Articoli di riviste: "Simulations HPC de plasma turbulent"

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Bouzat, Nicolas, Camilla Bressan, Virginie Grandgirard, Guillaume Latu e Michel Mehrenberger. "Targeting Realistic Geometry in Tokamak Code Gysela". ESAIM: Proceedings and Surveys 63 (2018): 179–207. http://dx.doi.org/10.1051/proc/201863179.

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In magnetically confined plasmas used in Tokamak, turbulence is respon-sible for specific transport that limits the performance of this kind of reactors. Gyroki-netic simulations are able to capture ion and electron turbulence that give rise to heat losses, but require also state-of-the-art HPC techniques to handle computation costs. Such simulations are a major tool to establish good operating regime in Tokamak such as ITER, which is currently being built. Some of the key issues to address more re- alistic gyrokinetic simulations are: efficient and robust numerical schemes, accurate geometric description, good parallelization algorithms. The framework of this work is the Semi-Lagrangian setting for solving the gyrokinetic Vlasov equation and the Gy-sela code. In this paper, a new variant for the interpolation method is proposed that can handle the mesh singularity in the poloidal plane at r = 0 (polar system is used for the moment in Gysela). A non-uniform meshing of the poloidal plane is proposed instead of uniform one in order to save memory and computations. The interpolation method, the gyroaverage operator, and the Poisson solver are revised in order to cope with non-uniform meshes. A mapping that establish a bijection from polar coordinates to more realistic plasma shape is used to improve realism. Convergence studies are provided to establish the validity and robustness of our new approach.

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Veltri, P., G. Nigro, F. Malara, V. Carbone e A. Mangeney. "Intermittency in MHD turbulence and coronal nanoflares modelling". Nonlinear Processes in Geophysics 12, n. 2 (9 febbraio 2005): 245–55. http://dx.doi.org/10.5194/npg-12-245-2005.

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Abstract. High resolution numerical simulations, solar wind data analysis, and measurements at the edges of laboratory plasma devices have allowed for a huge progress in our understanding of MHD turbulence. The high resolution of solar wind measurements has allowed to characterize the intermittency observed at small scales. We are now able to set up a consistent and convincing view of the main properties of MHD turbulence, which in turn constitutes an extremely efficient tool in understanding the behaviour of turbulent plasmas, like those in solar corona, where in situ observations are not available. Using this knowledge a model to describe injection, due to foot-point motions, storage and dissipation of MHD turbulence in coronal loops, is built where we assume strong longitudinal magnetic field, low beta and high aspect ratio, which allows us to use the set of reduced MHD equations (RMHD). The model is based on a shell technique in the wave vector space orthogonal to the strong magnetic field, while the dependence on the longitudinal coordinate is preserved. Numerical simulations show that injected energy is efficiently stored in the loop where a significant level of magnetic and velocity fluctuations is obtained. Nonlinear interactions give rise to an energy cascade towards smaller scales where energy is dissipated in an intermittent fashion. Due to the strong longitudinal magnetic field, dissipative structures propagate along the loop, with the typical speed of the Alfvén waves. The statistical analysis on the intermittent dissipative events compares well with all observed properties of nanoflare emission statistics. Moreover the recent observations of non thermal velocity measurements during flare occurrence are well described by the numerical results of the simulation model. All these results naturally emerge from the model dynamical evolution without any need of an ad-hoc hypothesis.

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Cranmer, Steven R., e Momchil E. Molnar. "Magnetohydrodynamic Mode Conversion in the Solar Corona: Insights from Fresnel-like Models of Waves at Sharp Interfaces". Astrophysical Journal 955, n. 1 (1 settembre 2023): 68. http://dx.doi.org/10.3847/1538-4357/acee6c.

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Abstract The solar atmosphere is known to contain many different types of wave-like oscillation. Waves and other fluctuations (e.g., turbulent eddies) are believed to be responsible for at least some of the energy transport and dissipation that heats the corona and accelerates the solar wind. Thus, it is important to understand the behavior of magnetohydrodynamic (MHD) waves as they propagate and evolve in different regions of the Sun’s atmosphere. In this paper, we investigate how MHD waves can affect the overall plasma state when they reflect and refract at sharp, planar interfaces in density. First, we correct an error in a foundational paper (Stein) that affects the calculation of wave energy-flux conservation. Second, we apply this model to reflection-driven MHD turbulence in the solar wind, where the presence of density fluctuations can enhance the generation of inward-propagating Alfvén waves. This model reproduces the time-averaged Elsässer imbalance fraction (i.e., the ratio of inward to outward Alfvénic power) from several published numerical simulations. Lastly, we model how the complex magnetic field threading the transition region (TR) between the chromosphere and corona helps convert a fraction of upward-propagating Alfvén waves into fast-mode and slow-mode MHD waves. These magnetosonic waves dissipate in a narrow region around the TR and produce a sharp peak in the heating rate. This newly found source of heating sometimes exceeds the expected heating rate from Alfvénic turbulence by an order of magnitude. It may explain why some earlier models seemed to require an additional ad hoc heat source at this location.

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Sharma, A. Y., M. D. J. Cole, T. Görler, Y. Chen, D. R. Hatch, W. Guttenfelder, R. Hager et al. "Global gyrokinetic study of shaping effects on electromagnetic modes at NSTX aspect ratio with ad hoc parallel magnetic perturbation effects". Physics of Plasmas 29, n. 11 (novembre 2022): 112503. http://dx.doi.org/10.1063/5.0106925.

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Plasma shaping may have a stronger effect on global turbulence in tight-aspect-ratio tokamaks than in conventional-aspect-ratio tokamaks due to the higher toroidicity and more acute poloidal asymmetry in the magnetic field. In addition, previous local gyrokinetic studies have shown that it is necessary to include parallel magnetic field perturbations in order to accurately compute growth rates of electromagnetic modes in tight-aspect-ratio tokamaks. In this work, the effects of elongation and triangularity on global, ion-scale, linear electromagnetic modes are studied at National Spherical Torus Experiment (NSTX) aspect ratio and high plasma β using the global gyrokinetic particle-in-cell code XGC. The effects of compressional magnetic perturbations are approximated via a well-known modification to the particle drifts that was developed for flux-tube simulations [Joiner et al., Phys. Plasmas 17, 072104 (2010)], without proof of its validity in a global simulation, with the gyrokinetic codes GENE and GEM being used for local verification and global cross-verification. Magnetic equilibria are re-constructed for each distinct plasma profile that is used. Coulomb collision effects are not considered. Within the limitations imposed by the present study, it is found that linear growth rates of electromagnetic modes (collisionless microtearing modes and kinetic ballooning modes) are significantly reduced in a high-elongation and high-triangularity NSTX-like geometry compared to a circular NSTX-like geometry. For example, growth rates of kinetic ballooning modes at high- β are reduced to the level of that of collisionless trapped electron modes.

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Baudoin, Camille, Patrick Tamain, Hugo Bufferand, Guido Ciraolo, Nicolas Fedorczak, Davide Galassi, Philippe Ghendrih e Nicolas Nace. "Turbulent heat transport in TOKAM3X edge plasma simulations". Contributions to Plasma Physics 58, n. 6-8 (6 giugno 2018): 484–89. http://dx.doi.org/10.1002/ctpp.201700168.

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Rincon, François, Francesco Califano, Alexander A. Schekochihin e Francesco Valentini. "Turbulent dynamo in a collisionless plasma". Proceedings of the National Academy of Sciences 113, n. 15 (29 marzo 2016): 3950–53. http://dx.doi.org/10.1073/pnas.1525194113.

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Magnetic fields pervade the entire universe and affect the formation and evolution of astrophysical systems from cosmological to planetary scales. The generation and dynamical amplification of extragalactic magnetic fields through cosmic times (up to microgauss levels reported in nearby galaxy clusters, near equipartition with kinetic energy of plasma motions, and on scales of at least tens of kiloparsecs) are major puzzles largely unconstrained by observations. A dynamo effect converting kinetic flow energy into magnetic energy is often invoked in that context; however, extragalactic plasmas are weakly collisional (as opposed to magnetohydrodynamic fluids), and whether magnetic field growth and sustainment through an efficient turbulent dynamo instability are possible in such plasmas is not established. Fully kinetic numerical simulations of the Vlasov equation in a 6D-phase space necessary to answer this question have, until recently, remained beyond computational capabilities. Here, we show by means of such simulations that magnetic field amplification by dynamo instability does occur in a stochastically driven, nonrelativistic subsonic flow of initially unmagnetized collisionless plasma. We also find that the dynamo self-accelerates and becomes entangled with kinetic instabilities as magnetization increases. The results suggest that such a plasma dynamo may be realizable in laboratory experiments, support the idea that intracluster medium turbulence may have significantly contributed to the amplification of cluster magnetic fields up to near-equipartition levels on a timescale shorter than the Hubble time, and emphasize the crucial role of multiscale kinetic physics in high-energy astrophysical plasmas.

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Gleize, Vincent, Michel Costes e Ivan Mary. "Numerical simulation of NACA4412 airfoil in pre-stall conditions". International Journal of Numerical Methods for Heat & Fluid Flow 32, n. 4 (30 novembre 2021): 1375–97. http://dx.doi.org/10.1108/hff-07-2021-0514.

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Purpose The purpose of this paper is to study turbulent flow separation at the airfoil trailing edge. This work aims to improve the knowledge of stall phenomenon by creating a QDNS database for the NACA412 airfoil. Design/methodology/approach Quasi-DNS simulations of the NACA 4412 airfoil in pre-stall conditions have been completed. The Reynolds number based on airfoil chord and freestream velocity is equal to 0.35 million, and the freestream Mach number to 0.117. Transition is triggered on both surfaces for avoiding the occurrence of laminar separation bubbles and to ensure turbulent mixing in the wake. Four incidences have been considered, 5, 8 10 and 11 degrees. Findings The results obtained show a reasonably good correlation of the present simulations with classical MSES airfoil simulations and with RANS computations, both in terms of pressure and skin-friction distribution, with an earlier and more extended flow separation in the QDNS. The database thus generated will be deeply analysed and enriched for larger incidences in the future. Originality/value No experimental or HPC numerical database at reasonable Reynolds number exists in the literature. The current work is the first step in that direction.

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Timofeev, I. V., e A. V. Terekhov. "Simulations of turbulent plasma heating by powerful electron beams". Physics of Plasmas 17, n. 8 (agosto 2010): 083111. http://dx.doi.org/10.1063/1.3474952.

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Timofeev, I. V., e A. V. Terekhov. "Simulations of Turbulent Plasma Heating by Powerful Electron Beams". Fusion Science and Technology 59, n. 1T (gennaio 2011): 70–73. http://dx.doi.org/10.13182/fst11-a11577.

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Kitiashvili, I. N., A. G. Kosovichev, A. A. Wray e N. N. Mansour. "Realistic MHD simulations of magnetic self-organization in solar plasma". Proceedings of the International Astronomical Union 6, S274 (settembre 2010): 120–24. http://dx.doi.org/10.1017/s1743921311006703.

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AbstractFilamentary structure is a fundamental property of the magnetized solar plasma. Recent high-resolution observations and numerical simulations have revealed close links between the filamentary structures and plasma dynamics in large-scale solar phenomena, such as sunspots and magnetic network. A new emerging paradigm is that the mechanisms of the filamentary structuring and large-scale organization are natural consequences of turbulent magnetoconvection on the Sun. We present results of 3D radiative MHD large-eddy simulations (LES) of magnetic structures in the turbulent convective boundary layer of the Sun. The results show how the initial relatively weak and uniformly distributed magnetic field forms the filamentary structures, which under certain conditions gets organized on larger scales, creating stable long-living magnetic structures. We discuss the physics of magnetic self-organization in the turbulent solar plasma, and compare the simulation results with observations.

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Nunami, M., S. Toda, M. Nakata e H. Sugama. "Improved prediction scheme for ion heat turbulent transport". Physics of Plasmas 29, n. 10 (ottobre 2022): 102505. http://dx.doi.org/10.1063/5.0103447.

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A novel scheme to predict the turbulent transport of ion heat of magnetic confined plasmas is developed by combining mathematical optimization techniques employed in data analysis approaches and first-principle gyrokinetic simulations. Gyrokinetic simulation, as a first-principle approach, is a reliable way to predict turbulent transport. However, in terms of the flux-matching [Candy et al., Phys. Plasmas 16, 060704 (2009)], quantitative transport estimates by gyrokinetic simulations incur extremely heavy computational costs. In order to reduce the costs of quantitative transport prediction based on the gyrokinetic simulations, we develop a scheme with the aid of a reduced transport model. In the scheme, optimization techniques are applied to find relevant input parameters for nonlinear gyrokinetic simulations, which should be performed to obtain relevant transport fluxes and to optimize the reduced transport model for a target plasma. The developed scheme can reduce the numbers of the gyrokinetic simulations to perform the quantitative estimate of the turbulent transport levels and plasma profiles. Utilizing the scheme, the predictions for the turbulent transport can be realized by performing the first-principle simulations once for each radial position.

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Pucci, F., M. Viviani, F. Valentini, G. Lapenta, W. H. Matthaeus e S. Servidio. "Turbulent Magnetogenesis in a Collisionless Plasma". Astrophysical Journal Letters 922, n. 1 (1 novembre 2021): L18. http://dx.doi.org/10.3847/2041-8213/ac36cf.

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Abstract We demonstrate an efficient mechanism for generating magnetic fields in turbulent, collisionless plasmas. By using fully kinetic, particle-in-cell simulations of an initially nonmagnetized plasma, we inspect the genesis of magnetization, in a nonlinear regime. The complex motion is initiated via a Taylor–Green vortex, and the plasma locally develops strong electron temperature anisotropy, due to the strain tensor of the turbulent flow. Subsequently, in a domino effect, the anisotropy triggers a Weibel instability, localized in space. In such active wave–particle interaction regions, the seed magnetic field grows exponentially and spreads to larger scales due to the interaction with the underlying stirring motion. Such a self-feeding process might explain magnetogenesis in a variety of astrophysical plasmas, wherever turbulence is present.

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Oyarzun, Guillermo, e Athanassios Dimas. "TURBULENT OSCILLATORY FLOW OVER RIPPLES AT HIGH REYNOLDS NUMBERS FOR PETA-SCALE SIMULATIONS". Coastal Engineering Proceedings, n. 36 (30 dicembre 2018): 95. http://dx.doi.org/10.9753/icce.v36.sediment.95.

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Surface waves in the coastal zone induce oscillatory flow motions in the vicinity of the seabed. These wave-induced coastal flows interact with the sandy seabed and modify the bed shape by generating coherent small-scale bed structures, which are generally known as ripples. The presence of ripples in oscillatory flows is important due to the impact they have on the seabed roughness and how they affect the near-bed boundary layer hydrodynamics. Simulations of higher and more real-scale Reynolds number (Re) require the use of supercomputers in order to obtain results in a reasonable amount of time. However, the constant evolution of the computing facilities makes the development of parallel algorithms a rather difficult task. The objective of the proposed research is to advance in the comprehension of coastal processes utilizing high performance computing (HPC) for the numerical simulation of the three-dimensional, turbulent flow, which is induced in the coastal zone by wave propagation. In particular, our CFD code (SimuCoast) has been developed using a hybrid MPI+OpenACC execution model that increases its scalability and allows it to engage the vast majority of high-end supercomputers. Special attention has been paid in the parallelization strategy of the Poisson solver that is the most computational demanding operation.

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SHAIKH, DASTGEER, e G. P. ZANK. "Turbulent spectra in the solar wind plasma". Journal of Plasma Physics 76, n. 2 (29 luglio 2009): 183–91. http://dx.doi.org/10.1017/s0022377809990237.

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AbstractObservations of interstellar scintillations at radio wavelengths reveal a Kolmogorov-like scaling of the electron density spectrum with a spectral slope of −5/3 over six decades in wavenumber space. A similar turbulent density spectrum in the solar wind plasma has been reported. The energy transfer process in the magnetized solar wind plasma over such extended length scales remains an unresolved paradox of modern turbulence theories, raising the especially intriguing question of how a compressible magnetized solar wind exhibits a turbulent spectrum that is a characteristic of an incompressible hydrodynamic fluid. To address these questions, we have undertaken three-dimensional time-dependent numerical simulations of a compressible magnetohydrodynamic fluid describing super-Alfvénic, supersonic and strongly magnetized plasma. It is shown that the observed Kolmogorov-like (−5/3) spectrum can develop in the solar wind plasma by supersonic plasma motions that dissipate into highly subsonic motion that passively convect density fluctuations.

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Thévenin, Sébastien, Nicolas Valade, Benoît-Joseph Gréa, Gilles Kluth e Olivier Soulard. "Modeling compressed turbulent plasma with rapid viscosity variations". Physics of Plasmas 29, n. 11 (novembre 2022): 112310. http://dx.doi.org/10.1063/5.0115272.

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We propose two-equation models in order to capture the dynamics of a turbulent plasma undergoing compression and experiencing large viscosity variations. The models account for possible relaminarization phases and rapid viscosity changes through closures dependent on the turbulent Reynolds and on the viscosity Froude numbers. These closures are determined from a data-driven approach using eddy-damped quasi-normal Markovian simulations. The best model is able to mimic the various self-similar regimes identified in Viciconte et al. [Phys. Rev. E 97, 023201 (1998)] and to recover the rapid transition limits identified by G. N. Coleman and N. N. Mansour [Phys. Fluids A 3, 2255 (1991)].

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Theilhaber, K., G. Laval e D. Pesme. "Numerical simulations of turbulent trapping in the weak beam–plasma instability". Physics of Fluids 30, n. 10 (1987): 3129. http://dx.doi.org/10.1063/1.866488.

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Reynolds-Barredo, J. M., D. E. Newman, R. Sanchez, D. Samaddar, L. A. Berry e W. R. Elwasif. "Mechanisms for the convergence of time-parallelized, parareal turbulent plasma simulations". Journal of Computational Physics 231, n. 23 (ottobre 2012): 7851–67. http://dx.doi.org/10.1016/j.jcp.2012.07.028.

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NISHIKAWA, K. I., J. NIMIEC, M. MEDVEDEV, B. ZHANG, P. HARDEE, Y. MIZUNO, Å. NORDLUND et al. "RADIATION FROM RELATIVISTIC SHOCKS WITH TURBULENT MAGNETIC FIELDS". International Journal of Modern Physics D 19, n. 06 (giugno 2010): 715–21. http://dx.doi.org/10.1142/s0218271810016865.

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Using our new 3D relativistic electromagnetic particle (REMP) code parallelized with MPI, we investigated long-term particle acceleration associated with a relativistic electron–positron jet propagating in an unmagnetized ambient electron–positron plasma. We have also performed simulations with electron-ion jets. The simulations were performed using a much longer simulation system than our previous simulations in order to investigate the full nonlinear stage of the Weibel instability for electron–positron jets and its particle acceleration mechanism. Cold jet electrons are thermalized and ambient electrons are accelerated in the resulting shocks for pair plasma case. Acceleration of ambient electrons leads to a maximum ambient electron density three times larger than the original value for pair plasmas. Behind the bow shock in the jet shock strong electromagnetic fields are generated. These fields may lead to time-dependent afterglow emission. We calculated radiation from electrons propagating in a uniform parallel magnetic field to verify the technique. We also used the new technique to calculate emission from electrons based on simulations with a small system with two different cases for Lorentz factors (15 and 100). We obtained spectra which are consistent with those generated from electrons propagating in turbulent magnetic fields with red noise. This turbulent magnetic field is similar to the magnetic field generated at an early nonlinear stage of the Weibel instability.

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Bañón Navarro, A., A. Di Siena, J. L. Velasco, F. Wilms, G. Merlo, T. Windisch, L. L. LoDestro, J. B. Parker e F. Jenko. "First-principles based plasma profile predictions for optimized stellarators". Nuclear Fusion 63, n. 5 (22 marzo 2023): 054003. http://dx.doi.org/10.1088/1741-4326/acc3af.

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Abstract In the present Letter, first-of-its-kind computer simulations predicting plasma profiles for modern optimized stellarators—while self-consistently retaining neoclassical transport, turbulent transport with 3D effects, and external physical sources—are presented. These simulations exploit a newly developed coupling framework involving the global gyrokinetic turbulence code GENE-3D, the neoclassical transport code KNOSOS, and the 1D transport solver TANGO. This framework is used to analyze the recently observed degradation of energy confinement in electron-heated plasmas in the Wendelstein 7-X stellarator, where the central ion temperature was ‘clamped’ to T i ≈ 1.5 keV regardless of the external heating power. By performing first-principles based simulations, we provide key evidence to understand this effect, namely the inefficient thermal coupling between electrons and ions in a turbulence-dominated regime, which is exacerbated by the large T e / T i ratios, and show that a more efficient ion heat source, such as direct ion heating, will increase the on-axis ion temperature. This work paves the way towards the use of high-fidelity models for the development of the next generation of stellarators, in which neoclassical and turbulent transport are optimized simultaneously.

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Dyrud, L. P., J. Urbina, J. T. Fentzke, E. Hibbit e J. Hinrichs. "Global variation of meteor trail plasma turbulence". Annales Geophysicae 29, n. 12 (16 dicembre 2011): 2277–86. http://dx.doi.org/10.5194/angeo-29-2277-2011.

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Abstract. We present the first global simulations on the occurrence of meteor trail plasma irregularities. These results seek to answer the following questions: when a meteoroid disintegrates in the atmosphere, will the resulting trail become plasma turbulent? What are the factors influencing the development of turbulence? and how do these trails vary on a global scale? Understanding meteor trail plasma turbulence is important because turbulent meteor trails are visible as non-specular trails to coherent radars. Turbulence also influences the evolution of specular radar meteor trails; this fact is important for the inference of mesospheric temperatures from the trail diffusion rates, and their usage for meteor burst communication. We provide evidence of the significant effect that neutral atmospheric winds and ionospheric plasma density have on the variability of meteor trail evolution and on the observation of non-specular meteor trails. We demonstrate that trails are far less likely to become and remain turbulent in daylight, explaining several observational trends for non-specular and specular meteor trails.

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Fulat, Karol, Artem Bohdan, Gabriel Torralba Paz e Martin Pohl. "Kinetic Simulations of Nonrelativistic High-mach-number Perpendicular Shocks Propagating in a Turbulent Medium". Astrophysical Journal 959, n. 2 (1 dicembre 2023): 119. http://dx.doi.org/10.3847/1538-4357/ad04dc.

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Abstract Strong nonrelativistic shocks are known to accelerate particles up to relativistic energies. However, for diffusive shock acceleration, electrons must have a highly suprathermal energy, implying the need for very efficient preacceleration. Most published studies consider shocks propagating through homogeneous plasma, which is an unrealistic assumption for astrophysical environments. Using 2D3V particle-in-cell simulations, we investigate electron acceleration and heating processes at nonrelativistic high-Mach-number shocks in electron-ion plasma with a turbulent upstream medium. For this purpose, slabs of plasma with compressive turbulence are simulated separately and then inserted into shock simulations, which require matching of the plasma slabs at the interface. Using a novel procedure of matching electromagnetic fields and currents, we perform simulations of perpendicular shocks setting different intensities of density fluctuations (≲10%) in the upstream region. The new simulation technique provides a framework for studying shocks propagating in turbulent media. We explore the impact of the fluctuations on electron heating, the dynamics of upstream electrons, and the driving of plasma instabilities. Our results indicate that while the presence of turbulence enhances variations in the upstream magnetic field, their levels remain too low to significantly influence the behavior of electrons at perpendicular shocks.

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da Silva, F., E. Ricardo, J. Ferreira, J. Santos, S. Heuraux, A. Silva, T. Ribeiro et al. "Benchmarking 2D against 3D FDTD codes for the assessment of the measurement performance of a low field side plasma position reflectometer applicable to IDTT". Journal of Instrumentation 17, n. 01 (1 gennaio 2022): C01017. http://dx.doi.org/10.1088/1748-0221/17/01/c01017.

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Abstract O-mode reflectometry, a technique to diagnose fusion plasmas, is foreseen as a source of real-time (RT) plasma position and shape measurements for control purposes in the coming generation of machines such as DEMO. It is, thus, of paramount importance to predict the behavior and capabilities of these new reflectometry systems using synthetic diagnostics. Finite-difference time-domain (FDTD) time-dependent codes allow for a comprehensive description of reflectometry but are computationally demanding, especially when it comes to three-dimensional (3D) simulations, which requires access to High Performance Computing (HPC) facilities, making the use of two-dimensional (2D) codes much more common. It is important to understand the compromises made when using a 2D model in order to decide if it is applicable or if a 3D approach is required. This work attempts to answer this question by comparing simulations of a potential plasma position reflectometer (PPR) at the Low Field-Side (LFS) on the Italian Divertor Tokamak Test facility (IDTT) carried out using two full-wave FDTD codes, REFMULF (2D) and REFMUL3 (3D). In particular, the simulations consider one of IDTT’s foreseen plasma scenarios, namely, a Single Null (SN) configuration, at the Start Of Flat-top (SOF) of the plasma current.

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Asai, N., N. Fukuda e R. Matsumoto. "Three-Dimensional MHD Simulations of a Subcluster Plasma Moving in Turbulent ICM". Proceedings of the International Astronomical Union 2, S235 (agosto 2006): 189. http://dx.doi.org/10.1017/s1743921306005953.

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AbstractWe carried out 3D magnetohydrodynamic simulations of a subcluster moving in turbulent ICM by including anisotropic heat conduction. Since magnetic fields stretched along the subcluster surface suppress the heat conduction across the front, cold fronts are formed and sustained.

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Yang, Yan, Francesco Pecora, William H. Matthaeus, Sohom Roy, Manuel Enrique Cuesta, Alexandros Chasapis, Tulasi Parashar et al. "Quantifying the Agyrotropy of Proton and Electron Heating in Turbulent Plasmas". Astrophysical Journal 944, n. 2 (1 febbraio 2023): 148. http://dx.doi.org/10.3847/1538-4357/acb25a.

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Abstract An important aspect of energy dissipation in weakly collisional plasmas is that of energy partitioning between different species (e.g., protons and electrons) and between different energy channels. Here we analyse pressure–strain interaction to quantify the fractions of isotropic compressive, gyrotropic, and nongyrotropic heating for each species. An analysis of kinetic turbulence simulations is compared and contrasted with corresponding observational results from Magnetospheric Multiscale Mission data in the magnetosheath. In assessing how protons and electrons respond to different ingredients of the pressure–strain interaction, we find that compressive heating is stronger than incompressive heating in the magnetosheath for both electrons and protons, while incompressive heating is stronger in kinetic plasma turbulence simulations. Concerning incompressive heating, the gyrotropic contribution for electrons is dominant over the nongyrotropic contribution, while for protons nongyrotropic heating is enhanced in both simulations and observations. Variations with plasma β are also discussed, and protons tend to gain more heating with increasing β.

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Bott, A. F. A., L. Chen, P. Tzeferacos, C. A. J. Palmer, A. R. Bell, R. Bingham, A. Birkel et al. "Insensitivity of a turbulent laser-plasma dynamo to initial conditions". Matter and Radiation at Extremes 7, n. 4 (1 luglio 2022): 046901. http://dx.doi.org/10.1063/5.0084345.

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It has recently been demonstrated experimentally that a turbulent plasma created by the collision of two inhomogeneous, asymmetric, weakly magnetized, laser-produced plasma jets can generate strong stochastic magnetic fields via the small-scale turbulent dynamo mechanism, provided the magnetic Reynolds number of the plasma is sufficiently large. In this paper, we compare such a plasma with one arising from two pre-magnetized plasma jets whose creation is identical save for the addition of a strong external magnetic field imposed by a pulsed magnetic field generator. We investigate the differences between the two turbulent systems using a Thomson-scattering diagnostic, x-ray self-emission imaging, and proton radiography. The Thomson-scattering spectra and x-ray images suggest that the external magnetic field has a limited effect on the plasma dynamics in the experiment. Although the external magnetic field induces collimation of the flows in the colliding plasma jets and although the initial strengths of the magnetic fields arising from the interaction between the colliding jets are significantly larger as a result of the external field, the energies and morphologies of the stochastic magnetic fields post-amplification are indistinguishable. We conclude that, for turbulent laser-plasmas with supercritical magnetic Reynolds numbers, the dynamo-amplified magnetic fields are determined by the turbulent dynamics rather than the seed fields or modest changes in the initial flow dynamics of the plasma, a finding consistent with theoretical expectations and simulations of turbulent dynamos.

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Bhide, Kalyani, Kiran Siddappaji, Shaaban Abdallah e Kurt Roberts. "Improved Supersonic Turbulent Flow Characteristics Using Non-Linear Eddy Viscosity Relation in RANS and HPC-Enabled LES". Aerospace 8, n. 11 (18 novembre 2021): 352. http://dx.doi.org/10.3390/aerospace8110352.

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A majority of the eddy viscosity models for supersonic turbulent flow are based on linear relationship between Reynolds stresses and mean strain rate. The validity of these models can be improved by introducing non-linearity in relation as RANS models offer advantages in terms of reduced turnaround times typical of industry applications. With these benefits, the present work utilizes quadratic constitutive relation (QCR) with Menter’s k omega SST model to characterize the flowfield of rectangular jets. The sensitivity of this model with QCR, weighted towards diffusion, dissipation, and a combination of both, is addressed. Viscous large eddy simulations (LES) with WALE subgrid scale models are employed for qualitative comparisons using a commercial solver. Massively parallel LES are enabled by the new in-house 1088-core computing cluster at the University of Cincinnati and are also used for benchmarking. The nearfield results are validated with available experimental data and show good agreement in both fidelities. Flow characteristics, including the shear layer profiles, Reynolds stresses, and turbulence kinetic energy (TKE) and its production are compared. LES reveal higher TKE production in the regions with highest Reynolds stresses. It is comparatively lower in QCR RANS. As a special case of TKE analysis in jets, a preliminary investigation of retropropulsion is outlined for rectangular nozzles for the first time. Improved flow behavior by implementation of a non-linear relationship between Reynolds stresses and mean strain rate is demonstrated.

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Hasan, Mahdi, e Michael Atkinson. "Investigation of a Dielectric Barrier Discharge Plasma Actuator to Control Turbulent Boundary Layer Separation". Applied Sciences 10, n. 6 (11 marzo 2020): 1911. http://dx.doi.org/10.3390/app10061911.

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A numerical investigation was carried out to explore the effects of a dielectric barrier discharge (DBD) plasma actuator on a three-dimensional incompressible, separated flow. The test article selected for the simulations was the National Aeronautical and Space Administration (NASA) wall-mounted hump model. The simulations were run at a Reynolds number of 936,000, based on hump chord length, and a freestream Mach number of 0.1. Hybrid partially averaged Navier–Stokes/large-eddy simulations (PANS/LES) were completed using CALC-LES, a well-validated computational fluid dynamics (CFD) code, developed by Chalmers University of Technology. The baseline code was modified to simulate the effects of the actuator, which were modeled as source terms in the momentum equation and were assumed to be steady and constant in the span-wise direction. The numerical simulations were carried out for a baseline (no control) case and five plasma control cases. To optimize the performance of the actuator, the variation of actuator location and voltage frequency was investigated. For the baseline case, a comparison of time-averaged skin friction, the coefficient of pressure, and velocity profiles was made of the available experimental results. The results of the baseline case showed good agreement for a hybrid turbulence model, thus strengthening the solver’s ability to predict a three-dimensional separated flow with reasonable accuracy. The results with the plasma actuator turned on showed improved flow characteristics compared to the baseline simulations by reducing the region of separated flow. The actuator placed just downstream of the separation point at an operational frequency of 5kHz completely eliminated the separated flow for our test conditions.

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Trotta, Domenico, Francesco Pecora, Adriana Settino, Denise Perrone, Heli Hietala, Timothy Horbury, William Matthaeus, David Burgess, Sergio Servidio e Francesco Valentini. "On the Transmission of Turbulent Structures across the Earth’s Bow Shock". Astrophysical Journal 933, n. 2 (1 luglio 2022): 167. http://dx.doi.org/10.3847/1538-4357/ac7798.

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Abstract Collisionless shocks and plasma turbulence are crucial ingredients for a broad range of astrophysical systems. The shock–turbulence interaction, and in particular the transmission of fully developed turbulence across the quasi-perpendicular Earth’s bow shock, is here addressed using a combination of spacecraft observations and local numerical simulations. An alignment between the Wind (upstream) and Magnetospheric Multiscale (downstream) spacecraft is used to study the transmission of turbulent structures across the shock, revealing an increase of their magnetic helicity content in its downstream. Local kinetic simulations, in which the dynamics of turbulent structures are followed through their transmission across a perpendicular shock, confirm this scenario, revealing that the observed magnetic helicity increase is associated with the compression of turbulent structures at the shock front.

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Acosta, Belén, Denisse Pastén e Pablo S. Moya. "Reversibility of Turbulent and Non-Collisional Plasmas: Solar Wind". Proceedings of the International Astronomical Union 15, S354 (giugno 2019): 363–66. http://dx.doi.org/10.1017/s1743921320000137.

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AbstractWe have studied turbulent plasma as a complex system applying the method known as Horizontal Visibility Graph (HVG) to obtain the Kullback-Leibler Divergence (KLD) as a first approach to characterize the reversibility of the time series of the magnetic fluctuations. For this, we have developed the method on Particle In Cell (PIC) simulations for a magnetized plasma and on solar wind magnetic time series, considering slow and fast wind. Our numerical results show that low irreversibility values are verified for magnetic field time series associated with Maxwellian distributions. In addition, considering the solar wind plasma, our preliminary results seem to indicate that greater irreversibility degrees are reached by the magnetic field associated with slow solar wind.

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Arró, G., F. Califano e G. Lapenta. "Statistical properties of turbulent fluctuations associated with electron-only magnetic reconnection". Astronomy & Astrophysics 642 (ottobre 2020): A45. http://dx.doi.org/10.1051/0004-6361/202038696.

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Context. Recent satellite measurements in the turbulent magnetosheath of Earth have given evidence of an unusual reconnection mechanism that is driven exclusively by electrons. This newly observed process was called electron-only reconnection, and its interplay with plasma turbulence is a matter of great debate. Aims. By using 2D-3V hybrid Vlasov–Maxwell simulations of freely decaying plasma turbulence, we study the role of electron-only reconnection in the development of plasma turbulence. In particular, we search for possible differences with respect to the turbulence associated with standard ion-coupled reconnection. Methods. We analyzed the structure functions of the turbulent magnetic field and ion fluid velocity fluctuations to characterize the structure and the intermittency properties of the turbulent energy cascade. Results. We find that the statistical properties of turbulent fluctuations associated with electron-only reconnection are consistent with those of turbulent fluctuations associated with standard ion-coupled reconnection, and no peculiar signature related to electron-only reconnection is found in the turbulence statistics. This result suggests that the turbulent energy cascade in a collisionless magnetized plasma does not depend on the specific mechanism associated with magnetic reconnection. The properties of the dissipation range are discussed as well, and we claim that only electrons contribute to the dissipation of magnetic field energy at sub-ion scales.

31

Bott, Archie F. A., Petros Tzeferacos, Laura Chen, Charlotte A. J. Palmer, Alexandra Rigby, Anthony R. Bell, Robert Bingham et al. "Time-resolved turbulent dynamo in a laser plasma". Proceedings of the National Academy of Sciences 118, n. 11 (8 marzo 2021): e2015729118. http://dx.doi.org/10.1073/pnas.2015729118.

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Understanding magnetic-field generation and amplification in turbulent plasma is essential to account for observations of magnetic fields in the universe. A theoretical framework attributing the origin and sustainment of these fields to the so-called fluctuation dynamo was recently validated by experiments on laser facilities in low-magnetic-Prandtl-number plasmas (Pm<1). However, the same framework proposes that the fluctuation dynamo should operate differently when Pm≳1, the regime relevant to many astrophysical environments such as the intracluster medium of galaxy clusters. This paper reports an experiment that creates a laboratory Pm≳1 plasma dynamo. We provide a time-resolved characterization of the plasma’s evolution, measuring temperatures, densities, flow velocities, and magnetic fields, which allows us to explore various stages of the fluctuation dynamo’s operation on seed magnetic fields generated by the action of the Biermann-battery mechanism during the initial drive-laser target interaction. The magnetic energy in structures with characteristic scales close to the driving scale of the stochastic motions is found to increase by almost three orders of magnitude and saturate dynamically. It is shown that the initial growth of these fields occurs at a much greater rate than the turnover rate of the driving-scale stochastic motions. Our results point to the possibility that plasma turbulence produced by strong shear can generate fields more efficiently at the driving scale than anticipated by idealized magnetohydrodynamics (MHD) simulations of the nonhelical fluctuation dynamo; this finding could help explain the large-scale fields inferred from observations of astrophysical systems.

32

Tamain, P., Ph Ghendrih, H. Bufferand, G. Ciraolo, C. Colin, N. Fedorczak, N. Nace, F. Schwander e E. Serre. "Multi-scale self-organisation of edge plasma turbulent transport in 3D global simulations". Plasma Physics and Controlled Fusion 57, n. 5 (15 aprile 2015): 054014. http://dx.doi.org/10.1088/0741-3335/57/5/054014.

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33

Galassi, Davide, Guido Ciraolo, Patrick Tamain, Hugo Bufferand, Philippe Ghendrih, Nicolas Nace e Eric Serre. "Tokamak Edge Plasma Turbulence Interaction with Magnetic X-Point in 3D Global Simulations". Fluids 4, n. 1 (15 marzo 2019): 50. http://dx.doi.org/10.3390/fluids4010050.

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Turbulence in the edge plasma of a tokamak is a key actor in the determination of the confinement properties. The divertor configuration seems to be beneficial for confinement, suggesting an effect on turbulence of the particular magnetic geometry introduced by the X-point. Simulations with the 3D fluid turbulence code TOKAM3X are performed here to evaluate the impact of a diverted configuration on turbulence in the edge plasma, in an isothermal framework. The presence of the X-point is found, locally, to affect both the shape of turbulent structures and the amplitude of fluctuations, in qualitative agreement with recent experimental observations. In particular, a quiescent region is found in the divertor scrape-off layer (SOL), close to the separatrix. Globally, a mild transport barrier spontaneously forms in the closed flux surfaces region near the separatrix, differently from simulations in limiter configuration. The effect of turbulence-driven Reynolds stress on the formation of the barrier is found to be weak by dedicated simulations, while turbulence damping around the X-point seems to globally reduce turbulent transport on the whole flux surface. The magnetic shear is thus pointed out as a possible element that contributes to the formation of edge transport barriers.

34

Bustard, Chad, e S. Peng Oh. "Turbulent Reacceleration of Streaming Cosmic Rays". Astrophysical Journal 941, n. 1 (1 dicembre 2022): 65. http://dx.doi.org/10.3847/1538-4357/aca021.

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Abstract Subsonic, compressive turbulence transfers energy to cosmic rays (CRs), a process known as nonresonant reacceleration. It is often invoked to explain the observed ratios of primary to secondary CRs at ∼GeV energies, assuming wholly diffusive CR transport. However, such estimates ignore the impact of CR self-confinement and streaming. We study these issues in stirring box magnetohydrodynamic (MHD) simulations using Athena++, with field-aligned diffusive and streaming CR transport. For diffusion only, we find CR reacceleration rates in good agreement with analytic predictions. When streaming is included, reacceleration rates depend on plasma β. Due to streaming-modified phase shifts between CR and gas variables, they are slower than canonical reacceleration rates in low-β environments like the interstellar medium but remain unchanged in high-β environments like the intracluster medium. We also quantify the streaming energy-loss rate in our simulations. For sub-Alfvénic turbulence, it is resolution dependent (hence unconverged in large-scale simulations) and heavily suppressed compared to the isotropic loss rate v A · ∇P CR/P CR ∼ v A/L 0, due to misalignment between the mean field and isotropic CR gradients. Unlike acceleration efficiencies, CR losses are almost independent of magnetic field strength over β ∼ 1–100 and are, therefore, not the primary factor behind lower acceleration rates when streaming is included. While this paper is primarily concerned with how turbulence affects CRs, in a follow-up paper we consider how CRs affect turbulence by diverting energy from the MHD cascade, altering the pathway to gas heating and steepening the turbulent spectrum.

35

Shaikh, D., e G. P. Zank. "Three-dimensional simulations of turbulent spectra in the local interstellar medium". Nonlinear Processes in Geophysics 14, n. 4 (6 luglio 2007): 351–59. http://dx.doi.org/10.5194/npg-14-351-2007.

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Abstract. Three-dimensional time dependent numerical simulations of compressible magnetohydrodynamic fluids describing super-Alfvénic, supersonic and strongly magnetized space and laboratory plasmas show a nonlinear relaxation towards a state of near incompressibility. The latter is characterized essentially by a subsonic turbulent Mach number. This transition is mediated dynamically by disparate spectral energy dissipation rates in compressible magnetosonic and shear Alfvénic modes. Nonlinear cascades lead to super-Alfvénic turbulent motions decaying to a sub-Alfvénic regime that couples weakly with (magneto)acoustic cascades. Consequently, the supersonic plasma motion is transformed into highly subsonic motion and density fluctuations experience a passive convection. This model provides a self-consistent explaination of the ubiquitous nature of incompressible magnetoplasma fluctuations in the solar wind and the interstellar medium.

36

Wang, Bei, Stephane Ethier, William Tang, Khaled Z. Ibrahim, Kamesh Madduri, Samuel Williams e Leonid Oliker. "Modern gyrokinetic particle-in-cell simulation of fusion plasmas on top supercomputers". International Journal of High Performance Computing Applications 33, n. 1 (29 giugno 2017): 169–88. http://dx.doi.org/10.1177/1094342017712059.

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The gyrokinetic toroidal code at Princeton (GTC-P) is a highly scalable and portable particle-in-cell (PIC) code. It solves the 5-D Vlasov–Poisson equation featuring efficient utilization of modern parallel computer architectures at the petascale and beyond. Motivated by the goal of developing a modern code capable of dealing with the physics challenge of increasing problem size with sufficient resolution, new thread-level optimizations have been introduced as well as a key additional domain decomposition. GTC-P’s multiple levels of parallelism, including internode 2-D domain decomposition and particle decomposition, as well as intranode shared memory partition and vectorization, have enabled pushing the scalability of the PIC method to extreme computational scales. In this article, we describe the methods developed to build a highly parallelized PIC code across a broad range of supercomputer designs. This particularly includes implementations on heterogeneous systems using NVIDIA GPU accelerators and Intel Xeon Phi (MIC) coprocessors and performance comparisons with state-of-the-art homogeneous HPC systems such as Blue Gene/Q. New discovery science capabilities in the magnetic fusion energy application domain are enabled, including investigations of ion–temperature–gradient driven turbulence simulations with unprecedented spatial resolution and long temporal duration. Performance studies with realistic fusion experimental parameters are carried out on multiple supercomputing systems spanning a wide range of cache capacities, cache-sharing configurations, memory bandwidth, interconnects, and network topologies. These performance comparisons using a realistic discovery-science-capable domain application code provide valuable insights on optimization techniques across one of the broadest sets of current high-end computing platforms worldwide.

37

Yelles Chaouche, L., R. H. Cameron, S. K. Solanki, T. L. Riethmüller, L. S. Anusha, V. Witzke, A. I. Shapiro et al. "Power spectrum of turbulent convection in the solar photosphere". Astronomy & Astrophysics 644 (30 novembre 2020): A44. http://dx.doi.org/10.1051/0004-6361/202037545.

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The solar photosphere provides us with a laboratory for understanding turbulence in a layer where the fundamental processes of transport vary rapidly and a strongly superadiabatic region lies very closely to a subadiabatic layer. Our tools for probing the turbulence are high-resolution spectropolarimetric observations such as have recently been obtained with the two balloon-borne SUNRISE missions, and numerical simulations. Our aim is to study photospheric turbulence with the help of Fourier power spectra that we compute from observations and simulations. We also attempt to explain some properties of the photospheric overshooting flow with the help of its governing equations and simulations. We find that quiet-Sun observations and smeared simulations are consistent with each other and exhibit a power-law behavior in the subgranular range of their Doppler velocity power spectra with a power-law index of ≈ − 2. The unsmeared simulations exhibit a power law that extends over the full range between the integral and Taylor scales with a power-law index of ≈ − 2.25. The smearing, reminiscent of observational conditions, considerably reduces the extent of the power-law-like portion of the power spectra. This suggests that the limited spatial resolution in some observations might eventually result in larger uncertainties in the estimation of the power-law indices. The simulated vertical velocity power spectra as a function of height show a rapid change in the power-law index (at the subgranular range) from roughly the optical depth unity layer, that is, the solar surface, to 300 km above it. We propose that the cause of the steepening of the power-law index is the transition from a super- to a subadiabatic region, in which the dominant source of motions is overshooting convection. A scale-dependent transport of the vertical momentum occurs. At smaller scales, the vertical momentum is more efficiently transported sideways than at larger scales. This results in less vertical velocity power transported upward at small scales than at larger scales and produces a progressively steeper vertical velocity power law below 180 km. Above this height, the gravity work progressively gains importance at all relevant scales, making the atmosphere progressively more hydrostatic and resulting in a gradually less steep power law. Radiative heating and cooling of the plasma is shown to play a dominant role in the plasma energetics in this region, which is important in terms of nonadiabatic damping of the convective motions.

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Carlevaro, Nakia, Giovanni Montani e Fabio Moretti. "On the Effects of Tokamak Plasma Edge Symmetries on Turbulence Relaxation". Symmetry 15, n. 9 (11 settembre 2023): 1745. http://dx.doi.org/10.3390/sym15091745.

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The plasma edge of a tokamak configuration is characterized by turbulent dynamics leading to enhanced transport. We construct a simplified 3D Hasegawa–Wakatani model reducing to a single partial differential equation for the turbulent electric potential dynamics. Simulations demonstrate how the 3D turbulence relaxes on a 2D axisymmetric profile, corresponding to the so-called interchange turbulence. The spectral features of this regime are found to be strongly dependent on the initialization pattern. We outline that the emergence of axisymmetric turbulence is also achieved when the corresponding mode amplitude is not initialized. Then, we introduce the symmetries of the magnetic X-point of a tokamak configuration. We linearize the governing equation by treating the poloidal field as a small correction. We show that it is not always possible to solve the electric potential dynamics following a perturbative approach. This finding, which is due to resonance between the modes of the background and the poloidal perturbation, confirms that the X-point symmetries can alter the properties of turbulent transport in the edge region.

39

Keskinen, M. J. "Theory of Strongly Turbulent Two-Dimensional Cross Field Convection of Current Carrying Space Plasmas". Symposium - International Astronomical Union 107 (1985): 475. http://dx.doi.org/10.1017/s0074180900075963.

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The “direct interaction approximation” of Kraichnan as modified by Kadomtsev is employed to develop a two-dimensional strong turbulence theory which predicts both nonlinear frequency broadening and a power law for the spectrum of a convecting plasma containing a gravitationally induced cross field current. These results are favorably compared with experimental observations, numerical simulations, and previous studies1 of turbulent cross field convection of current-carrying plasma.

40

WIECHEN, HEINZ M. "Simulations of Kelvin–Helmholtz modes in the dusty plasma environment of noctilucent clouds". Journal of Plasma Physics 73, n. 5 (ottobre 2007): 649–58. http://dx.doi.org/10.1017/s0022377806006088.

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AbstractWe present results of quantitative multi-fluid simulations of the nonlinear dynamics of Kelvin–Helmholtz modes in the partially ionized dusty plasma of noctilucent clouds. Noctilucent clouds are a typical example of dusty plasmas in the Earth's mesosphere/lower thermosphere. A specific feature observed in noctilucent clouds is wavy, turbulent structure. Possible explanations for these structures, which are discussed in the literature, are based on hydrodynamical models. The dusty plasma aspect has been widely neglected, so far. In this paper we examine the nonlinear dynamics of Kelvin–Helmholtz modes in noctilucent clouds from the viewpoint of dusty plasma dynamics. The corresponding results are in good qualitative and quantitative agreement with observations.

41

GHOSH, SHANKAR, e KRISHNAN MAHESH. "DNS of the thermal effects of laser energy deposition in isotropic turbulence". Journal of Fluid Mechanics 654 (14 maggio 2010): 387–416. http://dx.doi.org/10.1017/s0022112010000649.

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The interaction of a laser-induced plasma with isotropic turbulence is studied using numerical simulations. The simulations use air as the working fluid and assume local thermodynamic equilibrium. The numerical method is fully spectral and uses a shock-capturing scheme in a corrector step. A model problem involving the effect of energy deposition on an isolated vortex is studied as a first step towards plasma/turbulence interaction. Turbulent Reynolds number Reλ = 30 and fluctuation Mach numbers Mt = 0.001 and 0.3 are considered. A tear-drop-shaped shock wave is observed to propagate into the background, and progressively become spherical in time. The turbulence experiences strong compression due to the shock wave and strong expansion in the core. This behaviour is spatially inhomogeneous and non-stationary in time. Statistics are computed as functions of radial distance from the plasma axis and angular distance across the surface of the shock wave. For Mt = 0.001, the shock wave propagates on a much faster time scale compared to the turbulence evolution. At Mt of 0.3, the time scale of the shock wave is comparable to that of the background. For both cases the mean flow is classified into shock formation, shock propagation and subsequent collapse of the plasma core, and the effect of turbulence on each of these phases is studied in detail. The effect of mean vorticity production on the turbulent vorticity field is also discussed. Turbulent kinetic energy budgets are presented to explain the mechanism underlying the transfer of energy between the mean flow and background turbulence.

42

Makwana, Kirit, Hui Li, Fan Guo e Xiaocan Li. "Dissipation and particle energization in moderate to low beta turbulent plasma via PIC simulations". Journal of Physics: Conference Series 837 (30 maggio 2017): 012004. http://dx.doi.org/10.1088/1742-6596/837/1/012004.

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González, C. A., T. N. Parashar, D. Gomez, W. H. Matthaeus e P. Dmitruk. "Turbulent electromagnetic fields at sub-proton scales: Two-fluid and full-kinetic plasma simulations". Physics of Plasmas 26, n. 1 (gennaio 2019): 012306. http://dx.doi.org/10.1063/1.5054110.

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Ghanbari, Keyvan, e Vladimir Florinski. "Simulation of Solar Wind Turbulence near Corotating Interaction Regions: Superposed Epoch Analysis of Simulations and Observations". Astrophysical Journal 943, n. 2 (27 gennaio 2023): 87. http://dx.doi.org/10.3847/1538-4357/acabc4.

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Abstract The effect of the turbulence that is associated with solar wind corotating interaction regions (CIRs) on transport of galactic cosmic rays remains an outstanding problem in space science. Observations show that the intensities of the plasma and magnetic fluctuations are enhanced within a CIR. The velocity shear layer between the slow and fast wind embedded in a CIR is thought to be responsible for this enhancement in turbulent energy. We perform physics-based magnetohydrodynamic simulations of the plasma background and turbulent fluctuations in the solar wind dominated by CIRs for radial distances between 0.3 and 5 au. A simple but effective approach is used to incorporate the inner boundary conditions for the solar wind and magnetic field for the periods 2007–2008 and 2017–2018. Legendre coefficients at the source surface obtained from the Wilcox Solar Observatory library are utilized for dynamic reconstructions of the current sheet and the fast and slow streams at the inner boundary. The dynamic inner boundary enables our simulations to generate CIRs that are reasonably comparable with observations near Earth. While the magnetic field structure is reasonably well reproduced, the enhancements in the turbulent energy at the stream interfaces are smaller than observed. A superposed epoch analysis is performed over several CIRs from the simulation and compared to the superposed epoch analysis of the observed CIRs. The results for the turbulent energy and correlation length are used to estimate the diffusion tensor of galactic cosmic rays. The derived diffusion coefficients could be used for more realistic modeling of cosmic rays in a dynamically evolving inner heliosphere.

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Lin, Z., G. Rewoldt, S. Ethier, T. S. Hahm, W. W. Lee, J. L. V. Lewandowski, Y. Nishimura e W. X. Wang. "Particle-in-cell simulations of electron transport from plasma turbulence: recent progress in gyrokinetic particle simulations of turbulent plasmas". Journal of Physics: Conference Series 16 (1 gennaio 2005): 16–24. http://dx.doi.org/10.1088/1742-6596/16/1/002.

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Echeverría, Sebastián, Pablo S. Moya e Denisse Pastén. "On the multifractality of plasma turbulence in the solar wind". Proceedings of the International Astronomical Union 15, S354 (giugno 2019): 371–74. http://dx.doi.org/10.1017/s1743921320000514.

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AbstractIn this work we have analyzed turbulent plasma in the kinetic scale by the characterization of magnetic fluctuations time series. Considering numerical Particle-In-Cell (PIC) simulations we apply a method known as MultiFractal Detrended Fluctuation Analysis (MFDFA) to study the fluctuations of solar-wind-like plasmas in thermodynamic equilibrium (represented by Maxwellian velocity distribution functions), and out of equilibrium plasma represented by Tsallis velocity distribution functions, characterized by the kappa (κ) parameter, to stablish relations between the fractality of magnetic fluctuation and the kappa parameter.

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Yeates, A. R., A. J. B. Russell e G. Hornig. "Physical role of topological constraints in localized magnetic relaxation". Proceedings of the Royal Society A: Mathematical, Physical and Engineering Sciences 471, n. 2178 (giugno 2015): 20150012. http://dx.doi.org/10.1098/rspa.2015.0012.

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Predicting the final state of turbulent plasma relaxation is an important challenge, both in astro-physical plasmas such as the Sun's corona and in controlled thermonuclear fusion. Recent numerical simulations of plasma relaxation with braided magnetic fields identified the possibility of a novel constraint, arising from the topological degree of the magnetic field-line mapping. This constraint implies that the final relaxed state is drastically different for an initial configuration with topological degree 1 (which allows a Taylor relaxation) and one with degree 2 (which does not reach a Taylor state). Here, we test this transition in numerical resistive-magnetohydrodynamic simulations, by embedding a braided magnetic field in a linear force-free background. Varying the background force-free field parameter generates a sequence of initial conditions with a transition between topological degree 1 and 2. For degree 1, the relaxation produces a single twisted flux tube, whereas for degree 2 we obtain two flux tubes. For predicting the exact point of transition, it is not the topological degree of the whole domain that is relevant, but only that of the turbulent region.

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Montagud-Camps, Victor, Petr Hellinger, Andrea Verdini, Emanuele Papini, Luca Franci e Simone Landi. "Quantification of the Cross-helicity Turbulent Cascade in Compressible MHD Simulations". Astrophysical Journal 938, n. 2 (1 ottobre 2022): 90. http://dx.doi.org/10.3847/1538-4357/ac9281.

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Abstract In plasma turbulence, energy and cross helicity are transferred across scales at a constant rate as a consequence of nonlinear interactions. In incompressible magnetohydrodynamics (MHD), the energy cascade rate of both quantities can be computed by means of the temporal evolution of second-order structure functions, known as Karman–Howarth–Monin (KHM) equations. In the present work, we derive the KHM equation to compute the energy cascade rate of cross helicity in compressible MHD. Using three-dimensional direct numerical simulations, we validate the equation and use it to measure the cross-helicity turbulence properties. Our results show a slower development of the cross-helicity cascade with respect to the energy one and the presence of inverse cascades of energy and cross helicity at large scales when in the presence of a strong mean field. We propose the relation of these phenomena with the longer duration of geomagnetic storms after the arrival of solar winds with large cross helicity and the observation of patchy inertial ranges displaying positive and negative cascade rates for certain solar wind intervals.

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Trotta, Domenico, Francesco Valentini, David Burgess e Sergio Servidio. "Phase space transport in the interaction between shocks and plasma turbulence". Proceedings of the National Academy of Sciences 118, n. 21 (18 maggio 2021): e2026764118. http://dx.doi.org/10.1073/pnas.2026764118.

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The interaction of collisionless shocks with fully developed plasma turbulence is numerically investigated. Hybrid kinetic simulations, where a turbulent jet is slammed against an oblique shock, are employed to address the role of upstream turbulence on plasma transport. A technique, using coarse graining of the Vlasov equation, is proposed, showing that the particle transport strongly depends on upstream turbulence properties, such as strength and coherency. These results might be relevant for the understanding of acceleration and heating processes in space plasmas.

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Marscher, Alan P., e Svetlana G. Jorstad. "Frequency and Time Dependence of Linear Polarization in Turbulent Jets of Blazars". Galaxies 9, n. 2 (27 aprile 2021): 27. http://dx.doi.org/10.3390/galaxies9020027.

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Time-variable polarization is an extremely valuable observational tool to probe the dynamical physical conditions of blazar jets. Since 2008, we have been monitoring the flux and linear polarization of a sample of gamma-ray bright blazars at optical frequencies. Some of the observations were performed on nightly or intra-night time-scales in four optical bands, providing information on the frequency and time dependence of the polarization. The observed behavior is similar to that found in simulations of turbulent plasma in a relativistic jet that contains a standing shock and/or a helical background magnetic field. Similar simulations predict the characteristics of X-ray synchrotron polarization of blazars that will be measured in the future by the Imaging X-ray Polarimetry Explorer (IXPE).

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