Anisotropic Collisions Research Articles

Numerical simulation of non-spherical microparticles' deposition on single fiber

As a classical gas-solid two-phase flow system, the processes of fiber filtering microparticles are prevalent in nature and engineering. However, the impact of microparticle shape on fiber filtration processes is still largely unexplored. Herein, using the self-developed spheropolyhedral-based discrete element lattice Boltzmann method, the filtration process of non-spherical microparticles through a single fiber is investigated. Results show that the single fiber efficiency (SFE) for non-spherical particles exhibits a trend of initially increasing and subsequently decreasing trend with the increase in Stokes number (St), which is similar to the case of spherical particles. However, it is interesting to note that the peak values of SFE increase significantly with decreasing particle sphericity (ψ) and the corresponding St values become larger. As ψ decreases from 1.0 (sphere) to 0.671 (tetrahedron), the SFE increase from 0.205 to 0.49 and the corresponding St rises from 1.0 to 1.75. The enhanced SFE can be explained by elevated collision probability and adhesion probability, based on detailed particle kinematics and dynamics behavior analysis as well as microscopic depositional structure evaluation. The depositional structures of the non-spherical particles have larger capture areas, leading to higher initial collision probabilities. Meanwhile, the anisotropic collisions between non-spherical particles and fibers greatly contribute to higher secondary collision probabilities. In addition, compared to spherical particles of the same volume, the non-spherical particles experience greater fluid resistance, resulting in lower initial collision velocities and larger initial adhesion probabilities. The face-to-face contacts between non-spherical particles also lead to stronger interparticle adhesion and enhanced adhesion probabilities.

The plasma–sheath transition and Bohm criterion in a high recycling divertor

The high recycling regime of a divertor is characterized by high plasma particle fluxes and low temperature at the target, where a strong hydrogen recirculation loop exists. Atomic processes in the high recycling regime, such as ion–neutral friction and radiation, can affect the plasma momentum and energy transport in the sheath transition region. Here, the plasma–sheath transition near a high recycling wall is investigated. The Bohm speed, which constraints the ion exit flow speed, is evaluated from a transport model that accounts for the effect of the anisotropic transport and atomic collisions in the transition layer. A first principles kinetic code vector particle-in-cell with the atomic collision package is used to investigate a 1D self-consistent slab plasma with a high recycling boundary for the tungsten and carbon divertors. The results demonstrate the accuracy of the Bohm speed model in predicting the ion exit flow speed in the transition region, as well as the reduction of the Bohm speed due to the ion–neutral friction. The effect of different wall materials, tungsten, and carbon, on the Bohm speed and near-wall plasma profile is shown.

Tomography of Feshbach resonance states

Feshbach resonances are fundamental to interparticle interactions and become particularly important in cold collisions with atoms, ions, and molecules. In this work, we present the detection of Feshbach resonances in a benchmark system for strongly interacting and highly anisotropic collisions: molecular hydrogen ions colliding with noble gas atoms. The collisions are launched by cold Penning ionization, which exclusively populates Feshbach resonances that span both short- and long-range parts of the interaction potential. We resolved all final molecular channels in a tomographic manner using ion-electron coincidence detection. We demonstrate the nonstatistical nature of the final-state distribution. By performing quantum scattering calculations on ab initio potential energy surfaces, we show that the isolation of the Feshbach resonance pathways reveals their distinctive fingerprints in the collision outcome.

Effect of anisotropic collisions on solar scattering polarization

Scattering of anisotropic radiation by atoms, ions or molecules is sufficient to generate linear polarization observable in stars’ and planets’ atmospheres, circumstellar environments, and in particular in the Sun’s atmosphere. This kind of polarization is called scattering polarization (SP) or second solar spectrum (SSS) if it is formed near the limb of the solar photosphere. Generation of linear SP can typically be reached more easily than circular SP. Interestingly, the latter is often absent in observations and theories. Intrigued by this, we propose to demonstrate how circular SP can be created by anisotropic collisions if a magnetic field is present. We also demonstrate how anisotropic collisions can result in the creation of circular SP if the radiation field is anisotropic. We show that under certain conditions, linear SP creation is accompanied by the emergence of circular SP which can be useful for diagnostics of solar and astrophysical plasmas. We treat an example and calculate the density matrix elements of tensorial order k = 1 which are directly associated with the presence of circular SP. This work should encourage theoretical and observational research to be increasingly oriented towards circular SP profiles in addition to linear SP in order to improve our analysis tools of astrophysical and solar observations.

Mass-balance and locality versus accuracy with the new boundary and interface-conjugate approaches in advection-diffusion lattice Boltzmann method

We introduce two new approaches, called A-LSOB and N-MR, for boundary and interface-conjugate conditions on flat or curved surface shapes in the advection-diffusion lattice Boltzmann method (LBM). The Local Second-Order, single-node A-LSOB enhances the existing Dirichlet and Neumann normal boundary treatments with respect to locality, accuracy, and Péclet parametrization. The normal-multi-reflection (N-MR) improves the directional flux schemes via a local release of their nonphysical tangential constraints. The A-LSOB and N-MR restore all first- and second-order derivatives from the nodal non-equilibrium solution, and they are conditioned to be exact on a piece-wise parabolic profile in a uniform arbitrary-oriented tangential velocity field. Additionally, the most compact and accurate single-node parabolic schemes for diffusion and flow in grid-inclined pipes are introduced. In simulations, the global mass-conservation solvability condition of the steady-state, two-relaxation-time (S-TRT) formulation is adjusted with either (i) a uniform mass-source or (ii) a corrective surface-flux. We conclude that (i) the surface-flux counterbalance is more accurate than the bulk one, (ii) the A-LSOB Dirichlet schemes are more accurate than the directional ones in the high Péclet regime, (iii) the directional Neumann advective-diffusive flux scheme shows the best conservation properties and then the best performance both in the tangential no-slip and interface-perpendicular flow, and (iv) the directional non-equilibrium diffusive flux extrapolation is the least conserving and accurate. The error Péclet dependency, Neumann invariance over an additive constant, and truncation isotropy guide this analysis. Our methodology extends from the d2q9 isotropic S-TRT to 3D anisotropic matrix collisions, Robin boundary condition, and the transient LBM.

Evidence of Nonrigidity Effects in the Description of Low-Energy Anisotropic Molecular Collisions of Hydrogen Molecules with Excited Metastable Helium Atoms.

Cold collisions serve as a sensitive probe of the interaction potential. In the recent study of Klein et al. (Nature Phys.2017, 13, 35–38), the one-parameter scaling of the interaction potential was necessary to obtain agreement between theoretical and observed patterns of the orbiting resonances for excited metastable helium atoms colliding with hydrogen molecules. Here, we show that the effect of nonrigidity of the H2 molecule on the resonant structure, absent in the previous study, is critical to predict the correct positions of the resonances in that case. We have complemented the theoretical description of the interaction potential and revised reaction rate coefficients by proper inclusion of the flexibility of the molecule. The calculated reaction rate coefficients are in remarkable agreement with the experimental data without empirical adjustment of the interaction potential. We have shown that even state-of-the-art calculations of the interaction energy cannot ensure agreement with the experiment if such an important physical effect as flexibility of the interacting molecule is neglected. Our findings about the significance of the nonrigidity effects can be especially crucial in cold chemistry, where the quantum nature of molecules is pronounced.

Modeling of Lyman-α line polarization in fusion plasma due to anisotropic electron collisions

We have derived a formula for calculating the polarization state of the Lyman-α line in plasma caused by anisotropy in the electron velocity distribution function. Calculation results under the condition of the magnetically confined fusion plasma show that the longitudinal alignment of 0.01 is caused by approximately 10%–20% difference in the electron temperature in the directions parallel and perpendicular to the magnetic field. It is also found that the polarization decreases with the increasing electron density, ne, due to the collisional population averaging effect over the magnetic sublevels in the range of ne > 1018 m−3 and is virtually extinguished at ne = 1021 m−3.

Polarization measurement of hydrogen Lyman-α in the Large Helical Device

We have investigated the polarization of the Lyman-α line due to anisotropic electron collisions in the Large Helical Device (LHD). Optical components provided by the CLASP (Chromospheric Lyman-αlpha Spectro-Polarimeter) project [Kano R 2017 ApJL 839 L10] team have been incorporated into a VUV spectrometer so that linearly polarized light spectra at different angles can be obtained sequentially. The steady-state phase of an ECH (electron cyclotron heating) discharge has been analyzed, and it is confirmed that the Lyman-α line is polarized. From a temporal variation analysis of the intensity, integrated over the line profile, the polarization degree is evaluated to be 3.4%. A simple theoretical model calculation shows that this polarization degree can be explained by approximately 10% anisotropy in the electron temperature between the parallel and perpendicular directions with respect to the magnetic field.

Study of Lyman-α Polarization due to Anisotropic Electron Collisions in LHD

Study of Lyman-α Polarization due to Anisotropic Electron Collisions in LHD

Kinetics of two-pulsed photon echo formed on the 0–1 transition: isotropic and anisotropic collisions

The photon echo formed at the intercombination (6s2) 1S0 (6s6p) 3P1 (type 0–1) transition of ytterbium atoms 174Yb by two pulses of resonance radiation was investigated as a function of the time delay between exciting pulses (the photon echo kinetics). For exciting radiation pulses of the same linear polarization in the delay region 70–250 ns, the photon echo kinetics is close to the exponential one both in pure ytterbium and in its mixtures with buffer gases Ar and Xe. The relaxation cross sections were determined for all three cases, taking into account the isotopic composition of xenon and ytterbium. For the first time, the complex nonmonotonous kinetics of the collision-induced photon echo was observed in the same mixtures for the forming pulses of linear mutually orthogonal polarization. The relative efficiency of buffer atoms Yb (other isotopes than 174Yb), Xe and Ar in the collision-induced photon echo formation was obtained.

Anisotropic collisions of dipolar Bose–Einstein condensates in the universal regime

We report the measurement of collisions between two Bose–Einstein condensates with strong dipolar interactions. The collision velocity is significantly larger than the internal velocity distribution widths of the individual condensates, and thus, with the condensates being sufficiently dilute, a halo corresponding to the two-body differential scattering cross section is observed. The results demonstrate a novel regime of quantum scattering, relevant to dipolar interactions, in which a large number of angular momentum states become coupled during the collision. We perform Monte-Carlo simulations to provide a detailed comparison between theoretical two-body cross sections and the experimental observations.

Adiabatic theory for anisotropic cold molecule collisions.

We developed an adiabatic theory for cold anisotropic collisions between slow atoms and cold molecules. It enables us to investigate the importance of the couplings between the projection states of the rotational motion of the atom about the molecular axis of the diatom. We tested our theory using the recent results from the Penning ionization reaction experiment (4)He(1s2s (3)S) + HD(1s(2)) → (4)He(1s(2)) + HD(+)(1s) + e(-) [Lavert-Ofir et al., Nat. Chem. 6, 332 (2014)] and demonstrated that the couplings have strong effect on positions of shape resonances. The theory we derived provides cross sections which are in a very good agreement with the experimental findings.

The prospects of sympathetic cooling of NH molecules with Li atoms

We calculate the quartet potential energy surface for Li+NH and use it to calculate elastic and spin-relaxation cross sections for collisions in magnetically trappable spin-stretched states. The potential is strongly anisotropic but spin-relaxation collisions are still suppressed by centrifugal barriers when both species are in spin-stretched states. In the ultracold regime, both the elastic and inelastic cross sections fluctuate dramatically as the potential is varied because of Feshbach resonances. The potential-dependence is considerably reduced at higher energies. The major effect of using an unconverged basis set in the scattering calculations is to shift the resonances without changing their general behaviour. We have calculated the ratio of elastic and spin-relaxation cross sections, as a function of collision energy and magnetic field, for a variety of potential energy surfaces. Most of the surfaces produce ratios that are favorable for sympathetic cooling, at temperatures below about 20 mK.

Quantum theory of molecular collisions in a magnetic field: Efficient calculations based on the total angular momentum representation

An efficient method is presented for rigorous quantum calculations of atom-molecule and molecule-molecule collisions in a magnetic field. The method is based on the expansion of the wave function of the collision complex in basis functions with well-defined total angular momentum in the body-fixed coordinate frame. We outline the general theory of the method for collisions of diatomic molecules in the (2)Σ and (3)Σ electronic states with structureless atoms and with unlike (2)Σ and (3)Σ molecules. The cross sections for elastic scattering and Zeeman relaxation in low-temperature collisions of CaH((2)Σ(+)) and NH((3)Σ(-)) molecules with (3)He atoms converge quickly with respect to the number of total angular momentum states included in the basis set, leading to a dramatic (>10-fold) enhancement in computational efficiency compared to the previously used methods [A. Volpi and J. L. Bohn, Phys. Rev. A 65, 052712 (2002); R. V. Krems and A. Dalgarno, J. Chem. Phys. 120, 2296 (2004)]. Our approach is thus well suited for theoretical studies of strongly anisotropic molecular collisions in the presence of external electromagnetic fields.

Anisotropic collisions and impact circular polarization

Aims. In light of recent contradictory observational results concerning the atomic polarization of solar H i lines, our purpose is to present certain collisional effects that might contribute towards a possible explanation. In particular, we aim to draw attention to the possibility of creating the “impact circular polarization”. Methods. A general theoretical formulation of the problem of anisotropic collisions in the tensorial representation is obtained in an arbitrary symmetry of the relative velocity distribution. To try to understand this in concrete terms and estimate the effect of these collisions for creating and increasing the atomic orientation, we determine an explicit expression of the alignment-to-orientation transfer rates between two hydrogen levels in a particular case of symmetry. Results. The anisotropic collisions could play a role in creating and increasing the atomic orientation by an alignment-to-orientation conversion mechanism (impact circular polarization). Physically, this is due to coherence transfer by anisotropic collisions. This transfer, and hence the creation of atomic circular polarization, can be achieved in different ways, which we describe in the case of the Hα line. However, for given solar conditions, the alignment-to-orientation transfer rates seem to be 4 to 5 times lower than the population-to-alignment transfer rates which are basically responsible for generating the impact linear polarization.

Manifestation of higher polarization moments in the dipole emission under conditions of stepwise excitation and anisotropic collisions

The induction of higher polarization moments-the octupole orientation and the hexadecapole alignment—in the case of two-step excitation of atoms through dipole transitions and their manifestation in the polarization of dipole emission under conditions of anisotropic collisions are studied. For all channels of the two-step excitation process J0→J1J, the efficiencies of production of higher polarization moments with integer and half-integer angular momenta from J=2 to J=7 through the absorption of linearly or circularly polarized light are calculated. The rates of collisional relaxation and interconversion of higher polarization moments, for both orientation and alignment, as functions of frequency detuning of the laser line from the center of the Doppler profile are calculated. The theory is illustrated by an example of two channels of excitation of the atomic state J=2 by circularly polarized light: J0=0→J1=1J=2 and J0=3→J1=2 (or 3)→J=2. In the first case, the octupole orientation enhances or attenuates the signal of circular polarization depending on the laser frequency detuning. In the second case, it represents the only source producing orientation and circular polarization of the emitted light.

Signal of collisional beatings of polarization of light as a function of the angular momentum of an excited atomic state

A comparative study of the beats of circular polarization generated by anisotropic atomic collisions is carried out on the basis of theoretical calculations for the states J=1, 3/2, and 2 of neon atoms and ions excited by monochromatic laser radiation in xenon atmosphere. Contributions made by ordinary alignment and higher polarization moments (octupole orientation and dodecapole alignment) in the collisional generation of orientation and the signal of beatings of circular polarization are compared.

NONLINEAR OPTICAL PROPAGATION PHENOMENA IN NEAR-TRANSITION CENTROSYMMETRIC PHOTOREFRACTIVE CRYSTALS

We relate on a series of optical nonlinear propagation phenomena, mediated by photorefraction, present in ferroelectric crystals heated above the Curie point, in proximity of the critical regime. In particular, we discuss centrosymmetric screening solitons, diffusion-driven anisotropic nonlinear diffraction, ellipticity locking, diffusion-driven solitons, and spatial beam head-on collisions.

Auroral radar frequency broadening measurements at large aspect angles and comparison with theory

The observed characteristics of radio auroral backscatter at large aspect angles have never been adequately explained by theories of plasma turbulence that otherwise seem to be satisfactory. Recently, the first nonlinear theory on the subject has appeared in the open literature. In this theory, under the action of a driving electric field, the maximum aspect angle depends on the combined contribution of frequency broadening due to mode coupling and anisotropic anomalous collisions caused by the interaction of electrons with the fluctuating electric fields in the waves. In this paper an attempt is made to test the theoretical predictions by comparing them to observations of mean frequency broadening during typical auroral electrojet conditions. The analysis was based on a large number of Doppler spectra compiled from several VHF radar backscatter experiments with magnetic aspect angles in the range from about 1 to 10 deg. The theory fails to explain echo occurrence at directions perpendicular to the E × B flow, but otherwise it can account for most of the observations. The evidence shows that the effect on large aspect angle echoes of frequency broadening alone is insufficient and that the key role in fitting the data is played by the parallel anomalous collision frequency νe∥*. The agreement between theory and experiment, however, is possible only for large enhancements of νe∥* relative to the normal electron neutral collision frequency νe, with the increase ranging between 1 and 2 orders of magnitude. This imposes difficulties because the large νe∥*/νe ratios imply a pronounced ion acoustic plasma turbulence along B inside the electrojet layer. To our knowledge, there is no observational evidence to support this theoretical implication.

A simple approach to treat anisotropic elastic collisions in Monte Carlo calculations of the electron energy distribution function in cold plasmas

A simplified method allowing one to treat anisotropic electron heavy species elastic scattering in MonteCarlo models of gas discharges with the proper value for collision frequency is proposed The method is applied to an electric discharge in a Ne · Xe/HCl mixture, and the results are compared with the solution of the two-term expansion of the Boltzmann equation under the same conditions. Methods for reduction of computational time in Monte Carlo codes and the use of the Monte Carlo flux method are also discussed.