Atomic Collision Processes for Astrophysical and Laboratory Plasmas

The atomic data of medium- and high-<i>Z</i> elements, such as electron-impact ionization and excitation cross-sections, possess extensive applications in fields such as fusion science and X-ray interactions with matter. There are atoms and ions in high energy density plasma, with different charge states and energy states ranging from ground states to highly excited states, and the cross-sections of each charge state and energy state need to be calculated. The bottlenecks limiting computational performance are the inevitable relativistic effects of medium- and high-<i>Z</i> elements and the extremely complex electronic configurations. Taking tantalum (Ta) for example, by using the relativistic Dirac-Fock theory and distorted wave model, we compute the electron-impact ionization and excitation cross-sections of Ta from the ground state atom up to Ta<sup>72+</sup> with the incident electron energy range of 1–150 keV. The detailed configuration accounting (DCA) reaction channel cross-sections are derived by summing and weighting the original detailed level accounting (DLA) cross-sections. After examining the data, two regularities are found. In terms of DLA, the pre-averaging DCA cross-sections have varying initial DLA energy levels but are typically close to each other. There is not a straightforward function that can explain the discrepancies between them. In terms of DCA, inner subshells typically contribute very little to the total cross-section as their ionization and excitation cross-sections are orders of magnitude smaller than those of outer subshells. We provide two techniques to reduce the computational costs based on the regularities. To minimize the total number of DLA reaction channels used in the computation, the initial DLA energy levels can be randomly sampled. Through a Monte Carlo numerical experiment, we determine the appropriate number of sampling points that can reduce the total number of DLA channels by an order of magnitude while maintaining a 5% error margin. In terms of impact ionization, since small cross-section DCA channels are insignificant, only a tiny portion of the DCA channels are required to preserve a 95% accuracy of the entire cross-section. It is possible to use the analytical Binary Encounter Bethe (BEB) formula to determine which DCA channels should be neglected before the computation to reduce computational costs. In terms of electron-impact excitation, just the cross-sections of the same excited subshells as the preserved ionized subshells, which are determined in the previous electron-impact ionization (EII) calculations, are needed. Finally, we compare our EII results with theoretical and experimental results. In the low incident electron energy range of below 2 keV, our results accord with the theoretical result of the 6s EII cross-section of the Ta atom and the experimental result of the total EII cross-section of the Ta<sup>1+</sup> ion. In the high energy range of below 150 keV, our results are also consistent with the theoretical result of the 1s EII cross-section of the Ta atom and the experimental result of the 1s EII cross-section of the Cu atom. Our results reasonably match the previous experimental and theoretical results in low-energy range and high-energy range, inner subshell and outer subshell, indicating the accuracy of our calculation. The proposed optimizing strategy can be applied to various medium- to high-<i>Z</i> elements and is compatible to most computation codes.

We review the development of the time-dependent close-coupling method to study atomic and molecular few body dynamics. Applications include electron and photon collisions with atoms, molecules, and their ions.

Close-coupling (CC) calculations of electron-ion recombination cross sections using the R-matrix method are presented and benchmarked with available experimental measurements. The electron-ion recombination process, including resonant and non-resonant recombination may be unified as a natural extension of the coupled-channel approximation, as traditionally employed for photoionization and electron-ion scattering. Recombination cross sections can be calculated to the same accuracy by employing similar eigenfunction expansions for the target ion. Detailed results are obtained for electron recombination with C V, C VI, O VIII and Fe XXV. Several sets of theoretical calculations are reported and discussed: non-relativistic CC in LS coupling, relativistic CC in the Breit-Pauli approximation, with radiative attenuation and fine structure, and the relativistic distorted-wave approximation. The theoretical results are in very good agreement with highly accurate experimental measurements at the Heidelberg test storage ring for C V, C VI and O VIII, and the electron-ion beam trap at Livermore for Fe XXV. We discuss the overall effect of radiation damping of all resonances on effective cross sections and rates, important for H- and He-like ions. In addition to agreement with experimental data, the validity of the CC calculations is demonstrated by the continuity between the calculated photorecombination, dielectronic recombination and electron impact excitation cross sections. Certain issues related to the works of Badnell et al (1998 J. Phys. B: At. Mol. Opt. Phys. 31 L239) and Robicheaux (1998 J. Phys. B: At. Mol. Opt. Phys. 31 L109) are also addressed.

Diagnostics of Ar/N2 mixture plasma with detailed electron-impact argon fine-structure excitation cross sections

Cross section database for carbon atoms and ions: Electron-impact ionization, excitation, and charge exchange in collisions with hydrogen atoms

Cross sections for electron-impact excitation from the $5p^{5}6s$ $J=2$ metastable level of xenon to the lowest six levels of the $5p^{5}6p$ configuration have been measured. The cross sections generally have very large magnitudes $({10}^{\ensuremath{-}15}\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{2})$ and scale with the corresponding optical oscillator strengths. The substantial variations observed in the energy dependence of the cross sections for the six levels can also be related to the optical oscillator strengths. Cross sections for excitation out of the $J=2$ metastable level into the upper four levels of the $5p^{5}6p$ configuration are much smaller. The large disparity in cross sections between the upper and lower groups is explained in terms of the electronic structure of the excited states of xenon.

In the last few years it has been demonstrated, both by data analysis and by numerical simulations, that the transport of energetic particles in the presence of magnetic turbulence can be superdiffusive rather than normal diffusive (Gaussian). The term ‘superdiffusive’ refers to the mean square displacement of particle positions growing superlinearly with time, as compared to the normal linear growth. The so-called anomalous transport, which in general comprises both subdiffusion and superdiffusion, has gained growing attention during the last two decades in many fields including laboratory plasma physics, and recently in astrophysics and space physics. Here we show a number of examples, both from laboratory and from astrophysical plasmas, where superdiffusive transport has been identified, with a focus on what could be the main influence of superdiffusion on fundamental processes like diffusive shock acceleration and heliospheric energetic particle propagation. For laboratory plasmas, superdiffusion appears to be due to the presence of electrostatic turbulence which creates long-range correlations and convoluted structures in perpendicular transport: this corresponds to a similar phenomenon in the propagation of solar energetic particles (SEPs) which leads to SEP dropouts. For the propagation of energetic particles accelerated at interplanetary shocks in the solar wind, parallel superdiffusion seems to be prevailing; this is based on a pitch-angle scattering process different from that envisaged by quasi-linear theory, and this emphasizes the importance of nonlinear interactions and trapping effects. In the case of supernova remnant shocks, parallel superdiffusion is possible at quasi-parallel shocks, as occurring in the interplanetary space, and perpendicular superdiffusion is possible at quasi-perpendicular shocks, as corresponding to Richardson diffusion: therefore, cosmic ray acceleration at supernova remnant shocks should be formulated in terms of superdiffusion. The possible relations among anomalous transport in laboratory, heliospheric, and astrophysical plasmas will be indicated.

Results are presented for the inverse processes of photoionization and electron-ion recombination of (hν + Ne ↔ Ne + e) and (hν + Ne ↔ Ne + e) using the self-consistent unified method. The method employs an identical wave function expansion for both photoionization and recombination, and it includes both radiative and dielectronic recombination. Total, as well as level-specific, photoionization cross sections, σPI(E; nSLJ), and recombination rate coefficients, αR(T; nSLJ), are presented for all fine-structure levels up to n ≤ 10. These correspond to a total of 98 bound fine-structure levels of Ne VIII with 1/2 ≤ J ≤ 17/2, and 178 bound levels of Ne IX with 0 ≤ J ≤ 10. Total recombination cross sections and rates as functions of electron energy are also presented. The coupled channel wave function expansions for the core ions include 17 levels of Ne IX and 16 levels of Ne X. Relativistic fine structure is considered through the Breit-Pauli R-matrix (BPRM) method. The photoionization and recombination cross sections include important atomic effects such as radiation damping, channel coupling, and interference and should be of definitive accuracy. Level-specific σPI(nSLJ) and αR(T; nSLJ) are calculated for the first time. In addition, we describe the applicability of these comprehensive data sets to not only ionization balance and recombination-cascade models for astrophysical and laboratory plasmas, but also to (1) models of UV and X-ray lines in He-like ions from C V to Ne IX involving the 2 3P → 2 3S1 allowed triplet transitions in the UV, and 2(1P, 3P, 3S1) → 1 1S0 allowed, intercombination, and forbidden transitions in the X-ray, and (2) calculation of dielectronic satellite intensities from the highly resolved resonances in the unified recombination cross sections of He-like and Li-like ions through straightforward integration over a Maxwellian or other electron distribution functions.

Magnetic Relaxation and Self-Organization in Astrophysical and Laboratory Plasmas: Mean Field Dynamo Theory Origin, Structure and Stability of Astrophysical MHD Jets Turbulence and Turbulent Transport - The Agents of Relaxation and Structure Formation: A Tutorial on Basic Concepts in MHD Turbulence and Turbulent Transport Intermittency Like Phenomena in Plasma Turbulence Nonlinear Cascades and Spatial Structure of MHD Turbulence Scale Covariance and Scale-Ratio Covariance in Turbulent Front Propagation Transport Bifurcations and Relaxation: Transport Barrier Relaxations in Tokamak Edge Plasmas Dynamics of Edge Localized Modes On the Onset of Collapse Events in Toroidal Plasmas - Turbulence Trigger.

Magnetic reconnection is a fundamental plasma process responsible for the sometimes explosive release of magnetic energy in space and laboratory plasmas. Inside the diffusion regions of magnetic reconnection, the plasma becomes demagnetized and decouples from the magnetic field, enabling the change in magnetic topology necessary to power the energy release over larger scales. Since it was launched in 2015, the Magnetospheric MultiScale (MMS) mission has significantly advanced the understanding of the particle dynamics key to magnetic reconnection by providing high-resolution, in-situ measurements able to resolve ion and electron kinetic scales that have confirmed theoretical predictions, revealed new phenomena, and refined existing models. These breakthroughs are critical for understanding not only space plasmas but also laboratory and astrophysical plasmas where magnetic reconnection occurs. In this talk, we give a brief review and present some recent results of selected topics related to the ion and electron dynamics occurring within the diffusion regions.

The "International Topical Conference on Plasma Physics: New Frontiers in Nonlinear Sciences" was held at the University of Algarve (UA), Faro (Portugal), during the period 6–10 September 1999. The conference was organized by P K Shukla, R Bingham, J T Mendonça and L Stenflo with the help of an international advisory board and a program committee that included scientists from all over the world. The conference enters into a series of previous biennial activities that we have held at the Abdus Salam International Centre for Theoretical Physics, Trieste, since 1989.The purpose of the Faro meeting was to provide an informal forum for scientists who have dealt with various aspects ofnonlinear processes in space, astrophysical and laboratory plasmas, as well as in fluids and nonlinear optics. The selectedtopics, which were interdisciplinary, are expected to have a great deal of impact on the development of nonlinear sciencesin the coming millennium.The response of the conference was almost overwhelming. It was attended by approximately 120 delegates from Europe,USA, Japan, and developing countries. Many participants were young researchers from both the industrial and developingcountries, as the organizers tried to keep a good balance in inviting senior and younger generations of nonlinear scientiststo our Faro conference.The scientific program included five review talks (45 minutes) and thirty-five invited topical lectures (30 minutes). Inaddition, there were about eighty poster papers in three sessions. The latter gave opportunities to younger physicistsfor displaying the results of their recent work and to obtain comments from the other participants. During the five daysat the UA, we focused on fundamental aspects of: (i) the nonlinear physics in various branches of sciences (plasmas, fluids,and optics), (ii) nonlinearities in space and astrophysics, (iii) coherent processes in non-ideal systems (colloidal and dusty as well as pure electron plasmas), (iv) nonlinear charged particle beams and laser/neutrino-plasma interactions, and(v) nonlinearities in controlled laboratory devices. The focus was on nonlinear phenomena involving wave–wave and wave–particle interactions in ideal and non-ideal complex plasmas, intense short laser pulse interactions with plasmas and atomic clusters, the Kerr nonlinearity in optics, generation of harmonics as well as modulated wave packets in fluids, anomalous transport processes and complexities, etc. The nonlinear Schrödinger-like models for the propagation of particle and laser beams and optical pulses are still common in nonlinear dispersive media. The formation of coherent nonlinear structures (voids, envelope solitons, shocks, various types of solitary vortices, and vortex crystals) was exclusively demonstrated in data from low-temperature laboratory and space plasmas. For example, observations from the auroral ionosphere and microgravity dusty plasma experiments reveal the simultaneous presence of solitary vortex structures of various scale sizes as well as large scale density holes (voids). During the Faro meeting, some talks also concerned the role of self-organized criticality in nature as well as techniques for controlling chaos. Computer modeling of numerous kinetic instabilities and phase space vortex structures revived renewed interest at the conference. Furthermore, charged particle acceleration involving collective processes in ionospheric, astrophysical, and laboratory plasmas remained frontiers of the nonlinear sciences. Novel fields identified during the conference were dusty and neutrino plasma physics, which take advantage of knowledge from condensed matter and particle physics, making plasma physics truly cross-disciplinaryand very fascinating with wide ranging applications. Most of the contributions from the Faro meeting appear in this TopicalIssue of Physica Scripta, which will be distributed to all the participants. It is expected that the papers of the present proceedings, which systematically describe the advancement of a particular subject matter, shall be useful for understanding the many complex nonlinear phenomena that are occurring in science and technology.The organizers are grateful to Professor Adriano Pimpão, the President of the UA, for his generous support and warm hospitality in Faro. The excellent work of the scientific secretary Dr Rui Guerra is also acknowledged. The Editors want to express sincere gratitude to their colleagues and co-organizers Profs Tito Mendonça and Bob Bingham for their constant and wholehearted support in our endeavours. Thanks are also due to the Universidade do Algarve for providing the venue, and for administrative support, the Universidade Técnica de Lisbon for secretarial and other support, and theFCT-Fundação para a Ciência e a Tecnologia for a limited financial support to our conference at the UA, Faro.Finally, the organizers cordial thanks are extended to the speakers and the attendees for their contributions which resulted in the success of the Faro conference. Specifically, we appreciate the speakers for delivering excellent talks, supplying well prepared manuscripts for publication, and enhancing the nonlinear science activity at the UA, where we hope to create a new center of excellence for carrying out high quality research and for training young researchers in the fascinating new areas of nonlinear sciences which may have a tremendous impact on the development of new technologies andnew materials in the twenty-first century.

Recommended electron-impact excitation and ionization cross sections for Be I

The weighted total cross-section (WTCS) theory is used to calculate electron impact excitation, ionisation and dissociation cross-sections and rate coefficients of OH, H2, OH+, H2 +, OH- and H2 - diatomic molecules in the temperature range 1500–15000 K. Calculations are performed for H2(X, B, C), OH(X, A, B), H2 +(X), OH+(X, a, A, b, c), H2 -(X) and OH-(X) electronic states for which Dunham coefficients are available. Rate coefficients are calculated from WTCS assuming Maxwellian energy distribution functions for electrons and heavy particles. One and two temperature (θe and θg respectively for electron and heavy particles kinetic temperatures) results are presented and fitting parameters (a, b and c) are given for each reaction rate coefficient: k(θ) = a (θb)exp (-c/θ).

In this lecture I shall review recent developments which have occurred in theoretical methods for calculating atomic collision cross sections. The lecture is an up-dated (but self-contained) version of the review I gave at the previous NATO Advanced Study Institute on “Atomic and Molecular Processes in Controlled Thermonuclear Fusion”1. It is meant to provide a general introduction to subsequent lectures by other speakers, who will deal in more detail with the theoretical analysis of specific atomic and molecular collision processes of interest in fusion research.

SynopsisWe report measurements on differential cross sections (DCSs) for electron-impact excitation to the optically-allowed b1Πu state in molecular nitrogen in the impact energy range of 20 – 400 eV. Absolute excitation DCSs normalized by the relative flow technique were transformed to generalized oscillator strengths (GOSs) as a function of momentum transfer squared. Integral cross sections (ICSs) for the transitions to the b1Πu state were compared with the BEf-scaled plane-Born ICS as well as previous experimental studies.