| 1. |
- Bret, Antoine, et al.
(author)
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Hierarchy of instabilities for two counter-streaming magnetized pair beams: Influence of field obliquity
- 2017
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In: Physics of Plasmas. - : A I P Publishing LLC. - 1070-664X .- 1089-7674. ; 24:6
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Journal article (peer-reviewed)abstract
- The hierarchy of unstable modes when two counter-streaming pair plasmas interact over a flow-aligned magnetic field has been recently investigated [Phys. Plasmas 23, 062122 (2016)]. The analysis is here extended to the case of an arbitrarily tilted magnetic field. The two plasma shells are initially cold and identical. For any angle θ ∈ [0, π/2] between the field and the initial flow, the hierarchy of unstable modes is numerically determined in terms of the initial Lorentz factor of the shells γ0, and the field strength as measured by a parameter denoted σ. For θ = 0, four different kinds of mode are likely to lead the linear phase. The hierarchy simplifies for larger θ's, partly because the Weibel instability can no longer be cancelled in this regime. For θ > 0.78 (44°) and in the relativistic regime, the Weibel instability always govern the interaction. In the non-relativistic regime, the hierarchy becomes θ-independent because the interaction turns to be field-independent. As a result, the two-stream instability becomes the dominant one, regardless of the field obliquity.
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| 2. |
- Bret, Antoine, et al.
(author)
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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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In: Physics of Plasmas. - : American Institute of Physics. - 1070-664X .- 1089-7674. ; 17:3, s. 032109-
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Journal article (peer-reviewed)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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| 3. |
- Bret, Antoine, et al.
(author)
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Ions motion effects on the full unstable spectrum in relativistic electron beam plasma interaction
- 2008
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In: Physics of Plasmas. - : AIP Publishing. - 1070-664X .- 1089-7674. ; 15:1, s. 012104-1-12104-13
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Journal article (peer-reviewed)abstract
- A relativistic fluid model is implemented to assess the role of the ions motion in the linear phase of relativistic beam plasma electromagnetic instabilities. The all unstable wave vector spectrum is investigated, allowing us to assess how ion motions modify the competition between every possible instability. Beam densities up to the plasma one are considered. Due to the fluid approach, the temperatures must remain small, i.e., nonrelativistic. In the cold limit, ions motion affect the most unstable mode when the beam gamma factor bM/mi, being the beam to plasma density ratio, i the ion charge, M their mass, and m the electrons. The return current plays an important role by prompting Buneman-type instabilities which remain in the nonrelativistic regime up to high beam densities. Nonrelativistic temperatures only slightly affect these conclusions, except in the diluted beam regime where they can stabilize the Buneman modes.
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| 4. |
- Bret, Antoine, et al.
(author)
-
Multidimensional electron beam-plasma instabilities in the relativistic regime
- 2010
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In: Physics of Plasmas. - : AIP Publishing. - 1070-664X .- 1089-7674. ; 17:12, s. 120501-1-120501-36
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Research review (peer-reviewed)abstract
- The interest in relativistic beam-plasma instabilities has been greatly rejuvenated over the past two decades by novel concepts in laboratory and space plasmas. Recent advances in this long-standing field are here reviewed from both theoretical and numerical points of view. The primary focus is on the two-dimensional spectrum of unstable electromagnetic waves growing within relativistic, unmagnetized, and uniform electron beam-plasma systems. Although the goal is to provide a unified picture of all instability classes at play, emphasis is put on the potentially dominant waves propagating obliquely to the beam direction, which have received little attention over the years. First, the basic derivation of the general dielectric function of a kinetic relativistic plasma is recalled. Next, an overview of two-dimensional unstable spectra associated with various beam-plasma distribution functions is given. Both cold-fluid and kinetic linear theory results are reported, the latter being based on waterbag and Maxwell–Jüttner model distributions. The main properties of the competing modes (developing parallel, transverse, and oblique to the beam) are given, and their respective region of dominance in the system parameter space is explained. Later sections address particle-in-cell numerical simulations and the nonlinear evolution of multidimensional beam-plasma systems. The elementary structures generated by the various instability classes are first discussed in the case of reduced-geometry systems. Validation of linear theory is then illustrated in detail for large-scale systems, as is the multistaged character of the nonlinear phase. Finally, a collection of closely related beam-plasma problems involving additional physical effects is presented, and worthwhile directions of future research are outlined.
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| 5. |
- Bret, Antoine, et al.
(author)
-
Oblique electromagnetic instabilities for a hot relativistic beam interacting with a hot and magnetized plasma
- 2006
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In: Physics of Plasmas. - : AIP Publishing. - 1070-664X .- 1089-7674. ; 13:8, s. 082109-1-082109-8
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Journal article (peer-reviewed)abstract
- The temperature-dependent fluid model from Phys. Plasmas 13, 042106 (2006) is expanded in order to explore the oblique electromagnetic instabilities, which are driven by a hot relativistic electron beam that is interpenetrating a hot and magnetized plasma. The beam velocity vector is parallel to the magnetic-field direction. The results are restricted to nonrelativistic temperatures. The growth rates of all instabilities but the two-stream instability can be reduced by a strong magnetic field so that the distribution of unstable waves becomes almost one dimensional. For high beam densities, highly unstable oblique modes dominate the spectrum of unstable waves as long as omega(c)/omega(p)less than or similar to 2 gamma(3/2)(b), where omega(c) is the electron gyrofrequency, omega(p) is the electron plasma frequency, and gamma(b) is the relativistic factor of the beam. A uniform stabilization over the entire k space cannot be achieved.
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| 6. |
- Bret, Antoine, et al.
(author)
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Relativistic electron beam driven instabilities in the presence of an arbitrarily oriented magnetic field
- 2008
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In: Physics of Plasmas. - : AIP Publishing. - 1070-664X .- 1089-7674. ; 15:6, s. 062102-1-
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Journal article (peer-reviewed)abstract
- The electromagnetic instabilities driven by a relativistic electron beam, which moves through a magnetized plasma, are analyzed with a cold two-fluid model. It allows for any angle B between the beam velocity vector and the magnetic field vector and considers any orientation of the wavevector in the two-dimensional plane spanned by these two vectors. If the magnetic field is strong, the two-stream instability dominates if B=0 and the oblique modes grow faster at larger B. A weaker magnetic field replaces the two-stream modes with oblique modes as the fastest-growing waves. The threshold value separating both magnetic regimes is estimated. A further dimensionless parameter is identified, which determines whether or not the wavevector of the most unstable wave is changed continuously, as B is varied from 0 to /2. The fastest growing modes are always found for a transverse propagation of the beam with B=/2, irrespective of the magnetic field strength. ©2008 American Institute of Physics
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| 7. |
- Dieckmann, Mark E, 1969-, et al.
(author)
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Emergence of MHD structures in a collisionless PIC simulation plasma
- 2017
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In: Physics of Plasmas. - Melville, NY, United States : A I P Publishing LLC. - 1070-664X .- 1089-7674. ; 24:9
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Journal article (peer-reviewed)abstract
- The expansion of a dense plasma into a dilute plasma across an initially uniform perpendicular magnetic field is followed with a one-dimensional particle-in-cell simulation over magnetohydrodynamics time scales. The dense plasma expands in the form of a fast rarefaction wave. The accelerated dilute plasma becomes separated from the dense plasma by a tangential discontinuity at its back. A fast magnetosonic shock with the Mach number 1.5 forms at its front. Our simulation demonstrates how wave dispersion widens the shock transition layer into a train of nonlinear fast magnetosonic waves.
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| 8. |
- Dieckmann, Mark E, 1969-, et al.
(author)
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Evolution of the fastest-growing relativistic mixed mode instability driven by a tenuous plasma beam in one and two dimensions
- 2006
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In: Physics of Plasmas. - : AIP Publishing. - 1070-664X .- 1089-7674. ; 13:11, s. 112110-1-112110-8
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Journal article (peer-reviewed)abstract
- Particle-in-cell simulations confirm here that a mixed plasma mode is the fastest growing when a highly relativistic tenuous electron-proton beam interacts with an unmagnetized plasma. The mixed modes grow faster than the filamentation and two-stream modes in simulations with beam Lorentz factors Gamma of 4, 16, and 256, and are responsible for thermalizing the electrons. The mixed modes are followed to their saturation for the case of Gamma=4 and electron phase space holes are shown to form in the bulk plasma, while the electron beam becomes filamentary. The initial saturation is electrostatic in nature in the considered one- and two-dimensional geometries. Simulations performed with two different particle-in-cell simulation codes evidence that a finite grid instability couples energy into high-frequency electromagnetic waves, imposing simulation constraints.
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| 9. |
- Dieckmann, Mark E, 1969-, et al.
(author)
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Expansion of a radial plasma blast shell into an ambient plasma
- 2017
-
In: Physics of Plasmas. - Melville, NY, United States : A I P Publishing LLC. - 1070-664X .- 1089-7674. ; 24:9
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Journal article (peer-reviewed)abstract
- The expansion of a radial blast shell into an ambient plasma is modeled with a particle-in-cell simulation. The unmagnetized plasma consists of electrons and protons. The formation and evolution of an electrostatic shock is observed, which is trailed by ion-acoustic solitary waves that grow on the beam of the blast shell ions in the post-shock plasma. In spite of the initially radial symmetric outflow, the solitary waves become twisted and entangled and, hence, they break the radial symmetry of the flow. The waves and their interaction with the shocked ambient ions slow down the blast shell protons and bring the post-shock plasma closer to equilibrium.
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