The paper reviews the principles of interaction of energetic particles with solid carbon and carbon nanostructures. The reader is first introduced to the basic mechanisms of radiation effects in solids with particular emphasis on atom displacements by knock-on collisions. The influence of various parameters on the displacement cross sections of carbon atoms is discussed. The types of irradiation-induced defects and their migration are described as well as ordering phenomena which are observable under the non-equilibrium conditions of irradiation. The main part of this review deals with alterations of carbon nanostructures by the electron beam in an electron microscope. This type of experiment is of paramount importance because it allows in situ observation of dynamic processes on an atomic scale. In the second part, radiation effects in the modifications of elemental carbon, in particular in graphite which forms the crystallographic basis of most carbon nanostructures, are treated in detail. It follows a review of the available experimental results on radiation defects in carbon nanostructures such as fullerenes, nanotubes and carbon onions. Finally, the phenomena of structure formation under irradiation, in particular the self-assembling of spherical carbon onions and the irradiation-induced transformation of graphitic nanoparticles into diamond, are presented and discussed qualitatively in the context of non-equilibrium structure formation.

https://iopscience.iop.org/article/10.1088/0034-4885/62/8/201/pdf

Irradiation effects in carbon nanostructures

A review of recent developments in atomic processes for divertors and edge plasmas

Aims. The interpretation of astrophysical spectra depends directly on a knowledge of the ionization state of the emitting plasma. This is determined, in part, from collisional ionization rate coefficients. The most recent assessments of these were performed by Arnaud & Rothenflug (1985, A&AS, 60, 425) and Arnaud & Raymond (1992, ApJ, 398, 394). Since their work, new laboratory measurements of ionization cross sections have become available as well as the Flexible Atomic Code (FAC) which enables theoretical calculations of these rates. Our goal is to provide a complete set of ionization rate coefficients for the elements hydrogen through zinc. Methods. A scaling law, which assists the analysis of ionization cross sections and rate coefficients, has been developed following the approach of Burgess & Tully (1992, A&A, 254, 436). Essentially all available measured cross sections along each isoelectronic sequence have been examined and compared to cross sections calculated with the Flexible Atomic Code (FAC) and with other calculations. Two approaches has been taken to provide a complete set of ionization cross sections. In the first, fits to scaled measured ionization cross sections, particularly for neutral and singly ionized species, are performed. In the second, fits to scaled calculated direct ionization and excitation-autoionization cross sections are performed to provide the remainder of the set. The fits to the cross sections are then integrated over a Maxwellian velocity distribution to derive ionization rate coefficients. Results. A complete set of ground level ionization cross sections and rate coefficients has been developed through the combination of these two approaches. A tabulation of parameter fits to ground level ionization rate coefficients for all atoms and ions of the elements of H through Zn is provided.

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Ionization rate coefficients for the elements hydrogen through zinc

▪ Abstract Targeted laboratory astrophysics measurements are being conducted to address the needs of X-ray astronomy. The measurements are producing large sets of reliable atomic data, which include ionization and recombination cross sections for charge balance calculations, as well as line lists, excitation cross sections, and dielectronic recombination rates for interpreting X-ray line formation. Additional experiments focus on resolving specific puzzles posed by astrophysical observations, as well as on calibrating existing and developing new X-ray line diagnostics. We discuss the types of data produced and illustrate how the laboratory measurements support such missions as ASCA, EUVE, Chandra, XMM, and ASTRO-E2.

Laboratory X-Ray Astrophysics

Several experiments were conducted in ASDEX Upgrade to prove the suitability of tungsten as a divertor target material under the conditions of a high density and low temperature divertor. The observed fluxes from a tungsten tile into the plasma are low, in keeping with the extremely low sputtering yields. In addition, the very favourable effect of `prompt redeposition' (redeposition during the first gyration) could be confirmed by the experiments. Cooling of the edge region by neon injection seems permissible, i.e. neon impurity sputtering did not increase the eroded fluxes of tungsten. The transport and accumulation behaviour were investigated by means of the laser blow-off technique. No accumulation effects could be observed in ohmic discharges. In discharges with NBI heating but without ICRH, strong accumulation can occur. High heat flux tests were performed on graphite tiles coated with plasma sprayed tungsten, which withstood a thermal load of 15 MW/m2 lasting 2 s as well as 1000 cycles of 10 MW/m2 for 2 s without disabling damage. Owing to the encouraging results, an experiment using a tungsten divertor is planned in ASDEX Upgrade

https://iopscience.iop.org/article/10.1088/0029-5515/36/6/I01/pdf

Tungsten as target material in fusion devices

Due to their exceptionally high stability, fullerenes have been the object of extensive research over the last decade both experimentally and theoretically. In most experiments collisions between neutral fullerenes and different projectiles have been investigated. In the present studies, however, we look at collisions between electrons and fullerene ions.

Ionization and fragmentation of fullerene ions by electron impact.

Electron-impact ionization cross sections for He-like ${\mathrm{Li}}^{+}$ ions have been measured with extreme precision (0.01% statistics). Thus, two new mechanisms in the ionization of positive ions both involving two-electron excitations could be quantitatively investigated. We see (1) direct excitation of both K electrons into the L shell with subsequent autoionization and (2) resonant capture of the incident electron by the ion with correlated KK-LL excitation of both core electrons leading to a triply excited Li atom which then decays by the emission of two correlated electrons. Strong interference of process (2) with direct ionization is observed.

Correlated two-electron transitions in electron-impact ionization of Li+ ions.

We report narrow dielectronic capture resonances decaying by double autoionization observed for the first time in electron-impact ionization of ions. We also observe corresponding peaks in double and triple ionization leading us to postulate new processes where dielectronic capture is followed by triple or quadruple autoionization, respectively.

Resonances in electron-impact single, double, and triple ionization of heavy metal ions

Absolute measurements have been made of cross sections for the ionisation of Li2+ ions by electrons with energies which range from threshold (122.45 eV) to 1000 eV. The results are in good agreement with available distorted-wave exchange calculations. The scaling behaviour of ionisation cross sections along the hydrogen isoelectronic sequence is discussed.

https://iopscience.iop.org/article/10.1088/0953-4075/22/3/016/pdf

Experimental cross sections for electron impact ionisation of hydrogen-like Li2+ ions

Detailed measurements of electron-impact ionization cross sections have been made in the vicinity of the excitation-autoionization thresholds of Li-like B2+, C3+, N4+, O5+ and F6+ ions. With an energy spread of 0.4 to 1% of the electron energy and statistical uncertainties as low as 0.1% we could clearly resolve thresholds for excitation of individual terms in 1s2s2l configurations. Numerous resonance features were found which are due to dielectronic capture of the incident electron with subsequent two-electron emission. In particular, dielectronic capture processes involving Δn=2 excitations of a 1s electron provide the dominant resonance contributions to the measured cross sections. Rydberg series 1s2snln′l′ of resonances withn=3 (Δn=2) andn=4 (Δn=3) are resolved up to principle quantum numbersn′ equal to 6 or 7.

G. Hofmann

Papers

Ionization and fragmentation of fullerene ions by electron impact.

Correlated two-electron transitions in electron-impact ionization of Li+ ions.

Resonances in electron-impact single, double, and triple ionization of heavy metal ions

Experimental cross sections for electron impact ionisation of hydrogen-like Li2+ ions

Indirect processes in the electron impact ionization of Li-like ions