Articles de revues : « Collisional parameters »

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Chatterjee, A., et A. Ruina. « A New Algebraic Rigid-Body Collision Law Based on Impulse Space Considerations ». Journal of Applied Mechanics 65, n^o 4 (1 décembre 1998) : 939–51. http://dx.doi.org/10.1115/1.2791938.

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We present a geometric representation of the set of three-dimensional rigid-body collisional impulses that are reasonably permissible by the combination of non-negative post-collision separation rate, non-negative collisional compression impulse, non-negative energy dissipation and the Coulomb friction inequality. The construction is presented for a variety of special collisional situations involving special symmetry or extremes in the mass distribution, the friction coefficient, or the initial conditions. We review a variety of known friction laws and show how they do and do not fit in the permissible region in impulse space as well as comment on other attributes of these laws. We present a few parameterizations of the full permissible region of impulse space. We present a simple generalization to arbitrary three-dimensional point contact collisions of a simple law previously only applicable to objects with contact-inertia eigenvectors aligned with the surface normal and initial relative tangential velocity component (e.g., spheres and disks). This new algebraic collision law has two restitution parameters for general three-dimensional frictional single-point rigid-body collisions. The new law generates a collisional impulse that is a weighted sum of the impulses from a frictionless but nonrebounding collision and from a perfectly sticking, nonrebounding collision. We describe useful properties of our law; show geometrically the set of impulses it can predict for several collisional situations; and compare it with existing laws. For simultaneous collisions we propose that the new algebraic law be used by recursively breaking these collisions into a sequence ordered by the normal approach velocities of potential contact pairs.

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Arakawa, Sota, Hidekazu Tanaka et Eiichiro Kokubo. « Impacts of Viscous Dissipation on Collisional Growth and Fragmentation of Dust Aggregates ». Astrophysical Journal 933, n^o 2 (1 juillet 2022) : 144. http://dx.doi.org/10.3847/1538-4357/ac7460.

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Abstract Understanding the collisional behavior of dust aggregates consisting of submicron-sized grains is essential to unveiling how planetesimals formed in protoplanetary disks. It is known that the collisional behavior of individual dust particles strongly depends on the strength of viscous dissipation force; however, impacts of viscous dissipation on the collisional behavior of dust aggregates have not been studied in detail, especially for the cases of oblique collisions. Here we investigated the impacts of viscous dissipation on the collisional behavior of dust aggregates. We performed numerical simulations of collisions between two equal-mass dust aggregates with various collision velocities and impact parameters. We also changed the strength of viscous dissipation force systematically. We found that the threshold collision velocity for the fragmentation of dust aggregates barely depends on the strength of viscous dissipation force when we consider oblique collisions. In contrast, the size distribution of fragments changes significantly when the viscous dissipation force is considered. We obtained the empirical fitting formulae for the size distribution of fragments for the case of strong dissipation, which would be useful to study the evolution of size and spatial distributions of dust aggregates in protoplanetary disks.

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Arakawa, Sota, Hidekazu Tanaka et Eiichiro Kokubo. « Collisional Growth Efficiency of Dust Aggregates and Its Independence of the Strength of Interparticle Rolling Friction ». Astrophysical Journal 939, n^o 2 (1 novembre 2022) : 100. http://dx.doi.org/10.3847/1538-4357/ac96e1.

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Abstract The pairwise collisional growth of dust aggregates consisting of submicron-sized grains is the first step of planet formation, and understanding the collisional behavior of dust aggregates is therefore essential. It is known that the main energy dissipation mechanisms are the tangential frictions between particles in contact, namely, rolling, sliding, and twisting. However, there is great uncertainty for the strength of rolling friction, and the dependence of the collisional growth condition on the strength of rolling friction was poorly understood. Here we performed numerical simulations of collisions between two equal-mass porous aggregates with various collision velocities and impact parameters, and we also changed the strength of rolling friction systematically. We found that the threshold of the collision velocity for the fragmentation of dust aggregates is nearly independent of the strength of rolling friction. This is because the total amount of the energy dissipation by the tangential frictions is nearly constant even though the strength of rolling friction is varied.

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Seaton, M. J. « New Atomic Data for Astronomy : An Introductory Review ». Highlights of Astronomy 10 (1995) : 570–71. http://dx.doi.org/10.1017/s1539299600012065.

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Astronomers require the following basic atomic data: energy levels and wavelengths’, radiative transition probabilities; cross sections for photo-ionisation and for collisional processes; and line profile parameters. They also require processed data such as: level populations; opacities; radiation forces; line emissivities; and collisional rate-coefficients.Many of the data used by astronomers come from theoretical work. Experimental work is of importance in determining accurate wavelengths, in providing essential checks on theory for radiative probabilities and collision rates, and in the determination of line-profile parameters. Experimental studies are particularly important for processes of collisional ionisation.

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Jesus, Antônio D. C., Rafael S. Ribeiro, Alessandro Rossi et Ernesto Veira Neto. « Evasive Maneuvers in Space Debris Environment and Technological Parameters ». Mathematical Problems in Engineering 2012 (2012) : 1–15. http://dx.doi.org/10.1155/2012/126521.

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We present a study of collisional dynamics between space debris and an operational vehicle in LEO. We adopted an approach based on the relative dynamics between the objects on a collisional course and with a short warning time and established a semianalytical solution for the final trajectories of these objects. Our results show that there are angular ranges in 3D, in addition to the initial conditions, that favor the collisions. These results allowed the investigation of a range of technological parameters for the spacecraft (e.g., fuel reserve) that allow a safe evasive maneuver (e.g., time available for the maneuver). The numerical model was tested for different values of the impact velocity and relative distance between the approaching objects.

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Ngo, N. H., H. Tran, R. R. Gamache et J. M. Hartmann. « Pressure effects on water vapour lines : beyond the Voigt profile ». Philosophical Transactions of the Royal Society A : Mathematical, Physical and Engineering Sciences 370, n^o 1968 (13 juin 2012) : 2495–508. http://dx.doi.org/10.1098/rsta.2011.0272.

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A short overview of recent results on the effects of pressure (collisions) regarding the shape of isolated infrared lines of water vapour is presented. The first part of this study considers the basic collisional quantities, which are the pressure-broadening and -shifting coefficients, central parameters of the Lorentzian (and Voigt) profile and thus of any sophisticated line-shape model. Through comparisons of measured values with semi-classical calculations, the influences of the molecular states (both rotational and vibrational) involved and of the temperature are analysed. This shows the relatively unusual behaviour of H 2 O broadening, with evidence of a significant vibrational dependence and the fact that the broadening coefficient (in cm −1 atm −1 ) of some lines increases with temperature. In the second part of this study, line shapes beyond the Voigt model are considered, thus now taking ‘velocity effects’ into account. These include both the influence of collisionally induced velocity changes that lead to the so-called Dicke narrowing and the influence of the dependence of collisional parameters on the speed of the radiating molecule. Experimental evidence of deviations from the Voigt shape is presented and analysed. The interest of classical molecular dynamics simulations, to model velocity changes, together with semi-classical calculations of the speed-dependent collisional parameters for line-shape predictions from ‘first principles’, are discussed.

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Chernoff, David F., et Xiaolan Huang. « Frequency of Stellar Collisions in Three-Body Heating ». Symposium - International Astronomical Union 174 (1996) : 263–72. http://dx.doi.org/10.1017/s0074180900001601.

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The probability for collisional interaction of three body binaries is calculated as a function of the physical radius and mass of the stellar objects and the depth of the cluster potential well. For typical cluster parameters, there is a significant chance of physical collision for objects as small as white dwarfs. One consequence of the collisions is to lower the amount of heat produced from hardening a binary, thereby diminishing the efficiency of the three-body heating mechanism.

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Campo Bagatin, A., et P. Farinella. « Collisional reaccumulation of asteroids ». International Astronomical Union Colloquium 173 (1999) : 145–52. http://dx.doi.org/10.1017/s0252921100031341.

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AbstractWe have developed a new version of the code developed by Campo Bagatin (1994a, b) to model the collisional evolution of the asteroid size distribution. The new code distinguishes between “intact”, unfractured asteroids and asteroids converted by energetic collisions into “piles of rubble”. We have run a number of simulations of the collisional evolution process to assess the size range where reaccumulated bodies should be expected to be abundant in the main asteroid belt. We find that this diameter range ranges from about 10 to 100 km, but may extend to smaller or larger bodies depending on the prevailing collisional response parameters.

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FIORE, M., F. FIÚZA, M. MARTI, R. A. FONSECA et L. O. SILVA. « Relativistic effects on the collisionless–collisional transition of the filamentation instability in fast ignition ». Journal of Plasma Physics 76, n^o 6 (20 août 2010) : 813–32. http://dx.doi.org/10.1017/s0022377810000413.

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AbstractRelativistic collisional effects on the filamentation instability are analytically and numerically investigated by comparing collisionless and collisional scenarios for a fast ignition (FI) configuration. The theoretical kinetic model, including warm species and space charge effects, predicts the preferential formation of larger filaments and the inhibition/enhancement of the instability when collisions are accounted for. These collisional effects are qualitatively and quantitatively confirmed by 1D and 2D particle-in-cell (PIC) simulations, also providing a physical picture for the inhibition/enhancement regime due to collisions, based on the electron beam slowdown. By plugging typical FI parameters in the dispersion relation, the theoretical model predicts significant growth rates of the instability deep inside the FI target, thus showing the potential role of the filamentation instability as a mechanism for energy deposition into the pellet core.

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Mohammed, A. I., et C. S. Adams. « Ion shock layer formation during multi-ion-species plasma jet stagnation events ». Physics of Plasmas 29, n^o 7 (juillet 2022) : 072307. http://dx.doi.org/10.1063/5.0087509.

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We report the characteristics of collisional plasma shocks formed during interactions between low density ([Formula: see text] cm−3), low temperature ([Formula: see text] eV), high velocity (30 km s−1), plasma jets and stagnant plasma of similar parameters. This investigation seeks to probe the structure of shocks in multi-ion-species plasmas, in particular, the presence of gradient-driven ion species separation at the shock front. The railgun-accelerated jets utilized here have previously been shown to exist in a collisional regime with intra-jet collisional mean-free-path substantially smaller than jet size [Schneider et al., Plasma Sources Sci. Technol. 29, 045013 (2020)]. To induce collisions, a dielectric barrier is located downstream of the railgun to stagnate an initially supersonic plasma jet. Around the time of stagnation, the railgun emits a second jet which shortly collides with the stagnant plasma. The presence of a structure emitting in the UV-visible band is evident in high-speed photographs of the moments immediately following the arrival of the second jet at the stagnant plasma. Analysis of interferometric and spectroscopic data suggests that the observed increase in density from the jet to the post-collision plasma is consistent with the formation of a bow shock structure with a multi-millimeter-scale ion shock layer.

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Akter, Shahrina, et M. G. Hafez. « Head-on collision between two-counter-propagating electron acoustic soliton and double layer in an unmagnetized plasma ». AIP Advances 13, n^o 1 (1 janvier 2023) : 015005. http://dx.doi.org/10.1063/5.0124133.

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The head-on collision between two-counter-propagating electron acoustic solitons and double layers (DLs) in an unmagnetized collisionless multi-species plasma consisting of inertial cold electron fluid and ( α, q)-distributed hot electrons and positrons has been analyzed with the stationary background of massive positive ions. For nonlinear analysis of colliding wave phenomena, the coupled Korteweg–de Vries equation (KdVE), modified KdVE (mKdVE), and standard Gardner equation have been derived by adopting the extended Poincaré–Lighthill–Kuo technique. The effect of non-dimensional parameters on the collisional KdV, mKdV, and Gardner solitons (GSs) and DLs has been examined in detail by considering the limiting cases of ( α, q)-distributions. It is found that the plasma model supports (i) the compressive and rarefactive collisional KdV solitons and GSs, (ii) only compressive mKdV solitons, and (iii) only rarefactive collisional DLs. The rarefactive collisional solitons are more affected by nonextensivity and the increase of the temperature of electrons than their compressive counterpart, whereas the rarefactive collisional DLs only existed in the presence of nonthermality.

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Huang, Chenyang, Yang Yu, Zhijun Song, Bin Cheng et Wenyue Dai. « Understanding the Early Stage of Planet Formation : Design and Demonstration of the Space Experimental Apparatus ». Aerospace 10, n^o 3 (13 mars 2023) : 285. http://dx.doi.org/10.3390/aerospace10030285.

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Planet formation begins with the collision and growth of dust in protoplanetary disks. Concerning the basic cognition of the early stage of planet formation, a long-standing weakness of the research is a comprehensive physical model describing the collisional evolution of dust particles. Microgravity experiments providing original data are crucial in developing related theories. In this work, we propose an experimental scheme for observing the collisional growth of dust analogues under a unidirectional and continuous shearing process, aiming at a future implementation in space experiments. The experimental process is simulated using the discrete element method, and the atlas of the design parameter versus the evolutionary path is depicted. We notice fractal structures and growth stalling as remarkable outcomes in the process of collisional growth, which is analogous to the evolutionary mechanism in the ancient protoplanetary disks. Based on these phenomena, we determine the sensitive design parameters, i.e., the shear velocity and the filling factor, which serve as the recommended parameters in future space experiments. The validation using numerical experiments shows that the experimental scheme with proper design parameters is feasible, which promises to generate constructive data that will facilitate the development of planet formation theory.

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Wagner, G., M. Birk, R. R. Gamache et J. M. Hartmann. « Collisional parameters of lines : effect of temperature ». Journal of Quantitative Spectroscopy and Radiative Transfer 92, n^o 2 (mai 2005) : 211–30. http://dx.doi.org/10.1016/j.jqsrt.2004.07.023.

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Schräpler, Rainer R., Wolf A. Landeck et Jürgen Blum. « Collisional properties of cm-sized high-porosity ice and dust aggregates and their applications to early planet formation ». Monthly Notices of the Royal Astronomical Society 509, n^o 4 (23 novembre 2021) : 5641–56. http://dx.doi.org/10.1093/mnras/stab3348.

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ABSTRACT In dead zones of protoplanetary discs, it is assumed that micrometre-sized particles grow Brownian, sediment to the mid-plane and drift radially inward. When collisional compaction sets in, the aggregates collect slower and therefore dynamically smaller particles. This sedimentation and growth phase of highly porous ice and dust aggregates is simulated with laboratory experiments in which we obtained mm- to cm-sized ice aggregates with a porosity of 90 per cent as well as cm-sized dust agglomerates with a porosity of 85 per cent. We modelled the growth process during sedimentation in an analytical calculation to compute the agglomerate sizes when they reach the mid-plane of the disc. In the mid-plane, the dust particles form a thin dense layer and gain relative velocities by, e.g. the streaming instability or the onset of shear turbulence. To investigate these collisions, we performed additional laboratory drop tower experiments with the high-porosity aggregates formed in the sedimentary-growth experiments and determined their mechanical parameters, including their sticking threshold velocity, which is important for their further collisional evolution on their way to form planetesimals. Finally, we developed a method to calculate the packing-density-dependent fundamental properties of our dust and ice agglomerates, the Young’s modulus, the Poisson ratio, the shear viscosity, and the bulk viscosity from compression measurements. With these parameters, it was possible to derive the coefficient of restitution which fits our measurements. In order to physically describe these outcomes, we applied a collision model. With this model, predictions about general dust-aggregate collisions are possible.

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CADJAN, M. G., et M. F. IVANOV. « Langevin approach to plasma kinetics with Coulomb collisions ». Journal of Plasma Physics 61, n^o 1 (janvier 1999) : 89–106. http://dx.doi.org/10.1017/s0022377898007363.

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The Langevin approach to the kinetics of a collisional plasma is developed. Some collision models are considered, and the corresponding stochastic differential equations are derived. These equations can be regarded as an alternative to the description of a plasma in terms of a distribution function. The method developed here allows one to simulate plasma processes, taking account of both collective kinetics effects and Coulomb collisions. Results of the numerical simulation of the intervention of laser pulses with an overdense plasma are presented. The dependence of the absorption coefficient on the plasma parameters is calculated. The features of the plasma dynamics under the action of intense laser radiation are observed and discussed. The results of numerical tests of the validity of this method are also presented.

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Gamache, Robert R., et Jean-Michel Hartmann. « Collisional parameters of H2O lines : effects of vibration ». Journal of Quantitative Spectroscopy and Radiative Transfer 83, n^o 2 (janvier 2004) : 119–47. http://dx.doi.org/10.1016/s0022-4073(02)00296-0.

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Kim, M., S. Wolf, T. Löhne, F. Kirchschlager et A. V. Krivov. « Impact of planetesimal eccentricities and material strength on the appearance of eccentric debris disks ». Astronomy & ; Astrophysics 618 (octobre 2018) : A38. http://dx.doi.org/10.1051/0004-6361/201833061.

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Context. Since circumstellar dust in debris disks is short-lived, dust-replenishing requires the presence of a reservoir of planetesimals. These planetesimals in the parent belt of debris disks orbit their host star and continuously supply the disk with fine dust through their mutual collisions. Aims. We aim to understand effects of different collisional parameters on the observational appearance of eccentric debris disks. These parameters are the eccentricity of the planetesimal belt, dynamical excitation, and the material strength. Methods. The collisional evolution of selected debris disk configurations was simulated with the numerical code ACE. Subsequently, selected observable quantities are simulated with our newly developed code DMS. The impact of the eccentricity, dynamical excitation, and the material strength is discussed with respect to the grain size distribution, the spectral energy distribution, and spatially resolved images of debris disk systems. Results. The most recognizable features in different collisional evolutions are as follows. First, both the increase of dynamical excitation in the eccentric belt of the debris disk system and the decrease of the material strength of dust particles result in a higher production rate of smaller particles. This reduces the surface brightness differences between the periastron and the apastron sides of the disks. For very low material strengths, the “pericenter glow” phenomenon is reduced and eventually even replaced by the opposite effect, the “apocenter glow”. In contrast, higher material strengths and lower dynamical excitation of the system result in an enhancement of asymmetries in the surface brightness distribution. Second, it is possible to constrain the level of collisional activity from the appearance of the disk, for example, the wavelength-dependent apocenter-to-pericenter flux ratio. Within the considered parameter space, the impact of the material strength on the appearance of the disk is stronger than that of dynamical excitation of the system. Finally, we find that the impact of the collisional parameters on the net spectral energy distribution is weak.

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Kuanyshbaiuly, Y., K. Baiseitov et Т. Ramazanov. « Cornell potential in collisional quark-gluon plasma ». BULLETIN of the L.N. Gumilyov Eurasian National University. PHYSICS. ASTRONOMY Series 142, n^o 1 (30 mars 2023) : 16–25. http://dx.doi.org/10.32523/2616-6836-2023-142-1-16-25.

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In experimental studies of quark-gluon plasma at accelerators, charmonium and bottomonium play an important role, since their onset of dissociation is one of the few signs of a phase transition in a hot and superdense medium. These particles represent the bound states of the heavy quark and antiquark of charm and bottom flavour, respectively. One of the well-studied research methods is considered to be phenomenological models, where the interaction between quarks is described by the Cornell-type potentials, as well as its modifications. This article examines the influence of the movement of charmonium and bottomonium in an environment on the potential, using dynamic screening. For screening, the dielectric function of the wake field has been used by analogy with the electrodynamics of an ordinary plasma, in view of the similarity of description, which is characteristic only for these bound states. This dielectric function has been obtained from a collisional plasma based on the Boltzmann equation with the Bhatnagar-Gross-Krook collision operator. As a consequence, the equation describes the medium as rarefied gas with pair collisions between particles. Effective potentials between quarks in a given medium are constructed using the method of random-phase approximation. Potentials were obtained as a function of the relative motion of bound states in the medium and the collision frequency of counterpropagating particles. The characteristic changes in the potential in the direction of particle motion are graphically shown, depending on the magnitude of these parameters.

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Gilbert, RG. « Mechanism and Models for Collisional Energy Transfer in Highly Excited Large Polyatomic Molecules ». Australian Journal of Chemistry 48, n^o 11 (1995) : 1787. http://dx.doi.org/10.1071/ch9951787.

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Collisional energy transfer in highly excited molecules (say, 200-500 kJ mol-1 above the zero-point energy of reactant, or of product, for a recombination reaction) is reviewed. An understanding of this energy transfer is important in predicting and interpreting the pressure dependence of gas-phase rate coefficients for unimolecular and recombination reactions. For many years it was thought that this pressure dependence could be calculated from a single energy-transfer quantity, such as the average energy transferred per collision. However, the discovery of 'supercollisions' (a small but significant fraction of collisions which transfer abnormally large amounts of energy) means that this simplistic approach needs some revision. The 'ordinary' (non-super) component of the distribution function for collisional energy transfer can be quantified either by empirical models (e.g., an exponential-down functional form) or by models with a physical basis, such as biased random walk (applicable to monatomic or diatomic collision partners) or ergodic (for polyatomic collision partners) treatments. The latter two models enable approximate expressions for the average energy transfer to be estimated from readily available molecular parameters. Rotational energy transfer, important for finding the pressure dependence for recombination reactions, can for these purposes usually be taken as transferring sufficient energy so that the explicit functional form is not required to predict the pressure dependence. The mechanism of 'ordinary' energy transfer seems to be dominated by low-frequency modes of the substrate, whereby there is sufficient time during a vibrational period for significant energy flow between the collision partners. Supercollisions may involve sudden energy flow as an outer atom of the substrate is squashed between the substrate and the bath gas, and then is moved away from the interaction by large-amplitude motion such as a ring vibration or a rotation; improved experimental and theoretical understanding of this phenomenon is seen as an important area for future development.

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Pezzi, O., H. Liang, J. L. Juno, P. A. Cassak, C. L. Vásconez, L. Sorriso-Valvo, D. Perrone et al. « Dissipation measures in weakly collisional plasmas ». Monthly Notices of the Royal Astronomical Society 505, n^o 4 (27 mai 2021) : 4857–73. http://dx.doi.org/10.1093/mnras/stab1516.

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ABSTRACT The physical foundations of the dissipation of energy and the associated heating in weakly collisional plasmas are poorly understood. Here, we compare and contrast several measures that have been used to characterize energy dissipation and kinetic-scale conversion in plasmas by means of a suite of kinetic numerical simulations describing both magnetic reconnection and decaying plasma turbulence. We adopt three different numerical codes that can also include interparticle collisions: the fully kinetic particle-in-cell vpic, the fully kinetic continuum Gkeyll, and the Eulerian Hybrid Vlasov–Maxwell (HVM) code. We differentiate between (i) four energy-based parameters, whose definition is related to energy transfer in a fluid description of a plasma, and (ii) four distribution function-based parameters, requiring knowledge of the particle velocity distribution function. There is an overall agreement between the dissipation measures obtained in the PIC and continuum reconnection simulations, with slight differences due to the presence/absence of secondary islands in the two simulations. There are also many qualitative similarities between the signatures in the reconnection simulations and the self-consistent current sheets that form in turbulence, although the latter exhibits significant variations compared to the reconnection results. All the parameters confirm that dissipation occurs close to regions of intense magnetic stresses, thus exhibiting local correlation. The distribution function-based measures show a broader width compared to energy-based proxies, suggesting that energy transfer is co-localized at coherent structures, but can affect the particle distribution function in wider regions. The effect of interparticle collisions on these parameters is finally discussed.

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Kar, C., S. K. Majumdar et A. N. Sekar Iyengar. « Stabilization of collisional drift waves by kinetic Alfvén waves ». Journal of Plasma Physics 47, n^o 2 (avril 1992) : 249–60. http://dx.doi.org/10.1017/s002237780002420x.

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We have investigated a mode-coupling mechanism between kinetic Alfvén waves and a collisional drift wave in an inhomogeneous cylindrical plasma. Drift waves satisfying the condition k⊥D > 1/r0 (where r0 is the radius of the plasma cylinder) are stabilized by the low-frequency ponderomotive force generated by the kinetic Alfvén waves. For typical plasma parameters and a moderate level of Alfven-wave intensity the stabilization factor is comparable to the destabilization mechanism due to collisions.

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Jungwirth, Pavel, et Victoria Buch. « Van der Waals Attraction and Coalescence of Aqueous Salt Nanodroplets ». Collection of Czechoslovak Chemical Communications 68, n^o 12 (2003) : 2283–91. http://dx.doi.org/10.1135/cccc20032283.

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Collisions of aqueous salt nanodroplets at zero initial relative velocity are investigated by means of molecular dynamics simulations. The character of the van der Waals interactions, which bring the droplets together and cause coalescence, is described in detail, and the parameters of the droplet-droplet potential are extracted from the collisional trajectories. Concentration and size effects, together with implications for cloud and precipitation modeling are discussed.

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Frasca, A., R. Bruce, F. Cerutti, A. Ciccotelli et M. Patecki. « Optimizing Pb beam losses at the LHCb for maximum luminosity ». Journal of Physics : Conference Series 2687, n^o 2 (1 janvier 2024) : 022002. http://dx.doi.org/10.1088/1742-6596/2687/2/022002.

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Abstract In addition to the physics program with proton beams, the Large Hadron Collider (LHC) at CERN also provides collisions of fully-stripped Pb beams for about one month per year. When colliding Pb nuclei, electromagnetic interactions are the dominating processes because of the intense Coulomb field produced by the ions. These ’ultra-peripheral’ interactions give rise to ions with a changed magnetic rigidity. This causes losses in the machine that can impose limits on the luminosity. Among them, the bound-free pair production (BFPP) causes a localised power deposition downstream of each collision point, which could induce superconducting magnet quenches if not well controlled. These losses were studied and successfully mitigated for most LHC experiments, however the recent request by LHCb to increase the Pb-Pb luminosity requires a revision of BFPP collisional loss limitations. In this paper, the simulation of BFPP losses from Pb-Pb collisions around LHCb is presented. The loss patterns are discussed for different beam parameters. Finally, a mitigation strategy by means of an orbit bump is studied.

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Thébault, Philippe. « Planet signatures and Size Segregation in Debris Discs ». Proceedings of the International Astronomical Union 8, S299 (juin 2013) : 358–59. http://dx.doi.org/10.1017/s1743921313008946.

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The response of a debris disc to a planetary perturber is the result of the complex interplay between gravitational effects, grain collisions and stellar radiation pressure (Stark & Kuchner (2009). We investigate to what extent this response can depart from the pure gravitational case when including grain collisional production and radiation pressure. We use the DyCoSS code (Thébault (2012), designed to study the coupled effect of collisions and dynamics for systems at steady state with one perturbing body. We focus on two outcomes: the 2D surface density profile of the disc+planet system, and the way the Particle Size Distribution (PSD) is spatially segregated within the disc. We consider two set-ups: 1) a narrow ring with an exterior “shepherding” planet, and 2) an extended disc in which a planet is embedded. For each case, the planet mass and orbit are explored as free parameters, and an unperturbed “no-planet” case is also considered. Another parameter is the disc's collisional activity, as parameterized by its optical depth τ.

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Zhang, Geng, et Xiongping Xia. « Laser beam self-focusing in collisional plasma with periodical density ripple ». Laser and Particle Beams 38, n^o 1 (4 février 2020) : 45–53. http://dx.doi.org/10.1017/s0263034620000026.

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AbstractIn the paper, we applied the paraxial region theory and Wentzel–-Kramers–-Brillouin approximation to study laser beam self-focusing in the interaction of laser and collisional plasma with periodical density ripple. The results have shown that, under the influence of collision nonlinear effect, laser presents stable self-focusing, self-defocusing, and oscillational self-focusing in the plasma. Besides, the parameters of plasma with periodical density ripple have a greater impact on the effect of self-defocusing and oscillational self-focusing than stable self-focusing. In certain conditions, beam self-defocusing and oscillational self-focusing would decline and even disappear, and stable self-focusing would further be strengthened. Hence, selecting a suitable periodic plasma system is advantageous for separating self-defocusing and oscillational self-focusing, and for the formation of a more stable collisional self-focusing.

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Barway, Sudhanshu, Y. D. Mayya et Aitor Robleto-Orús. « Discovery of a near-infrared bar and a pseudo-bulge in the collisional ring galaxy Cartwheel ». Monthly Notices of the Royal Astronomical Society 497, n^o 1 (3 juillet 2020) : 44–51. http://dx.doi.org/10.1093/mnras/staa1887.

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ABSTRACT We report the discovery of a bar, a pseudo-bulge, and unresolved point source in the archetype collisional ring galaxy Cartwheel using careful morphological analysis of a near-infrared (NIR) Ks-band image of excellent quality (seeing = 0.42″) at the ESO archive. The bar is oval-shaped with a semi-major axis length of 3.23″ (∼2.09 kpc), with almost a flat light distribution along it. The bulge is almost round (ellipticity = 0.21) with an effective radius of 1.62″ (∼1.05 kpc) and a Sersic index of 0.99, parameters typical of pseudo-bulges in late-type galaxies. The newly discovered bar is not recognizable as such in the optical images even with more than a factor of 2 higher spatial resolution of the Hubble Space Telescope, due to a combination of its red colour and the presence of dusty features. The observed bar and pseudo-bulge most likely belonged to the pre-collisional progenitor of the Cartwheel. The discovery of a bar in an archetype collisional ring galaxy Cartwheel is the first observational evidence to confirm the prediction that bars can survive a drop-through collision along with the morphological structures like a central bulge (pseudo).

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Varshney, Prateek, Ajit Upadhayay, K. Madhubabu, Vivek Sajal et J. A. Chakera. « Strong terahertz radiation generation by cosh-Gaussian laser beams in axially magnetized collisional plasma under non-relativistic ponderomotive regime ». Laser and Particle Beams 36, n^o 2 (juin 2018) : 236–45. http://dx.doi.org/10.1017/s0263034618000216.

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AbstractWe propose a scheme for terahertz (THz) radiation generation by non-linear mixing of two cosh-Gaussian laser beams in axially magnetized plasma with spatially periodic density ripple where electron-neutral collisions have been taken into account. The laser beams exert a non-linear ponderomotive force due to spatial non-uniformity in the intensity. The plasma electrons acquire non-linear oscillatory velocity under the influence of ponderomotive force. This oscillatory velocity couples with preformed density ripples (n′ = n0αeiαz) to generate a strong transient non-linear current that resonantly derives THz radiation of frequency ~ωh (upper hybrid frequency). Laser frequencies (ω1 and ω2) are chosen such that the beat frequency (ω) lies in the THz region. The periodicity of density ripple provides phase-matching conditions (ω = ω1 − ω2 and $\vec k = \vec k_1 - \vec k_2 + {\rm \vec \alpha} $) to transfer maximum momentum from laser to THz radiation. The axially applied external magnetic field can be utilized to enhance the non-linear coupling and control various parameters of generated THz wave. The effects of decentered parameters (b), collisional frequency (νen), and magnetic field strength (B0 = ωcm/e) are analyzed for strong THz radiation generation. Analytical results show that the amplitude of THz wave enhances with decentered parameters as well as with the magnitude of axially applied magnetic field. The THz amplitude is found to be highly sensitive to collision frequency.

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Chakraborty, Priyanka, Rachel Hemmer, Adam R. Foster, John Raymond, Arnab Sarkar, Randall Smith et Nancy Brickhouse. « Investigating the Impact of Atomic Data Uncertainties on the Measured Physical Parameters of the Perseus Galaxy Cluster ». Astrophysical Journal 962, n^o 2 (1 février 2024) : 192. http://dx.doi.org/10.3847/1538-4357/ad17be.

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Abstract Accurate atomic data and plasma models are essential for interpreting the upcoming high-quality spectra from missions like XRISM and Athena. Estimating physical quantities, like temperature, abundance, turbulence, and the resonance scattering factor, is highly dependent on the underlying atomic data. We use the AtomDB tool variableapec to estimate the impact of atomic data uncertainties in Einstein A coefficients, collisional rate coefficients, and the ionization and recombination rates of H-, He-, and Li-like iron in modeling the spectrum of Perseus observed by Hitomi. The best-fit temperatures, abundances, resonance scattering factors, and turbulence parameters including atomic data uncertainties vary by approximately 17%, 35%, 30%, and 3%, respectively, from the best-fit temperatures, abundances, RS factors, and turbulence parameters estimated without atomic data uncertainties. These indicate that approximately 32%, 35%, and 25% of the best-fit temperatures, abundances, and resonance scattering factors when including uncertainties lie outside the 3σ error regions of their corresponding best-fit values computed with zero atomic data errors. Expanding the energy range to 1.8–20.0 keV shows less variability, with 26% of the abundances and 22% of the resonance scattering factors lying outside the 3σ errors of the best-fit values. We also studied correlations between physical parameters and atomic rate uncertainties to identify key atomic quantities requiring precise lab measurements. We report negative correlations between the best-fit temperatures and the z (1s2s 3S1 → 1s2) collisional rate coefficients, abundances and y (1s2p 3P1 → 1s2) collisional rate coefficients, and abundances and z collisional rate coefficients, and a positive correlation between the resonance scattering factors and the w (1s2p 1P1 → 1s2) collisional rate coefficients.

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Ezzeddine, R., T. Merle, B. Plez, M. Gebran, F. Thévenin et M. Van der Swaelmen. « An empirical recipe for inelastic hydrogen-atom collisions in non-LTE calculations ». Astronomy & ; Astrophysics 618 (octobre 2018) : A141. http://dx.doi.org/10.1051/0004-6361/201630352.

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Context. Determination of high-precision abundances of late-type stars has been and always will be an important goal of spectroscopic studies, which requires accurate modeling of their stellar spectra with non-local thermodynamic equilibrium (NLTE) radiative transfer methods. This entails using up-to-date atomic data of the elements under study, which are still subject to large uncertainties.Aims. We investigate the role of hydrogen collisions in NLTE spectral line synthesis, and introduce a new general empirical recipe to determine inelastic charge transfer (CT) and bound-bound hydrogen collisional rates. This recipe is based on fitting the energy functional dependence of published quantum collisional rate coefficients of several neutral elements (BeI, Na I, Mg I, Al I, Si Iand Ca I) using simple polynomial equations.Methods. We perform thorough NLTE abundance calculation tests using our method for four different atoms, Na, Mg, Al and Si, for a broad range of stellar parameters. We then compare the results to calculations computed using the published quantum rates for all the corresponding elements. We also compare to results computed using excitation collisional rates via the commonly used Drawin equation for different fudge factors,SMH, applied.Results. We demonstrate that our proposed method is able to reproduce the NLTE abundance corrections performed with the quantum rates for different spectral types and metallicities for representative Na Iand Al Ilines to within ≤0.05 dex and ≤0.03 dex, respectively. For Mg Iand Si Ilines, the method performs better for the cool giants and dwarfs, while larger discrepancies up to 0.2 dex could be obtained for some lines for the subgiants and warm dwarfs. We obtained larger NLTE correction differences between models incorporating Drawin rates relative to the quantum models by up to 0.4 dex. These large discrepancies are potentially due to ignoring either or both CT and ionization collisional processes by hydrogen in our Drawin models.Conclusions. Our general empirical fitting method (EFM) for estimating hydrogen collision rates performs well in its ability to reproduce, within narrow uncertainties, the abundance corrections computed with models incorporating quantum collisional rates. It performs generally best for the cool and warm dwarfs, with slightly larger discrepancies obtained for the giants and subgiants. It could possibly be extended in the future to transitions of the same elements for which quantum calculations do not exist, or, in the absence of published quantum calculations, to other elements as well.

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Martinović, Mihailo M., et Kristopher G. Klein. « Ion-driven Instabilities in the Inner Heliosphere. II. Classification and Multidimensional Mapping ». Astrophysical Journal 952, n^o 1 (1 juillet 2023) : 14. http://dx.doi.org/10.3847/1538-4357/acdb79.

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Abstract Linear theory is a well-developed framework for characterizing instabilities in weakly collisional plasmas, such as the solar wind. In the previous installment of this series, we analyzed ∼1.5M proton and α particle velocity distribution functions (VDFs) observed by Helios I and II to determine the statistical properties of the standard instability parameters such as the growth rate, frequency, the direction of wave propagation, and the power emitted or absorbed by each component, as well as to characterize their behavior with respect to the distance from the Sun and collisional processing. In this work, we use this comprehensive set of instability calculations to train a machine-learning algorithm consisting of three interlaced components that: (1) predict if an interval is unstable from observed VDF parameters; (2) predict the instability properties for a given unstable VDF; and (3) classify the type of the unstable mode. We use these methods to map the properties in multidimensional phase space to find that the parallel-propagating, proton-core-induced ion cyclotron mode dominates the young solar wind, while the oblique fast magnetosonic mode regulates the proton beam drift in the collisionally old plasma.

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Fajardo, D., C. Angioni, P. Maget et P. Manas. « Analytical model for collisional impurity transport in tokamaks at arbitrary collisionality ». Plasma Physics and Controlled Fusion 64, n^o 5 (12 avril 2022) : 055017. http://dx.doi.org/10.1088/1361-6587/ac5b4d.

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Abstract The physics governing the collisional transport of impurities in tokamak plasmas can change significantly depending on four main parameters, namely the collisionality, the impurity charge and mass, and the trapped particle fraction, which can vary widely from the core to the edge of a fusion device. We present an analytical model for collisional impurity transport with a consistent dependence on broad scans in these four parameters, showing good agreement with the drift-kinetic code NEO. Radial profiles of collisional fluxes are calculated for different impurity species using ASDEX Upgrade experimental profiles as well as ITER simulated profiles, and they are also compared to NEO. This model is well suited for fast integrated modelling applications due to its low computational cost.

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Karnopp, Júlia, Bernardo Magaldi, Julio Sagás et Rodrigo Pessoa. « The Effect of Excited Species on the Collisional Energy of Argon Inductively Coupled Plasmas : A Global Model Study ». Plasma 5, n^o 1 (4 janvier 2022) : 30–43. http://dx.doi.org/10.3390/plasma5010003.

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Global modeling of inductively coupled plasma (ICP) reactors is a powerful tool to investigate plasma parameters. In this article, the argon ICP global model is revisited to explore the effect of excited species on collisional energy through the study of different approaches to particle and energy balance equations. The collisional energy loss is much more sensitive to modifications in the balance equations than the electron temperature. According to the simulations, the multistep ionization reduces the collisional energy loss in all investigated reaction sets and the inclusion of heavy species reactions has negligible influence. The plasma parameters obtained, such as total energy loss and electron temperature, were compared with experimental results from the literature. The simulated cases that have more excited species and reactions in the energy balance are in better agreement with the experimental measurements.

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Farji, A., H. Aroui et J. Vander Auwera. « Air-induced collisional parameters in the ν3 band of methane ». Journal of Quantitative Spectroscopy and Radiative Transfer 275 (novembre 2021) : 107878. http://dx.doi.org/10.1016/j.jqsrt.2021.107878.

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Yang, Lei, J. T. (Johan) Padding et J. A. M. (Hans) Kuipers. « Investigation of collisional parameters for rough spheres in fluidized beds ». Powder Technology 316 (juillet 2017) : 256–64. http://dx.doi.org/10.1016/j.powtec.2016.12.090.

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Makarov, D. S., I. A. Koval, M. A. Koshelev, V. V. Parshin et M. Yu Tretyakov. « Collisional parameters of the 118-GHz oxygen line : Temperature dependence ». Journal of Molecular Spectroscopy 252, n^o 2 (décembre 2008) : 242–43. http://dx.doi.org/10.1016/j.jms.2008.08.005.

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Whyte, Andrew R., Kieran F. Lim, Robert G. Gilbert et William L. Hase. « The calculation and interpretation of average collisional energy transfer parameters ». Chemical Physics Letters 152, n^o 4-5 (novembre 1988) : 377–81. http://dx.doi.org/10.1016/0009-2614(88)80109-x.

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Nishiyama, Akiko, Grzegorz Kowzan, Dominik Charczun, Roman Ciuryło et Piotr Masłowski. « Comb-based FTIR spectroscopy of CO perturbed by N₂ at 4.6 µm ». EPJ Web of Conferences 287 (2023) : 07030. http://dx.doi.org/10.1051/epjconf/202328707030.

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Line-shape measurements of the fundamental vibrational band of CO at 4.6 µm perturbed by N2 with a mid-infrared frequency comb-based Fourier-transform spectrometer were performed. Precise collisional line-shape parameters for 41 lines were determined, including the pressure broadening and shifting and speed-dependence of the collisional width. The results were compared with sparse literature data available.

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Puchkov, V. A. « Thomson scattering from Langmuir fluctuations in a parabolic layer of a collisional plasma ». Canadian Journal of Physics 96, n^o 9 (septembre 2018) : 1053–58. http://dx.doi.org/10.1139/cjp-2017-0772.

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Thomson scattering of a probe wave by the Langmuir fluctuations inside a plasma layer with a parabolic density profile is considered. The collisional damping of plasma fluctuations is taken into account. The plasma line part of the scattered spectrum is calculated depending on the layer thickness, electron collision frequency, and the form of the distribution functions for the electrons and ions. Simple analytic expressions for the plasma line shape and characteristic spectrum width are found. It is shown that this plasma line is asymmetric, and the asymmetry depends on the layer type (maximum or minimum). Some important plasma parameters, such as the electron collision frequency and the sign of the electron density deviation inside the layer can be obtained from the plasma line spectrum calculated in this paper.

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Dagdigian, Paul J. « Collisional excitation of deuterated hydroxyl (OD) by molecular hydrogen ». Monthly Notices of the Royal Astronomical Society 505, n^o 2 (19 mai 2021) : 1987–91. http://dx.doi.org/10.1093/mnras/stab1412.

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ABSTRACT Cross sections and rate coefficients for transitions between hyperfine levels associated with the 14 lowest rotational/fine-structure levels of OD induced by collisions with ortho-H2 and para-H2 are presented. These collisional parameters have been computed in time-independent close-coupling quantum-scattering calculations with a potential energy surface (PES) describing the OD–H2 interaction, which was obtained by transformation of the OH–H2 PES. Rate coefficients have been computed for temperatures from 5 to 200 K. Cross sections for OD transitions between rotational/fine-structure levels are found to be significantly larger than the corresponding transitions in OH, mainly because of the reduced energy gaps in OD. The hyperfine-resolved rate coefficients were employed in simple radiative transfer calculations for OD and compared with analogous calculations for OH.

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Gilbert, RG, et MJ McEwan. « The Pressure Dependance of Ion-Molecule Reaction Rate Coefficients : CH3+ + HCN/He ». Australian Journal of Chemistry 38, n^o 2 (1985) : 231. http://dx.doi.org/10.1071/ch9850231.

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Illustrative calculations are presented on the application to termolecular ion-molecule reactions of methods recently developed for the study of fall-off effects in neutral thermal unimolecular reactions. The energy-dependent microscopic reaction rate, k(E), is obtained from RRKM theory with activated complex parameters first estimated by using ab initio and spectroscopic data and then refined to yield the appropriate pressure-saturated rate. The collisional energy transfer probability distribution function, P(E,E′), is obtained by fitting the fall-off data, guided by information from trajectory calculations. Overall rate coefficients are computed from accurate solutions to the appropriate integral master equation. The illustrative calculations are for the CH3+ + HCN+He → C2H4N+ +He system. It is shown that pressure-dependent data for ion-molecule systems can yield reliable information on P(E,E′). Collisions with the bath gas (He) are comparatively weak, with the average downward energy transferred per collision being c. 8 kJ mol-1. The product of the reaction before any isomerization can occur is shown to be protonated methyl isocyanide , H3CNCH+.

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Zubair, M., Muhammad Zeeshan, Syed Hasan et V. Oikonomou. « Impact of Collisional Matter on the Late-Time Dynamics of f(R,T) Gravity ». Symmetry 10, n^o 10 (5 octobre 2018) : 463. http://dx.doi.org/10.3390/sym10100463.

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We study the cosmic evolution of non-minimally coupled f ( R , T ) gravity in the presence of matter fluids consisting of collisional self-interacting dark matter and radiation. We study the cosmic evolution in the presence of collisional matter, and we compare the results with those corresponding to non-collisional matter and the Λ -cold-dark-matter ( Λ CDM) model. Particularly, for a flat Friedmann–Lema i ^ tre–Robertson–Walker Universe, we study two non-minimally coupled f ( R , T ) gravity models and we focus our study on the late-time dynamical evolution of the model. Our study is focused on the late-time behavior of the effective equation of the state parameter ω e f f and of the deceleration parameter q as functions of the redshift for a Universe containing collisional and non-collisional dark matter fluids, and we compare both models with the Λ CDM model. As we demonstrate, the resulting picture is well accommodated to the latest observational data on the basis of physical parameters.

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Sirohi, Kapil. « Collisional Alignment and Orientation Parameters of Ra Atom at 75 eV ». IOSR Journal of Applied Physics 2, n^o 1 (2012) : 19–23. http://dx.doi.org/10.9790/4861-0211923.

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Tran, H., D. Bermejo, J. L. Domenech, P. Joubert, R. R. Gamache et J. M. Hartmann. « Collisional parameters of H2O lines : Velocity effects on the line-shape ». Journal of Quantitative Spectroscopy and Radiative Transfer 108, n^o 1 (novembre 2007) : 126–45. http://dx.doi.org/10.1016/j.jqsrt.2007.03.009.

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Petrova, T. M., A. M. Solodov, A. A. Solodov et V. I. Starikov. « Broadening parameters of the H2O–He collisional system for astrophysical applications ». Journal of Molecular Spectroscopy 321 (mars 2016) : 50–58. http://dx.doi.org/10.1016/j.jms.2016.01.009.

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Duclous, R., J. P. Morreeuw, V. T. Tikhonchuk et B. Dubroca. « Reduced multi-scale kinetic models for the relativistic electron transport in solid targets : Effects related to secondary electrons ». Laser and Particle Beams 28, n^o 1 (mars 2010) : 165–77. http://dx.doi.org/10.1017/s0263034610000042.

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AbstractA reduced mathematical model for the transport of high current relativistic electron beams in a dense collisional plasma is developed. Based on the hypothesis that the density of relativistic electrons is much less than the plasma density and their energy is much higher than the plasma temperature, a model with two energy scales is proposed, where the beam and plasma electrons are considered as two coupled sub-systems, which exchange the energy and particles due to collisions. The process of energy exchange is described in the Fokker-Planck approximation, where the pitch angle electron-ion and electron-electron collisions dominate. The process of particle exchange between populations, leading to the production of secondary energetic electrons, is described with a Boltzmann term. The electron-electron collisions with small impact parameters make an important contribution in the overall dynamics of the beam electrons.

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Gonzalez-Ondina, Jose M., Luigi Fraccarollo et Philip L. F. Liu. « Two-level, two-phase model for intense, turbulent sediment transport ». Journal of Fluid Mechanics 839 (26 janvier 2018) : 198–238. http://dx.doi.org/10.1017/jfm.2017.920.

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The study of sediment transport requires in-depth investigation of the complex effects of sediment particles in fluid turbulence. In this paper we focus on intense sediment transport flows. None of the existing two-phase models in the literature properly replicates the liquid and solid stresses in the near bed region of high concentration of sediment. The reason for this shortcoming is that the physical processes occurring at the length scale of the particle collisions are different from those occurring at larger length scales and therefore, they must be modelled independently. We present here a two-level theoretical derivation of two-phase, Favre averaged Navier–Stokes equations (FANS). This approach treats two levels of energy fluctuations independently, those associated with a granular spatial scale (granular temperature and small-scale fluid turbulence) and those associated with the ensemble average (turbulent kinetic energy for the two phases). Although similar attempts have been made by other researchers, the two level approach ensures that the two relevant length scales are included independently in a more consistent manner. The model is endowed with a semi-empirical formulation for the granular scale fluid turbulence, which is important even in the dense collisional shear layer, as has been recently recognized. As a result of the large and small scale modelling of the liquid and solid fluctuations, predictions are promising to be reliable in a wide range of flow conditions, from collisional to turbulent suspensions. This model has been validated for steady state flows with intense, collisional or mixed collisional–turbulent sediment transport, using various sources of detailed experimental data. It compares well with the experimental results in the whole experimental range of Shields parameters, better than previous models, although at the cost of increased complexity in the equations. Further experiments on turbulent suspensions would be necessary to definitely assess the model capabilities.

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Pokorný, Petr, Althea V. Moorhead, Marc J. Kuchner, Jamey R. Szalay et David M. Malaspina. « How Long-lived Grains Dominate the Shape of the Zodiacal Cloud ». Planetary Science Journal 5, n^o 3 (1 mars 2024) : 82. http://dx.doi.org/10.3847/psj/ad2de8.

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Abstract Grain–grain collisions shape the three-dimensional size and velocity distribution of the inner Zodiacal Cloud and the impact rates of dust on inner planets, yet they remain the least understood sink of zodiacal dust grains. For the first time, we combine the collisional grooming method combined with a dynamical meteoroid model of Jupiter-family comets (JFCs) that covers 4 orders of magnitude in particle diameter to investigate the consequences of grain–grain collisions in the inner Zodiacal Cloud. We compare this model to a suite of observational constraints from meteor radars, the Infrared Astronomical Satellite, mass fluxes at Earth, and inner solar probes, and use it to derive the population and collisional strength parameters for the JFC dust cloud. We derive a critical specific energy of Q D * = 5 × 10 5 ± 4 × 10 5 R met − 0.24 J kg−1 for particles from JFC particles, making them 2–3 orders of magnitude more resistant to collisions than previously assumed. We find that the differential power-law size index −4.2 ± 0.1 for particles generated by JFCs provides a good match to observed data. Our model provides a good match to the mass-production rates derived from the Parker Solar Probe observations and their scaling with the heliocentric distance. The higher resistance to collisions of dust particles might have strong implications to models of collisions in solar and exosolar dust clouds. The migration via Poynting–Robertson drag might be more important for denser clouds, the mass-production rates of astrophysical debris disks might be overestimated, and the mass of the source populations might be underestimated. Our models and code are freely available online.

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Hematizadeh, A., S. M. Jazayeri et B. Ghafary. « Generation of terahertz radiation by beating of two laser beams in collisional magnetized plasma ». Laser and Particle Beams 34, n^o 4 (30 août 2016) : 569–75. http://dx.doi.org/10.1017/s0263034616000513.

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AbstractThis paper presents analytical calculations for terahertz (THz) radiation by beating of two cosh-Gaussian laser beams in a density rippled collisional magnetized plasma. Lasers beams exert a ponderomotive force on the electrons of plasma in beating frequency which generates THz waves. The magnetic field was considered parallel to the direction of lasers which leads to propagate right-hand circularly polarized or left-hand circularly polarized waves in the plasma depending on the phase matching conditions. Effects of collision frequency, decentered parameter of lasers and the magnetic field strength are analyzed for THz radiation generation. By the optimization of laser and plasma parameters, the efficiency of order 27% can be achieved.

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Candioti, Lorenzo G., Thibault Duretz, Evangelos Moulas et Stefan M. Schmalholz. « Buoyancy versus shear forces in building orogenic wedges ». Solid Earth 12, n^o 8 (10 août 2021) : 1749–75. http://dx.doi.org/10.5194/se-12-1749-2021.

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Abstract. The dynamics of growing collisional orogens are mainly controlled by buoyancy and shear forces. However, the relative importance of these forces, their temporal evolution and their impact on the tectonic style of orogenic wedges remain elusive. Here, we quantify buoyancy and shear forces during collisional orogeny and investigate their impact on orogenic wedge formation and exhumation of crustal rocks. We leverage two-dimensional petrological–thermomechanical numerical simulations of a long-term (ca. 170 Myr) lithosphere deformation cycle involving subsequent hyperextension, cooling, convergence, subduction and collision. Hyperextension generates a basin with exhumed continental mantle bounded by asymmetric passive margins. Before convergence, we replace the top few kilometres of the exhumed mantle with serpentinite to investigate its role during subduction and collision. We study the impact of three parameters: (1) shear resistance, or strength, of serpentinites, controlling the strength of the evolving subduction interface; (2) strength of the continental upper crust; and (3) density structure of the subducted material. Densities are determined by linearized equations of state or by petrological-phase equilibria calculations. The three parameters control the evolution of the ratio of upward-directed buoyancy force to horizontal driving force, FB/FD=ArF, which controls the mode of orogenic wedge formation: ArF≈0.5 causes thrust-sheet-dominated wedges, ArF≈0.75 causes minor wedge formation due to relamination of subducted crust below the upper plate, and ArF≈1 causes buoyancy-flow- or diapir-dominated wedges involving exhumation of crustal material from great depth (>80 km). Furthermore, employing phase equilibria density models reduces the average topography of wedges by several kilometres. We suggest that during the formation of the Pyrenees ArF⪅0.5 due to the absence of high-grade metamorphic rocks, whereas for the Alps ArF≈1 during exhumation of high-grade rocks and ArF⪅0.5 during the post-collisional stage. In the models, FD increases during wedge growth and subduction and eventually reaches magnitudes (≈18 TN m−1) which are required to initiate subduction. Such an increase in the horizontal force, required to continue driving subduction, might have “choked” the subduction of the European plate below the Adriatic one between 35 and 25 Ma and could have caused the reorganization of plate motion and subduction initiation of the Adriatic plate.

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Sharma, Mohit K., et Suresh Chandra. « Anomalous phenomena in cyclopropenylidene (c-C3H2) with accurate collisional rate coefficients ». Monthly Notices of the Royal Astronomical Society 514, n^o 2 (14 juin 2022) : 2116–21. http://dx.doi.org/10.1093/mnras/stac1360.

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ABSTRACT Collisional rate coefficients are important parameters, but their calculation is a tedious task. Accurate collisional rate coefficients for collisional transitions between pure rotational levels of cyclopropenylidene (c-C3H2), colliding with He atom, are now available. Strong anomalous absorption line 220–211 at 21.587 GHz of cyclopropenylidene is found ubiquitous in interstellar medium. It is therefore worth to analyse the line spectrum of c-C3H2 using accurate collisional rate coefficients. After performing the Sobolev analysis of c-C3H2, we have found four anomalous absorption lines namely 22,0–21,1, 44,0–43,1, 33,0–32,1, and 43,2–50,5. We have also found two weak MASER lines, 40,4–33,1 and 51,4–44,1, which may make the detection of c-C3H2 in a cosmic object more convenient. Even for low kinetic temperature and low column density of cyclopropenylidene, the detection of 22,0–21,1 transition is possible, and it may be the reason for its detection in a large number of cosmic objects. The stability of results is tested.

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