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arXiv2013

Symmetry breaking and universality of decaying MHD Taylor-Green flows

We investigate the evolution and stability of a decaying magnetohydrodynamic (MHD) Taylor-Green flow. The chosen flow has been shown to result in a steep total energy spectrum with power law behaviour $k^{-2}$. We investigate the symmetry breaking of this flow by exciting perturbations of different amplitudes. It is shown that for any finite amplitude perturbation there is a high enough Reynolds number for which the perturbation will grow enough at the peak of dissipation rate resulting to a non-linear feedback in the flow and subsequently break the Taylor-Green symmetries. In particular, we show that symmetry breaking at large scales occurs if the amplitude of the perturbation is $ρ_{crit} \sim Re^{-1}$ and at small scales occurs if $ρ_{crit} \sim Re^{-3/2}$. This symmetry breaking modifies the scaling laws of the energy spectra at the peak of dissipation rate away from the $k^{-2}$ scaling and towards the classical $k^{-5/3}$ and $k^{-3/2}$ power laws.

physics.flu-dyn↗

arXiv2013

Porting Large HPC Applications to GPU Clusters: The Codes GENE and VERTEX

We have developed GPU versions for two major high-performance-computing (HPC) applications originating from two different scientific domains. GENE is a plasma microturbulence code which is employed for simulations of nuclear fusion plasmas. VERTEX is a neutrino-radiation hydrodynamics code for "first principles"-simulations of core-collapse supernova explosions. The codes are considered state of the art in their respective scientific domains, both concerning their scientific scope and functionality as well as the achievable compute performance, in particular parallel scalability on all relevant HPC platforms. GENE and VERTEX were ported by us to HPC cluster architectures with two NVidia Kepler GPUs mounted in each node in addition to two Intel Xeon CPUs of the Sandy Bridge family. On such platforms we achieve up to twofold gains in the overall application performance in the sense of a reduction of the time to solution for a given setup with respect to a pure CPU cluster. The paper describes our basic porting strategies and benchmarking methodology, and details the main algorithmic and technical challenges we faced on the new, heterogeneous architecture.

physics.comp-ph↗

arXiv2013

Fast magnetosonic modes of cylindrical magnetotail

FMS modes are studied in the model of the magnetotail as a cylinder with plasma sheet. The presence of the plasma sheet leads to a significant modification of the modes existing in the magnetotail in the form of a cylinder with no plasma sheet. Azimuthal scales of the FMS modes differ significantly between the lobes and the plasma sheet. The azimuthal scale in the plasma sheet is much smaller than that in the magnetotail lobes. FMS waves with certain parameters are strongly reflected from the boundary between the lobes and the plasma sheet and are very weak in the plasma sheet.

physics.space-ph↗

arXiv2013

New foundations and unification of basic plasma physics by means of classical mechanics

The derivation of Debye shielding and Landau damping from the $N$-body description of plasmas requires many pages of heavy kinetic calculations in classical textbooks and is done in distinct, unrelated chapters. Using Newton's second law for the $N$-body system, we perform this derivation in a few steps with elementary calculations using standard tools of calculus, and no probabilistic setting. Unexpectedly, Debye shielding is encountered on the way to Landau damping. The theory is extended to accommodate a correct description of trapping or chaos due to Langmuir waves, and to avoid the small amplitude assumption for the electrostatic potential. Using the shielded potential, collisional transport is computed for the first time by a convergent expression including the correct calculation of deflections for all impact parameters. Shielding and collisional transport are found to be two related aspects of the repulsive deflections of electrons.

physics.plasm-ph↗

arXiv2013

Experimental structure of gravitational wave detection by bounded cold electronic plasma in a long pipe

In the previous paper, we introduced a new method of gravitational waves (GW) detection. In our proposal, we replaced usual Weber's metallic bar with a cold electronic plasma. We obtained a nonhomogenous differential equation for tangential electric field that on it GW is known as nonhomogenous term. In this paper we estimate, the dimension of pipe, the electron density and some other associated parameters for obtaining the best detection.

gr-qc↗

arXiv2013

Effect of laser polarization on QED cascading

Development of QED cascades in a standing electromagnetic wave for circular and linear polarizations is simulated numerically with a 3D PIC-MC code. It is demonstrated that for the same laser energy the number of particles produced in a circularly polarized field is greater than in a linearly polarized field, though the acquiring mean energy per particle is larger in the latter case. The qualitative model of laser-assisted QED cascades is extended by including the effect of polarization of the field. It turns out that cascade dynamics is notably more complicated in the case of linearly polarized field, where separation into the qualitatively different "electric" and "magnetic" regions (where the electric field is stronger than the magnetic field and vice versa) becomes essential. In the "electric" regions acceleration is suppressed and moreover the high-energy electrons are even getting cooled by photon emission. The volumes of the "electric" and "magnetic" regions evolve periodically in time, and so does the cascade growth rate. In contrast to the linear polarization the charged particles can be accelerated by circularly polarized wave even in "magnetic" region. The "electric" and "magnetic" regions do not evolve in time and cascade growth rate almost does not depend on time for circular polarization.

physics.plasm-ph↗

arXiv2014

Geometric view on noneikonal waves

An axiomatic theory of classical nondissipative waves is proposed that is constructed based on the definition of a wave as a multidimensional oscillator. Waves are represented as abstract vectors $|ψ\rangle$ in the appropriately defined space $Ψ$ with a Hermitian metric. The metric is usually positive-definite but can be more general in the presence of negative-energy waves (which are typically unstable and must not be confused with negative-frequency waves). The very form of wave equations is derived from properties of $Ψ$. The generic wave equation is shown to be a quantumlike Schrodinger equation; hence one-to-one correspondence with the mathematical framework of quantum mechanics is established, and the quantum-mechanical machinery becomes applicable to classical waves "as is". The classical wave action is defined as the density operator, $|ψ\rangle\langleψ|$. The coordinate and momentum spaces, not necessarily Euclidean, need not be postulated but rather emerge when applicable. Various kinetic equations flow as projections of the von Neumann equation for $|ψ\rangle\langleψ|$. The previously known action conservation theorems for noneikonal waves and the conventional Wigner-Weyl-Moyal formalism are generalized and subsumed under a unifying invariant theory. Whitham's equations are recovered as the corresponding fluid limit in the geometrical-optics approximation. The Liouville equation is also yielded as a special case, yet in a somewhat different limit; thus ray tracing, and especially nonlinear ray tracing, is found to be more subtle than commonly assumed. Applications of this axiomatization are also discussed, briefly, for some characteristic equations.

physics.plasm-ph↗

arXiv2013

Bright gamma-rays from betatron resonance acceleration in near critical density plasma

We show that electron betatron resonance acceleration by an ultra-intense ultra-short laser pulse in a near critical density plasma works as a high-brightness gamma-ray source. Compared with laser plasma X-ray sources in under-dense plasma, near critical density plasma provides three benefits for electron radiation: more radiation electrons, larger transverse amplitude, and higher betatron oscillation frequency. Three-dimensional particle-in-cell simulations show that, by using a 7.4J laser pulse, 8.3mJ radiation with critical photon energy 1MeV is emitted. The critical photon energy $E_c$ increases with the incident laser energy %faster than a linear relation. $W_I$ as $E_c \propto W_I^{1.5}$, and the corresponding photon number is proportional to $W_I$. A simple analytical synchrotron-like radiation model is built, which can explain the simulation results.

physics.acc-ph↗

arXiv2015

A proposal for measuring photon temporal coherence in continuum radiation

A technique complementary to those for spectral lines is proposed for the observation of continuum radiation. As, quantum mechanically, the radiation is a mixture of pure states, it should be possible to measure the temporal coherence of the states as a function of energy. We propose here experiments with just this possibility, showing the empirical grounds on which it is based. A simple means of measuring temporal coherence is with the extent of the interference pattern produced in a Young's double-slit experiment. Thus, we propose the use of such an experiment, but with a screen of energy measuring photon detectors, small enough and sensitive enough to delineate the interference patterns according to photon energy. We then only register a detection if the absorbed photon is found to have the energy of interest. In this way, we have allowed the interference, i.e., the wave aspect of the experiment, to proceed as normal, recovering the interference pattern by accumulating just those registered detections. We, also, explain why this form of filtering does not actually violate the conventional wisdom that, when using an optical filter with incident continuum radiation to separate out the photons of a particular energy, only the temporal coherence of the filter's passband would be measured. It naturally then introduces the concept of a detections filter, differentiating it from an optical filter.

quant-ph↗

arXiv2013

Long-Lived Vortex Structures in Collisional Pure and Gas-Discharge Nonneutral Electron Plasmas

The analysis of experimental investigations of equilibrium, interaction and dynamics of vortex structures in pure electron and gas-discharge electron nonneutral plasmas during the time much more than the electron-neutral collision time has been carried out. The problem of long confinement of the column of pure electron plasma in Penning-Malmberg trap is considered. The mechanism of stability of long-lived vortex structure in gas-discharge nonneutral electron plasma is investigated. The collapse of electron sheath in gas-discharge nonneutral electron plasma in Penning cell at high pressures of neutral gas is described. The interaction between the stable vortex structure and the annular electron sheath, and the action of vortex structures on the transport of electrons along and across the magnetic field are discussed.

physics.plasm-ph↗

arXiv2013

Enhancement of maximum attainable ion energy in the radiation pressure acceleration regime using a guiding structure

Radiation Pressure Acceleration relies on high intensity laser pulse interacting with solid target to obtain high maximum energy, quasimonoenergetic ion beams. Either extremely high power laser pulses or tight focusing of laser radiation is required. The latter would lead to the appearance of the maximum attainable ion energy, which is determined by the laser group velocity and is highly influenced by the transverse expansion of the target. Ion acceleration is only possible with target velocities less than the group velocity of the laser. The transverse expansion of the target makes it transparent for radiation, thus reducing the effectiveness of acceleration. Utilization of an external guiding structure for the accelerating laser pulse may provide a way of compensating for the group velocity and transverse expansion effects.

physics.plasm-ph↗

arXiv2013

160 MeV laser-accelerated protons from CH2 nano-targets for proton cancer therapy

Proton (and ion) cancer therapy has proven to be an extremely effective even supe-rior method of treatment for some tumors 1-4. A major problem, however, lies in the cost of the particle accelerator facilities; high procurement costs severely limit the availability of ion radiation therapy, with only ~26 centers worldwide. Moreover, high operating costs often prevent economic operation without state subsidies and have led to a shutdown of existing facilities 5,6. Laser-accelerated proton and ion beams have long been thought of as a way out of this dilemma, with the potential to provide the required ion beams at lower cost and smaller facility footprint 7-14. The biggest challenge has been the achievement of sufficient particle energy for therapy, in the 150-250 MeV range for protons 15,16. For the last decade, the maximum exper-imentally observed energy of laser-accelerated protons has remained at ~60 MeV 17. Here we the experimental demonstration of laser-accelerated protons to energies exceeding 150 MeV, reaching the therapy window. This was achieved through a dif-ferent acceleration regime rather than a larger laser, specifically a 150 TW laser with CH2 nano-targets in the relativistically transparent regime 18,19. We also demonstrate a clear scaling law with laser intensity based on analytical theory, computer simulations and experimental validation that will enable design of a pro-totype system spanning the full range of therapeutically desirable energies.

physics.plasm-ph↗

arXiv2013

Magnetic Generation due to Mass Difference between Charge Carriers

The possibility of spontaneous magnetization due to the "asymmetry in mass" of charge carriers in a system is investigated. Analysis shows that when the masses of positive and negative charge carriers are identical, no magnetization is predicted. However, if the masses of two species are different, spontaneous magnetic field would appear, either due to the equipartition of magnetic energy or due to fluctuations together with a feedback mechanism. The conditions for magnetization to occur are also obtained, in the form of n-T phase diagram. The theory proposed here, if confirmed by future observations and/or experiments, would provide a new insight on the origin of magnetic fields in the universe.

physics.plasm-ph↗

arXiv2014

Fabrication of high quality GaN nanopillar arrays by dry and wet chemical etching

We study strain relaxation and surface damage of GaN nanopillar arrays fabricated using inductively coupled plasma (ICP) etching and post etch wet chemical treatment. We controlled the shape and surface damage of such nanopillar structures through selection of etching parameters. We compared different substrate temperatures and different chlorine-based etch chemistries to fabricate high quality GaN nanopillars. Room temperature photoluminescence and Raman scattering measurements were carried to study the presence of surface defect and strain relaxation on these nanostructures, respectively. We found that wet KOH etching can remove the side wall damages caused by dry plasma etching, leading to better quality of GaN nanopillars arrays. The Si material underneath the GaN pillars was removed by KOH wet etching, leaving behind a fine Si pillar to support the GaN structure. Substantial strain relaxations were observed in these structures from room temperature Raman spectroscopy measurements. Room temperature Photoluminescence spectroscopy shows the presence of whispering gallery modes from these the nano disks structures.

cond-mat.mtrl-sci↗

arXiv2013

Modeling of radiative and quantum electrodynamics effects in PIC simulations of ultra-relativistic laser-plasma interaction

Next generation of ultra-intense laser facilities will lead to novel physical conditions ruled by collective and quantum electrodynamics effects, such as synchrotron-like emission of high-energy photons and electron-positron pair generation. In view of the future experiments performed in this regime, the latter processes have been implemented into the particle-in-cell code CALDER.

physics.plasm-ph↗

arXiv2013

Recent progress on phase-space turbulence and dynamical response in collisionless plasmas

In the presence of wave dissipation, phase-space structures emerge in nonlinear Vlasov dynamics. Their dynamics can lead to a nonlinear continuous shifting of the wave frequency (chirping). This report summarizes my personal contribution to these topics in the fiscal year 2012. The effects of collisions on chirping characteristics were investigated, with a one-dimensional beam-plasma kinetic model. The long-time nonlinear evolution was systematically categorized as damped, steady-state, periodic, chaotic and chirping. The chirping regime was sub-categorized as periodic, chaotic, bursty, and intermittent. Existing analytic theory was extended to account for Krook-like collisions. Relaxation oscillations, associated with chirping bursts, were investigated in the presence of dynamical friction and velocity-diffusion. The period increases with decreasing drag, and weakly increases with decreasing diffusion. A new theory gives a simple relation between the growth of phase-space structures and that of the wave energy. When dissipation is modeled by a linear term in the field equation, simple expressions of a single hole growth rate and of the initial perturbation threshold are in agreement with numerical simulations.

physics.plasm-ph↗

arXiv2013

Discrete Filters for Large Eddy Simulation of Forced Compressible MHD Turbulence

In present study, we discuss results of applicability of discrete filters for large eddy simulation (LES) method of forced compressible magnetohydrodynamic (MHD) turbulent flows with the scale-similarity model. Influences and effects of discrete filter shapes on the scale-similarity model are examined in physical space using a finite-difference numerical schemes. We restrict ourselves to the Gaussian filter and the top-hat filter. Representations of this subgrid-scale model which correspond to various 3- and 5-point approximations of both Gaussian and top-hat filters for different values of parameter $ε$ (the ratio of the mesh size to the cut-off lengthscale of the filter) are investigated. Discrete filters produce more discrepancies for magnetic field. It is shown that the Gaussian filter is more sensitive to the parameter $ε$ than the top-hat filter in compressible forced MHD turbulence. The 3-point filters at $ε=2$ and $ε=3$ give the least accurate results and the 5-point Gaussian filter shows the best results at $ε=2$.

physics.plasm-ph↗

arXiv2014

ELM Processes and Properties in 2T 2MA JET Plasmas

During July 2012, 150 almost identical H-mode plasmas were consecutively created in the Joint European Torus (JET), providing a combined total of approximately 8 minutes of steady-state plasma with 15,000 Edge Localised Modes (ELMs). In principle, each of those 15,000 ELMs are statistically equivalent. Here the changes in edge density and plasma energy associated with those ELMs are explored, using the spikes in Beryllium II (527 nm) radiation as an indicator for the onset of an ELM. Two timescales are found during the ELM process. The first timescale is associated with the density drop, and the second (longer) timescale is associated with a reduction in MHD energy that is consistent with a resistive relaxation of the plasma's edge. The statistical properties of the energy and density losses due to the ELMs are explored. Surprisingly the commonly reported link between ELM energy (E) and ELM frequency (f), of E proportional to 1/f, is not found. Instead beyond the first 0.02 seconds of waiting time between ELMs, the energy losses due to the ELMs are found to be statistically the same. Surprisingly no correlation is found between the energies of consecutive ELMs either. A weak link is found between the density drop and the ELM waiting time. Consequences of these results for ELM control and modelling are discussed.

physics.plasm-ph↗