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arXiv2014

A robust plasma-based laser amplifier via stimulated Brillouin scattering

It is shown here that Brillouin amplification can be used to produce picosecond pulses of petawatt power. Brillouin amplification is far more resilient to fluctuations in the laser and plasma parameters than Raman amplification, making it an attractive alternative to Raman amplification. Through analytic theory and multi-dimensional computer simulations, a novel, well-defined parameter regime has been found, distinct from that of Raman amplification, where pump-to-probe compression ratios of up to 100 and peak laser fluences over 1 kJ/cm$^2$ with 30\% efficiency have been achieved. High pulse quality has been maintained through control of parasitic instabilities.

physics.plasm-ph↗

arXiv2013

On the velocity space discretization for the Vlasov-Poisson system: comparison between Hermite spectral and Particle-in-Cell methods. Part 1: semi-implicit scheme

We discuss a spectral method for the numerical solution of the Vlasov-Poisson system where the velocity space is decomposed by means of an Hermite basis. We describe a semi-implicit time discretization that extends the range of numerical stability relative to an explicit scheme. We also introduce and discuss the effects of an artificial collisional operator, which is necessary to take care of the velocity space filamentation problem, unavoidable in collisionless plasmas. The computational efficiency and the cost-effectiveness of this method are compared to a Particle-in-Cell (PIC) method in the case of a two-dimensional phase space. The following examples are discussed: Langmuir wave, Landau damping, ion-acoustic wave, two-stream instability, and plasma echo. The Hermite spectral method can achieve solutions that are several orders of magnitude more accurate (at a fraction of the cost) with respect to the PIC method.

physics.plasm-ph↗

arXiv2014

Effects of Circulating Energetic Ions on Geodesic Acoustic Modes with Toroidal Rotation

Effects of circulating energetic ions (CEIs) on the geodesic acoustic modes (GAMs) in toroidally rotating tokamaks are theoretically analyzed utilizing the hybrid kinetic-fluid model. The frequencies of GAMs in the presence of toroidal rotation and CEIs are derived by first considering the rigid equilibrium condition. The unperturbed distribution function of CEIs depends on the poloidal angle, eventually influencing the frequencies significantly. The co-CEIs and counter-CEIs do not cancel each other in the balanced tangential neutral beam injection (NBI) case and show destabilizing effect on the zonal flows. The accurate mode frequency and the growth rate are presented.

physics.plasm-ph↗

arXiv2013

Landau damping: paraproducts and Gevrey regularity

We give a new, simpler, proof of nonlinear Landau damping on T^d in Gevrey-1/s regularity (s > 1/3) which matches the regularity requirement predicted by the formal analysis of Mouhot and Villani in the original proof of Landau damping [Acta Mathematica 2011]. Our proof combines in a novel way ideas from the original proof of Landau damping and the proof of inviscid damping in 2D Euler [arXiv:1306.5028]. As in the work on 2D Euler, we use paraproduct decompositions and controlled regularity loss to replace the Newton iteration scheme employed in the original proof. We perform time-response estimates adapted from the original proof to control the plasma echoes and couple them to energy estimates on the distribution function in the style of the work on 2D Euler.

math.AP↗

arXiv2013

The structure of warm dense matter modeled with an average atom model with ion-ion correlations

We present a new model of warm dense matter that represents an intermediate approach between the relative simplicity of ''one-ion'' average atom models and the more realistic but computationally expensive ab initio simulation methods. Physical realism is achieved primarily by including the correlations in the plasma that surrounds a central ion. The plasma is described with the Ornstein-Zernike integral equations theory of fluids, which is coupled to an average atom model for the central ion. In this contribution we emphasize the key elements and approximations and how they relate to and expand upon a typical average atom model. Besides being relatively inexpensive computationally, this approach offers several advantages over ab initio simulations but also has a number of limitations. The model is validated by comparisons with numerical solutions for the pair distribution function of the ions from ab initio simulations for several elements and a wide range of plasma conditions. Simulations results are reproduced remarkably well and simpler limiting theories are recovered as well. This model has many potential applications to calculation of the properties of warm dense matter such as the equation of state and conductivities for a wide range of temperatures and densities.

physics.plasm-ph↗

arXiv2013

The Model of Gas-Discharge Nonneutral Electron Plasma

The model of gas-discharge nonneutral electron plasma has been considered, in which the electron density is limited by non-linear processes initiated by diocotron instability and it does not depend on the mechanism of electron transport across the magnetic field. The model describes well the characteristics of electron sheath and the current characteristics of discharge both, in magnetron geometry and in the geometry of inversed magnetron, and it allows us to describe quantitatively the influence of anode misalignment on the discharge electron sheath for the first time. The scope of applicability of the proposed model, as well as its relation with other models of electron sheath is studied.

physics.plasm-ph↗

arXiv2013

Kinematic Constraints on Absorption of Ultraintense Laser Light

We derive upper and lower bounds on the absorption of ultraintense laser light by solids as a function of fundamental laser and plasma parameters. These limits emerge naturally from constrained optimization techniques applied to a generalization of the laser-solid interaction as a strongly-driven, relativistic, two degree of freedom Maxwell-Vlasov system. We demonstrate that the extrema and the phase-space-averaged absorption must always increase with intensity, and increase most rapidly when $10^{18} < I_L \ λ_L^2 < 10^{20}$ W $μ$m$^2/$cm$^{2}$. Our results indicate that the fundamental empirical trend towards increasing fractional absorption with irradiance therefore reflects the underlying phase space constraints.

physics.plasm-ph↗

arXiv2013

On_the_implementation_of_the_conditions_of_Inertial_Confinement_ Fusion by bombarding the target a macro particle

The acceleration of lithium tube segments with the length one centimeter, diameter sixteen microns wall thickness one nanometer is considered. These segments are electrically charged by proton beams produced by an electron beam source. Then, they are accelerated by the traveling wave field in a spiral waveguide. The segments are next sent to a target where they are compressed by three hundred times in the longitudinal direction and compressing target radially, so the conditions for thermonuclear fusion are realized.

physics.plasm-ph↗

arXiv2013

Shape effects in nonlinear Thomson and Compton processes: the quest for MeV high-order harmonics

Spectra of Thomson and Compton radiation, emitted during electron scattering off an intense laser beam, are calculated using the frameworks of classical and strong-field quantum electrodynamics, respectively. Both approaches use a plane-wave-fronted pulse approximation regarding the driving laser beam. Within this approximation, a very good agreement between Thomson and Compton frequency distributions is observed provided that frequencies of the emitted radiation is relatively low. The dependence of frequency spectra on the laser pulse envelope is analyzed. This becomes important in the context of ultra-short pulse generation, as illustrated by numerical examples.

physics.atom-ph↗

arXiv2014

Scalability of the plasma physics code GEM

We discuss a detailed weak scaling analysis of GEM, a 3D MPI-parallelised gyrofluid code used in theoretical plasma physics at the Max Planck Institute of Plasma Physics, IPP at Garching b. München, Germany. Within a PRACE Preparatory Access Project various versions of the code have been analysed on the HPC systems SuperMUC at LRZ and JUQUEEN at Jülich Supercomputing Centre (JSC) to improve the parallel scalability of the application. The diagnostic tool Scalasca has been used to filter out suboptimal routines. The code uses the electromagnetic gyrofluid model which is a superset of magnetohydrodynamic and drift-Alfvén microturbulance and also includes several relevant kinetic processes. GEM can be used with different geometries depending on the targeted use case, and has been proven to show good scalability when the computational domain is distributed amongst two dimensions. Such a distribution allows grids with sufficient size to describe small scale tokamak devices. In order to enable simulation of very large tokamaks (such as the next generation nuclear fusion device ITER in Cadarache, France) the third dimension has been parallelised and weak scaling has been achieved for significantly larger grids.

cs.DC↗

arXiv2013

An Introduction to Control of Chaos for Quasi-Integrable Hamiltonian Systems

Quasi-integrable Hamiltonian systems are of great interest in many research fields of physics and mathematics. In these systems, the phase space has regular and chaotic trajectories. These trajectories depend in part on the magnitude of perturbation that breaks the integrability of the system. The value of the critical perturbation responsible for this transition is a key element in the control of chaos . In this paper, we explore a procedure for the control in quasi-integrable Hamiltonian systems via canonical map. Initially, we present the basic tools for this study: Hamiltonian map, linearization of the map and Chirikov criterion. Subsequently, we investigated the behavior of a wave-particle interaction front perturbation. Finally, we confront with a numerical analytical approach (iteration of the map) results, showing a good agreement.

physics.plasm-ph↗

arXiv2013

Continuous Transitions between Discontinuous Magnetohydrodynamic Flows of Plasma and Its Heating

The possibility that the type of discontinuous flow changes as the conditions gradually (continuously) change is investigated in connection with the problems arising when the results of numerical simulations of magnetic reconnection in plasma are interpreted. The conservation laws at a discontinuity surface in magnetohydrodynamics admit such transitions, but the socalled transition solutions for the boundary conditions that simultaneously satisfy two types of discontinuities should exist in this case. The specific form of such solutions has been found, and a generalized scheme of permitted transitions has been constructed on their basis. An expression for the jump in internal energy at discontinuity is derived. The dependence of the plasma heating efficiency on the type of discontinuity is considered.

physics.plasm-ph↗

arXiv2014

Three-dimensional Iroshnikov-Kraichnan turbulence in a mean magnetic field

Forced, weak MHD turbulence with guide field is shown to adopt different regimes, depending on the magnetic excess of the large forced scales. When the magnetic excess is large enough, the classical perpendicular cascade with $5/3$ scaling is obtained, while when equipartition is imposed, an isotropic $3/2$ scaling appears in all directions with respect to the mean field (\cite{2010PhRvE..82b6406G} or GM10). We show here that the $3/2$ scaling of the GM10 regime is not ruled by a small-scale cross-helicity cascade, and propose that it is a 3D extension of a perpendicular weak Iroshnikov-Kraichnan (IK) cascade. We analyze in detail the structure functions in real space and show that they closely follow the critical balance relation both in the local frame and the global frame: we show that there is no contradiction between this and the isotropic $3/2$ scaling of the spectra. We propose a scenario explaining the spectral structure of the GM10 regime, that starts with a perpendicular weak IK cascade and extends to 3D by using quasi-resonant couplings. The quasi-resonance condition happens to reduce the energy flux in the same way as is done in the weak perpendicular cascade, so leading to a $3/2$ scaling in all directions. We discuss the possible applications of these findings to solar wind turbulence.

astro-ph.SR↗

arXiv2013

Propagation of an ultrashort electromagnetic pulse in solid-state plasma

The change of the shape of an ultrashort electromagnetic pulse in its propagation in solid-state plasma was calculated in the linear approximation. A case of solid-state silver plasma and of a "Mexican hat" wavelet pulse was considered. The dielectric permittivity of the medium was calculated in the Drude model. Strong dispersion spreading of a pulse at distances of the order of several microns was shown, and the comparison of evolution of the pulse shape for different center frequencies was carried out.

physics.plasm-ph↗

arXiv2014

Effects of Circulating Energetic Ions on Geodesic Acoustic Modes with Finite Wave Numbers

Effects of circulating energetic particles on the geodesic acoustic modes (GAMs) with finite wave numbers are analyzed by using the hybrid kinetic-fluid model. The dispersion relation is derived by adopting the slowing beam ions distribution function in the presence of finite parallel wave number (FPWN), which is shown to affect the GAM as the toroidal rotation to the classical GAMs with zero wave numbers. The effects of CEIs can be divided into two parts. One is contributed by the poloidal angle and kinetic energy dependencies of the unperturbed distribution function, and is independent of the poloidal and toroidal wave numbers. The other related to the radial gradient of the CEIs pressure is proportional to the poloidal wave number, and is shown to play destabilizing effect on the GAMs. The critical condition responsible for an instability is analytically presented.

physics.plasm-ph↗

arXiv2013

On the velocity space discretization for the Vlasov-Poisson system: comparison between Hermite spectral and Particle-in-Cell methods. Part 2: fully-implicit scheme

We describe a spectral method for the numerical solution of the Vlasov-Poisson system where the velocity space is decomposed by means of an Hermite basis, and the configuration space is discretized via a Fourier decomposition. The novelty of our approach is an implicit time discretization that allows exact conservation of charge, momentum and energy. The computational efficiency and the cost-effectiveness of this method are compared to the fully-implicit PIC method recently introduced by Markidis and Lapenta (2011) and Chen et al. (2011). The following examples are discussed: Langmuir wave, Landau damping, ion-acoustic wave, two-stream instability. The Fourier-Hermite spectral method can achieve solutions that are several orders of magnitude more accurate at a fraction of the cost with respect to PIC. This paper concludes the study presented in Camporeale et al. (2013) where the same method has been described for a semi-implicit time discretization, and was compared against an explicit PIC.

physics.plasm-ph↗

arXiv2013

Energy Loss of High-Energy Particles in Particle-in-Cell Simulation

When a charged particle moves through a plasma at a speed much higher than the thermal velocity of the plasma, it is subjected to the force of the electrostatic field induced in the plasma by itself and loses its energy. This process is well-known as the stopping power of a plasma. In this paper we show that the same process works in particle-in-cell (PIC) simulations as well and the energy loss rate of fast particles due to this process is mainly determined by the number of plasma electrons contained in the electron skin depth volume. However, since there are generally very few particles in that volume in PIC simulations compared with real plasmas, the energy loss effect can be exaggerated significantly and can affect the results. Therefore, especially for the simulations that investigate the particle acceleration processes, the number of particles used in the simulations should be chosen large enough to avoid this artificial energy loss.

physics.comp-ph↗

arXiv2013

On the microscopic mechanism of ion-extraction of a gridded ion propulsion thruster

The following paper includes a physical microscopic particle-description of the phenomena and mechanisms that lead to the extraction of ions with the aim to generate thrust. This theoretical treatise arose from the intention to visualize the behavior of the involved particles under effect of the involved electrical fields. By this way, an underlying basis for experimental investigations of the work of an ion thruster should be formed. So a foundation was created, which explains the ion extracting and so thrust generating function of an ion thruster. The theoretical work was related to the Radio-frequency Ion Thruster (RIT). But the model worked out can be generalized for all thruster types that use electrostatic fields to extract positively charged ions.

physics.plasm-ph↗