Collisionless energy absorption in the short-pulse intense laser-cluster interaction

In a previous Letter [Phys. Rev. Lett. 96, 123401 (2006)] we have shown by means of three-dimensional particle-in-cell simulations and a simple rigid-sphere model that nonlinear resonance absorption is the dominant collisionless absorption mechanism in the intense, short-pulse laser cluster interaction. In this paper we present a more detailed account of the matter. In particular we show that the absorption efficiency is almost independent of the laser polarization. In the rigid-sphere model, the absorbed energy increases by many orders of magnitude at a certain threshold laser intensity. The particle-in-cell results display maximum fractional absorption around the same intensity. We calculate the threshold intensity and show that it is underestimated by the common over-barrier ionization estimate.Comment: 12 pages, 13 figures, RevTeX

Attractive Potential around a Thermionically Emitting Microparticle

We present a simulation study of the charging of a dust grain immersed in a plasma, considering the effect of electron emission from the grain (thermionic effect). It is shown that the OML theory is no longer reliable when electron emission becomes large: screening can no longer be treated within the Debye-Huckel approach and an attractive potential well forms, leading to the possibility of attractive forces on other grains with the same polarity. We suggest to perform laboratory experiments where emitting dust grains could be used to create non-conventional dust crystals or macro-molecules.Comment: 3 figures. To appear on Physical Review Letter

Investigation of Particle-in-Cell Acceleration Techniques for Plasma Simulations

COLISEUM is an application framework that integrates plasma propagation schemes and arbitrary 3D surface geometries. Using Particle-in-Cell (PIC) schemes to model the plasma propagation high fidelity modeling of the plasma and its interactions with the surfaces is possible. In order to improve the computational performance of the Particle-in-Cell scheme with Direct Simulation Monte Carlo collision modeling (PIC-DSMC) within COLISEUM, AQUILA, acceleration techniques have been developed that significantly decrease the amount of CPU time needed to obtain a steady-state solution. These techniques have been demonstrated to decrease the CPU time from 3 to 24 times with little appreciable differences in the global particle properties and number densities. This work investigates the differences in the local plasma properties that result from the application of the different acceleration techniques. Results show that the subcycling acceleration scheme does accurately capture the macroscopic flow properties (such as particle counts and species number densities) and the velocity distributions in the lower density regions of the flow field. However, the higher density regions of the flow field (such as in the main beam of the plasma source) show significant differences that are believed to be associated with the simplifying assumptions used in the original collision modeling scheme within the PIC-DSMC module AQUILA

New combined PIC-MCC approach for fast simulation of a radio frequency discharge at low gas pressure

A new combined PIC-MCC approach is developed for accurate and fast simulation of a radio frequency discharge at low gas pressure and high density of plasma. Test calculations of transition between different modes of electron heating in a ccrf discharge in helium and argon show a good agreement with experimental data. We demonstrate high efficiency of the combined PIC-MCC algorithm, especially for the collisionless regime of electron heating.Comment: 6 paged, 8 figure

An alternative to the plasma emission model: Particle-In-Cell, self-consistent electromagnetic wave emission simulations of solar type III radio bursts

1.5D PIC, relativistic, fully electromagnetic (EM) simulations are used to model EM wave emission generation in the context of solar type III radio bursts. The model studies generation of EM waves by a super-thermal, hot beam of electrons injected into a plasma thread that contains uniform longitudinal magnetic field and a parabolic density gradient. In effect, a single magnetic line connecting Sun to earth is considered, for which several cases are studied. (i) We find that the physical system without a beam is stable and only low amplitude level EM drift waves (noise) are excited. (ii) The beam injection direction is controlled by setting either longitudinal or oblique electron initial drift speed, i.e. by setting the beam pitch angle. In the case of zero pitch angle, the beam excites only electrostatic, standing waves, oscillating at plasma frequency, in the beam injection spatial location, and only low level EM drift wave noise is also generated. (iii) In the case of oblique beam pitch angles, again electrostatic waves with same properties are excited. However, now the beam also generates EM waves with the properties commensurate to type III radio bursts. The latter is evidenced by the wavelet analysis of transverse electric field component, which shows that as the beam moves to the regions of lower density, frequency of the EM waves drops accordingly. (iv) When the density gradient is removed, electron beam with an oblique pitch angle still generates the EM radiation. However, in the latter case no frequency decrease is seen. Within the limitations of the model, the study presents the first attempt to produce simulated dynamical spectrum of type III radio bursts in fully kinetic plasma model. The latter is based on 1.5D non-zero pitch angle (non-gyrotropic) electron beam, that is an alternative to the plasma emission classical mechanism.Comment: Physics of Plasmas, in press, May 2011 issue (final accepted version

On particle acceleration and trapping by Poynting flux dominated flows

Using particle-in-cell (PIC) simulations, we study the evolution of a strongly magnetized plasma slab propagating into a finite density ambient medium. Like previous work, we find that the slab breaks into discrete magnetic pulses. The subsequent evolution is consistent with diamagnetic relativistic pulse acceleration of \cite{liangetal2003}. Unlike previous work, we use the actual electron to proton mass ratio and focus on understanding trapping vs. transmission of the ambient plasma by the pulses and on the particle acceleration spectra. We find that the accelerated electron distribution internal to the slab develops a double-power law. We predict that emission from reflected/trapped external electrons will peak after that of the internal electrons. We also find that the thin discrete pulses trap ambient electrons but allow protons to pass through, resulting in less drag on the pulse than in the case of trapping of both species. Poynting flux dominated scenarios have been proposed as the driver of relativistic outflows and particle acceleration in the most powerful astrophysical jets.Comment: 25 pages, Accepted by Plasma Physics and Controlled Fusio

Waves and instability in a one-dimensional microfluidic array

Motion in a one-dimensional (1D) microfluidic array is simulated. Water droplets, dragged by flowing oil, are arranged in a single row, and due to their hydrodynamic interactions spacing between these droplets oscillates with a wave-like motion that is longitudinal or transverse. The simulation yields wave spectra that agree well with experiment. The wave-like motion has an instability which is confirmed to arise from nonlinearities in the interaction potential. The instability's growth is spatially localized. By selecting an appropriate correlation function, the interaction between the longitudinal and transverse waves is described

Multi-GPU Acceleration of the iPIC3D Implicit Particle-in-Cell Code

iPIC3D is a widely used massively parallel Particle-in-Cell code for the simulation of space plasmas. However, its current implementation does not support execution on multiple GPUs. In this paper, we describe the porting of iPIC3D particle mover to GPUs and the optimization steps to increase the performance and parallel scaling on multiple GPUs. We analyze the strong scaling of the mover on two GPU clusters and evaluate its performance and acceleration. The optimized GPU version which uses pinned memory and asynchronous data prefetching outperform their corresponding CPU versions by 5-10x on two different systems equipped with NVIDIA K80 and V100 GPUs.Comment: Accepted for publication in ICCS 201

Statistical kinetic treatment of relativistic binary collisions

In particle-based algorithms, the effect of binary collisions is commonly described in a statistical way, using Monte Carlo techniques. It is shown that, in the relativistic regime, stringent constraints should be considered on the sampling of particle pairs for collision, which are critical to ensure physically meaningful results, and that nonrelativistic sampling criteria (e.g., uniform random pairing) yield qualitatively wrong results, including equilibrium distributions that differ from the theoretical J\"uttner distribution. A general procedure for relativistically consistent algorithms is provided, and verified with three-dimensional Monte Carlo simulations, thus opening the way to the numerical exploration of the statistical properties of collisional relativistic systems.Comment: Accepted for publication as a Rapid Communication in Phys. Rev.