168 research outputs found
Superdiffusive and Subdiffusive Transport of Energetic Particles in Solar Wind Anisotropic Magnetic Turbulence
The transport of energetic particles in a mean magnetic field and the presence of anisotropic magnetic turbulence are studied numerically, for parameter values relevant to the solar wind. A numerical realization of magnetic turbulence is set up in which we can vary the type of anisotropy by changing the correlation lengths lx, ly, lz. We find that for lx, ly lz, transport can be non-Gaussian, with superdiffusion along the average magnetic field and subdiffusion perpendicular to it. Decreasing the lx/lz ratio down to 0.3, Gaussian diffusion is obtained, showing that the transport regime depends on the turbulence anisotropy. Implications for energetic particle propagation in the solar wind and for diffusive shock acceleration are discussed
Model for the spatio-temporal intermittency of the energy dissipation in turbulent flows
Modeling the intermittent behavior of turbulent energy dissipation processes
both in space and time is often a relevant problem when dealing with phenomena
occurring in high Reynolds number flows, especially in astrophysical and space
fluids. In this paper, a dynamical model is proposed to describe the
spatio-temporal intermittency of energy dissipation rate in a turbulent system.
This is done by using a shell model to simulate the turbulent cascade and
introducing some heuristic rules, partly inspired by the well known -model,
to construct a spatial structure of the energy dissipation rate. In order to
validate the model and to study its spatially intermittency properties, a
series of numerical simulations have been performed. These show that the level
of spatial intermittency of the system can be simply tuned by varying a single
parameter of the model and that scaling laws in agreement with those obtained
from experiments on fully turbulent hydrodynamic flows can be recovered. It is
finally suggested that the model could represent a useful tool to simulate the
spatio-temporal intermittency of turbulent energy dissipation in those high
Reynolds number astrophysical fluids where impulsive energy release processes
can be associated to the dynamics of the turbulent cascade.Comment: 22 pages, 9 figure
Fourier-Hermite decomposition of the collisional Vlasov-Maxwell system: Implications for the velocity-space cascade
Turbulence at kinetic scales is an unresolved and ubiquitous phenomenon that characterizes both space and laboratory plasmas. Recently, new theories, {\it in-situ} spacecraft observations and numerical simulations suggest a novel scenario for turbulence, characterized by a so-called phase space cascade -- the formation of fine structures, both in physical and velocity space. This new concept is here extended by directly taking into account the role of inter-particle collisions, modeled through the nonlinear Landau operator or the simplified Dougherty operator. The characteristic times, associated with inter-particle correlations, are derived in the above cases. The implications of introducing collisions on the phase space cascade are finally discussed
ViDA: a VlasovDArwin solver for plasma physics at electron scales
We present a VlasovâDArwin numerical code (ViDA) specifically designed to
address plasma physics problems, where small-scale high accuracy is requested
even during the nonlinear regime to guarantee a clean description of the plasma
dynamics at fine spatial scales. The algorithm provides a low-noise description of
proton and electron kinetic dynamics, by splitting in time the multi-advection Vlasov
equation in phase space. Maxwell equations for the electric and magnetic fields are
reorganized according to the Darwin approximation to remove light waves. Several
numerical tests show that ViDA successfully reproduces the propagation of linear and
nonlinear waves and captures the physics of magnetic reconnection. We also discuss
preliminary tests of the parallelization algorithm efficiency, performed at CINECA
on the Marconi-KNL cluster. ViDA will allow the running of Eulerian simulations
of a non-relativistic fully kinetic collisionless plasma and it is expected to provide
relevant insights into important problems of plasma astrophysics such as, for instance,
the development of the turbulent cascade at electron scales and the structure and
dynamics of electron-scale magnetic reconnection, such as the electron diffusion
region
Les droits disciplinaires des fonctions publiques : « unification », « harmonisation » ou « distanciation ». A propos de la loi du 26 avril 2016 relative à la déontologie et aux droits et obligations des fonctionnaires
The production of tt⟠, W+bb⟠and W+cc⟠is studied in the forward region of protonâproton collisions collected at a centre-of-mass energy of 8 TeV by the LHCb experiment, corresponding to an integrated luminosity of 1.98±0.02 fbâ1 . The W bosons are reconstructed in the decays WââÎœ , where â denotes muon or electron, while the b and c quarks are reconstructed as jets. All measured cross-sections are in agreement with next-to-leading-order Standard Model predictions.The production of , and is studied in the forward region of proton-proton collisions collected at a centre-of-mass energy of 8 TeV by the LHCb experiment, corresponding to an integrated luminosity of 1.98 0.02 \mbox{fb}^{-1}. The bosons are reconstructed in the decays , where denotes muon or electron, while the and quarks are reconstructed as jets. All measured cross-sections are in agreement with next-to-leading-order Standard Model predictions
Measurement of the J/Ï pair production cross-section in pp collisions at TeV
The production cross-section of J/Ï pairs is measured using a data sample of pp collisions collected by the LHCb experiment at a centre-of-mass energy of TeV, corresponding to an integrated luminosity of 279 ±11 pb. The measurement is performed for J/Ï mesons with a transverse momentum of less than 10 GeV/c in the rapidity range 2.0 < y < 4.5. The production cross-section is measured to be 15.2 ± 1.0 ± 0.9 nb. The first uncertainty is statistical, and the second is systematic. The differential cross-sections as functions of several kinematic variables of the J/Ï pair are measured and compared to theoretical predictions.The production cross-section of pairs is measured using a data sample of collisions collected by the LHCb experiment at a centre-of-mass energy of , corresponding to an integrated luminosity of . The measurement is performed for mesons with a transverse momentum of less than in the rapidity range . The production cross-section is measured to be . The first uncertainty is statistical, and the second is systematic. The differential cross-sections as functions of several kinematic variables of the pair are measured and compared to theoretical predictions
Measurement of forward production in collisions at TeV
A measurement of the cross-section for production in collisions is presented using data corresponding to an integrated luminosity of fb collected by the LHCb experiment at a centre-of-mass energy of TeV. The electrons are required to have more than GeV of transverse momentum and to lie between 2.00 and 4.25 in pseudorapidity. The inclusive production cross-sections, where the decays to , are measured to be \begin{align*} \begin{split} \sigma_{W^{+} \to e^{+}\nu_{e}}&=1124.4\pm 2.1\pm 21.5\pm 11.2\pm 13.0\,\mathrm{pb},\\ \sigma_{W^{-} \to e^{-}\bar{\nu}_{e}}&=\,\,\,809.0\pm 1.9\pm 18.1\pm\,\,\,7.0\pm \phantom{0}9.4\,\mathrm{pb}, \end{split} \end{align*} where the first uncertainties are statistical, the second are systematic, the third are due to the knowledge of the LHC beam energy and the fourth are due to the luminosity determination. Differential cross-sections as a function of the electron pseudorapidity are measured. The cross-section ratio and production charge asymmetry are also reported. Results are compared with theoretical predictions at next-to-next-to-leading order in perturbative quantum chromodynamics. Finally, in a precise test of lepton universality, the ratio of boson branching fractions is determined to be \begin{align*} \begin{split} \mathcal{B}(W \to e\nu)/\mathcal{B}(W \to \mu\nu)=1.020\pm 0.002\pm 0.019, \end{split} \end{align*} where the first uncertainty is statistical and the second is systematic.A measurement of the cross-section for production in collisions is presented using data corresponding to an integrated luminosity of fb collected by the LHCb experiment at a centre-of-mass energy of TeV. The electrons are required to have more than GeV of transverse momentum and to lie between 2.00 and 4.25 in pseudorapidity. The inclusive production cross-sections, where the decays to , are measured to be \begin{equation*} \sigma_{W^{+} \to e^{+}\nu_{e}}=1124.4\pm 2.1\pm 21.5\pm 11.2\pm 13.0\,\mathrm{pb}, \end{equation*} \begin{equation*} \sigma_{W^{-} \to e^{-}\bar{\nu}_{e}}=\,\,\,809.0\pm 1.9\pm 18.1\pm\,\,\,7.0\pm \phantom{0}9.4\,\mathrm{pb}, \end{equation*} where the first uncertainties are statistical, the second are systematic, the third are due to the knowledge of the LHC beam energy and the fourth are due to the luminosity determination. Differential cross-sections as a function of the electron pseudorapidity are measured. The cross-section ratio and production charge asymmetry are also reported. Results are compared with theoretical predictions at next-to-next-to-leading order in perturbative quantum chromodynamics. Finally, in a precise test of lepton universality, the ratio of boson branching fractions is determined to be \begin{equation*} \mathcal{B}(W \to e\nu)/\mathcal{B}(W \to \mu\nu)=1.020\pm 0.002\pm 0.019, \end{equation*} where the first uncertainty is statistical and the second is systematic.A measurement of the cross-section for W â eÎœ production in pp collisions is presented using data corresponding to an integrated luminosity of 2 fb collected by the LHCb experiment at a centre-of-mass energy of TeV. The electrons are required to have more than 20 GeV of transverse momentum and to lie between 2.00 and 4.25 in pseudorapidity. The inclusive W production cross-sections, where the W decays to eÎœ, are measured to be where the first uncertainties are statistical, the second are systematic, the third are due to the knowledge of the LHC beam energy and the fourth are due to the luminosity determination
Measurements of prompt charm production cross-sections in pp collisions at TeV
Production cross-sections of prompt charm mesons are measured using data from collisions at the LHC at a centre-of-mass energy of TeV. The data sample corresponds to an integrated luminosity of pb collected by the LHCb experiment. The production cross-sections of , , , and mesons are measured in bins of charm meson transverse momentum, , and rapidity, . They cover the rapidity range and transverse momentum ranges for and and for and mesons. The inclusive cross-sections for the four mesons, including charge-conjugate states, within the range of are determined to be \begin{equation*} \sigma(pp\rightarrow D^0 X) = 1190 \pm 3 \pm 64\,\mu\text{b} \end{equation*} \begin{equation*} \sigma(pp\rightarrow D^+ X) = 456 \pm 3 \pm 34\,\mu\text{b} \end{equation*} \begin{equation*} \sigma(pp\rightarrow D_s^+ X) = 195 \pm 4 \pm 19\,\mu\text{b} \end{equation*} \begin{equation*} \sigma(pp\rightarrow D^{*+} X)= 467 \pm 6 \pm 40\,\mu\text{b} \end{equation*} where the uncertainties are statistical and systematic, respectively.Production cross-sections of prompt charm mesons are measured using data from pp collisions at the LHC at a centre-of-mass energy of 5 TeV. The data sample corresponds to an integrated luminosity of 8.60 ± 0.33 pb collected by the LHCb experiment. The production cross-sections of D, D, D , and D mesons are measured in bins of charm meson transverse momentum, p, and rapidity, y. They cover the rapidity range 2.0 < y < 4.5 and transverse momentum ranges 0 < p < 10 GeV/c for D and D and 1 < p < 10 GeV/c for D and D mesons. The inclusive cross-sections for the four mesons, including charge-conjugate states, within the range of 1 < p < 8 GeV/c are determined to be where the uncertainties are statistical and systematic, respectively.Production cross-sections of prompt charm mesons are measured using data from collisions at the LHC at a centre-of-mass energy of TeV. The data sample corresponds to an integrated luminosity of pb collected by the LHCb experiment. The production cross-sections of , , , and mesons are measured in bins of charm meson transverse momentum, , and rapidity, . They cover the rapidity range and transverse momentum ranges for and and for and mesons. The inclusive cross-sections for the four mesons, including charge-conjugate states, within the range of are determined to be \sigma(pp\rightarrow D^0 X) = 1004 \pm 3 \pm 54\,\mu\text{b} \sigma(pp\rightarrow D^+ X) = 402 \pm 2 \pm 30\,\mu\text{b} \sigma(pp\rightarrow D_s^+ X) = 170 \pm 4 \pm 16\,\mu\text{b} \sigma(pp\rightarrow D^{*+} X)= 421 \pm 5 \pm 36\,\mu\text{b} where the uncertainties are statistical and systematic, respectively
Improvements of the magnetic field design for SPIDER and MITICA negative ion beam sources
The design of the magnetic field configuration in the SPIDER and MITICA negative ion beam sources has evolved considerably during the past four years. This evolution was driven by three factors: 1) the experimental results of the large RF-driven ion sources at IPP, which have provided valuable indications on the optimal magnetic configurations for reliable RF plasma source operation and for large negative ion current extraction, 2) the comprehensive beam optics and heat load simulations, which showed that the magnetic field configuration in the accelerator is crucial for keeping the heat load due to electrons on the accelerator grids within tolerable limits, without compromising the optics of the negative ion beam in the foreseen operating scenarios, 3) the progress of the detailed mechanical design of the accelerator, which stimulated the evaluation of different solutions for the correction of beamlet deflections of various origin and for beamlet aiming. On this basis, new requirements and solution concepts for the magnetic field configuration in the SPIDER and MITICA beam sources have been progressively introduced and updated until the design converged. The paper presents how these concepts have been integrated into a final design solution based on a horizontal "long- range" field (few mT) in combination with a "local" vertical field of some tens of mT on the acceleration grids
Cancellation of the ion deflection due to electron-suppression magnetic field in a negative-ion accelerator
A new magnetic configuration is proposed for the suppression of co-extracted electrons in a negative- ion accelerator. This configuration is produced by an arrangement of permanent magnets embedded in one accelerator grid and creates an asymmetric local magnetic field on the upstream and downstream sides of this grid. Thanks to the \u201cconcentration\u201d of the magnetic field on the upstream side of the grid, the resulting deflection of the ions due to magnetic field can be \u201cintrinsically\u201d cancelled by calibrating the configuration of permanent magnets. At the same time, the suppression of co-extracted electrons can be improved
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