both ion beams and the electrons saturate by the formation of phase space holes by the beam-aligned modes. The slower oblique modes accelerate some electrons, but they cannot heat up the electrons significantly before they are trapped by the faster beam-aligned modes. Two circular electron velocity distributions develop, which are centred around the velocity of each ion beam. They develop due to the scattering of the electrons by the electrostatic wave potentials. The growth of magnetic fields is observed, but their amplitude remains low.
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| 5. |
- Dieckmann, Mark Eric, et al.
(författare)
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PIC simulation of a thermal anisotropy-driven Weibel instability in a circular rarefaction wave
- 2012
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Ingår i: New Journal of Physics. - London : Institute of Physics (IOP). - 1367-2630. ; 14:023007
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Tidskriftsartikel (refereegranskat)abstract
- The expansion of an initially unmagnetized planar rarefaction wave has recently been shown to trigger a thermal anisotropy-driven Weibel instability (TAWI), which can generate magnetic fields from noise levels. It is examined here whether the TAWI can also grow in a curved rarefaction wave. The expansion of an initially unmagnetized circular plasma cloud, which consists of protons and hot electrons, into a vacuum is modelled for this purpose with a two-dimensional particle-in-cell (PIC) simulation. It is shown that the momentum transfer from the electrons to the radially accelerating protons can indeed trigger a TAWI. Radial current channels form and the aperiodic growth of a magnetowave is observed, which has a magnetic field that is oriented orthogonal to the simulation plane. The induced electric field implies that the electron density gradient is no longer parallel to the electric field. Evidence is presented here that this electric field modification triggers a second magnetic instability, which results in a rotational low-frequency magnetowave. The relevance of the TAWI is discussed for the growth of small-scale magnetic fields in astrophysical environments, which are needed to explain the electromagnetic emissions by astrophysical jets. It is outlined how this instability could be examined experimentally.
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| 6. |
- 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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| 7. |
- Dieckmann, Mark Eric, et al.
(författare)
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The Weibel instability in a circular rarefaction wave
- 2012
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Ingår i: Proceedings of the 39th European Physical Society Conference & 16th Int. Congress on Plasma Physics. ; , s. P1.176-1-P1.176-4
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Konferensbidrag (refereegranskat)abstract
- Instabilities behind the front of a cylindrically expanding plasma have been investigated experimentally and with a particle-in-cell simulation. Tubelike filamentary structures form behind the front of a plasma created by irradiating wire targets with a ps-duration and intense ( 1019 W cm-2) laser pulse. These filaments exhibit coherent magnetic fields with a remarkable stability ( 103 / wp: plasma frequency). PIC simulations indicate that an instability driven by a thermal anisotropy of the electron population is the cause. This instability requires a plasma density gradient and hot electrons. It can thus contribute to the generation of strongsustained magnetic fields in astrophysical jets.
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| 8. |
- Jenab, Mehdi, 1982, et al.
(författare)
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Ultrafast electron holes in plasma phase space dynamics
- 2021
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Ingår i: Scientific Reports. - : Springer Science and Business Media LLC. - 2045-2322 .- 2045-2322. ; 11:1
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Tidskriftsartikel (refereegranskat)abstract
- Electron holes (EH) are localized modes in plasma kinetic theory which appear as vortices in phase space. Earlier research on EH is based on the Schamel distribution function (df). A novel df is proposed here, generalizing the original Schamel df in a recursive manner. Nonlinear solutions obtained by kinetic simulations are presented, with velocities twice the electron thermal speed. Using 1D-1V kinetic simulations, their propagation characteristics are traced and their stability is established by studying their long-time evolution and their behavior through mutual collisions.
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| 9. |
- Koen, Etienne, 1985-
(författare)
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A Simulation Approach to Plasma Waves
- 2014
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Electrostatic waves in the form of Broadband Electrostatic Noise (BEN) have been observed inthe Earth’s auroral region associated with high geomagnetic activity. Their broad frequencyspectrum consists of three electrostatic modes, namely electron plasma, electron acoustic andbeam-driven modes.A 1D Particle-in-Cell (PIC) simulation was developed to investigate the characteristics ofthe electrostatic waves found in such a plasma. Dispersion, phase space and spatial electricfield diagrams were constructed from the output of the PIC simulation which was used todescribe the wave dispersion and spatial field structures found in a plasma. A three electroncomponent plasma was studied using a Maxwellian distribution function to model their ve-locities. Beam-driven waves were found to dominate the frequency spectrum while electronplasma and electron acoustic waves were damped for a high beam velocity. Furthermore, for ahigh beam velocity, solitary waves are generated by electron holes (positive potentials), givingrise to a bipolar spatial electric field structure moving in the direction of the beam. Increasingthe beam temperature allows the beam electrons to mix more freely with the hot and coolelectrons, which leads to electron plasma and electron acoustic waves being enhanced whilebeam-driven waves are damped. Decreasing the beam density and velocity leads to dampingof beam-driven waves, while electron plasma and electron acoustic waves are enhanced.The electron acoustic mode was studied with the addition of a static background magneticfield. When the angle of wave propagation is perfectly perpendicular to the backgroundmagnetic field, a set of harmonics, called Bernstein modes, were produced. These modesare characterized by their nodes being furtherly displaced along the wave vector axis for anincrease in the node (harmonic) number. The model was further generalized by allowing theangle of wave propagation, θ, with respect to the magnetic field to be varied, thus enabling thestudy of the obliquely propagating electron acoustic mode. Both the amplitude and frequencyof the electron acoustic mode was found to decrease as θ increases.Measurements in Saturn’s magnetosphere have shown the co-existence of two electron (hotand cool) components. The electron velocities are best described by a κ-distribution (insteadof a Maxwellian) which has a high-energy tail. Using an adapted PIC simulation, the study ofelectron plasma and electron acoustic waves was extended by using a κ-distribution to describethe electron velocities with low κ indices. Electron acoustic waves are damped over most wavenumber ranges while electron plasma waves are weakly damped at low wave numbers anddamped for all other wave numbers. Furthermore, the study was extended by introducingthe motion of ions to study the ion acoustic waves in Saturn’s magnetosphere. While the ionacoustic mode was found to be relatively insensitive to the κ indices of the electrons, it isfound to be sensitive to the electron temperature and density ratios.
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| 10. |
- Sarri, Gianluca, et al.
(författare)
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Dynamics of Self-Generated, Large Amplitude Magnetic Fields Following High-Intensity Laser Matter Interaction
- 2012
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Ingår i: Physical Review Letters. - : American Physical Society. - 0031-9007 .- 1079-7114. ; 109:20, s. 205002-
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Tidskriftsartikel (refereegranskat)abstract
- The dynamics of magnetic fields with an amplitude of several tens of megagauss, generated at both sides of a solid target irradiated with a high-intensity (∼1019 W/cm2) picosecond laser pulse, has been spatially and temporally resolved using a proton imaging technique. The amplitude of the magnetic fields is sufficiently large to have a constraining effect on the radial expansion of the plasma sheath at the target surfaces. These results, supported by numerical simulations and simple analytical modeling, may have implications for ion acceleration driven by the plasma sheath at the rear side of the target as well as for the laboratory study of self-collimated high-energy plasma jets.
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