PLATEAU FORMATION ON ACCELERATING WAKEFIELD FOR ELECTRON-WITNESS-BUNCH AND ON DECELERATING WAKEFIELD FOR DRIVER-BUNCHES IN A PLASMA

Plasma wakefield acceleration promises compact sources of high-brightness relativistic electron and positron beams. Applications (particle colliders and free-electron lasers) of plasma wakefield accelerators demand low ener-gy spread beams and high-efficiency operation. Achieving both requires plateau formation on both the accelerating field for witness-bunch and the decelerating fields for driver-bunches by controlled beam loading of the plasma wave with careful tailored current profiles. We demonstrate by numerical simulation by 2.5D PIC code LCODE such optimal beam loading in a linear and blowout electron-driven plasma accelerator with RF generated low and high beam charge and high beam quality.

Bunch Expansion as a Cause for Pulsar Radio Emissions

Electromagnetic waves due to electron–positron clouds (bunches), created by cascading processes in pulsar magnetospheres, have been proposed to explain the pulsar radio emission. In order to verify this hypothesis, we utilized for the first time Particle-in-Cell (PIC) code simulations to study the nonlinear evolution of electron–positron bunches dependant on the initial relative drift speeds of electrons and positrons, plasma temperature, and distance between the bunches. For this sake, we utilized the PIC code ACRONYM with a high-order field solver and particle weighting factor, appropriate to describe relativistic pair plasmas. We found that the bunch expansion is mainly determined by the relative electron–positron drift speed. Finite drift speeds were found to cause the generation of strong electrostatic superluminal waves at the bunch density gradients that reach up to E ∼ 7.5 × 105 V cm−1 (E/(m e c ω p e −1) ∼ 4.4) and strong plasma heating. As a result, up to 15% of the initial kinetic energy is transformed into the electric field energy. Assuming the same electron and positron distributions, we found that the fastest (in the bunch reference frame) particles of consecutively emitted bunches eventually overlap in momentum (velocity) space. This overlap causes two-stream instabilities that generate electrostatic subluminal waves with electric field amplitudes reaching up to E ∼ 1.9 × 104 V cm−1 (E/(m e c ω p e −1) ∼ 0.11). We found that in all simulations the evolution of electron–positron bunches may lead to the generation of electrostatic superluminal or subluminal waves, which, in principle, can be behind the observed electromagnetic emissions of pulsars in the radio wave range.

Numerical Aspects of Particle-in-Cell Simulations for Plasma-Motion Modeling of Electric Thrusters

The present work is focused on a detailed description of an in-house, particle-in-cell code developed by the authors, whose main aim is to perform highly accurate plasma simulations on an off-the-shelf computing platform in a relatively short computational time, despite the large number of macro-particles employed in the computation. A smart strategy to set up the code is proposed, and in particular, the parallel calculation in GPU is explored as a possible solution for the reduction in computing time. An application on a Hall-effect thruster is shown to validate the PIC numerical model and to highlight the strengths of introducing highly accurate schemes for the electric field interpolation and the macroparticle trajectory integration in the time. A further application on a helicon double-layer thruster is presented, in which the particle-in-cell (PIC) code is used as a fast tool to analyze the performance of these specific electric motors.

Kinetic Modeling of the Impact of Solar Wind Discontinuities on the Magnetopause

<p>An important step in comprehending the effects of solar wind structures on the magnetosphere is to develop an understanding of their impact on the dayside magnetopause.  While most of the time global magnetohydrodynamic (MHD) models describe adequately the large-scale effects of solar wind structures on the magnetopause, recent spacecraft observations in the near Earth solar wind indicate that solar wind discontinuities have plasma features that are often not accurately described by MHD.  In this presentation, we report our progress in gaining a comprehensive understanding of kinetic processes occurring at the magnetopause as solar wind structures impact the dayside magnetosphere. Our approach combines implicit PIC simulations with global MHD simulations of the solar wind-magnetosphere-ionosphere system. The global simulation sets the overall configuration of the magnetosphere, while fields and plasma moments of a sub-domain of the global simulation are used to set initial and boundary conditions of the PIC code. Results are discussed in the context of spacecraft observations.</p>

ION ACOUSTIC INSTABILITY IN DUSTY PLASMA

The computer simulation results of the ion acoustic instability evolution in the dusty plasma are presented. The dusty plasma consists from electrons, ions, massive charged dust grains and neutral atoms. In the simulation the implicit PIC-code is used. To this code the implicit scheme of particles movement is applied, namely the direct implicit method of Langdon-Fridman. Realization of the algorithm is presented too.