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Synopsis Electron-impact ionisation cross sections of tungsten ions in the energy range from the ionisation threshold to 1000 eV were measured. The experimental cross sections are compared with results from sub-configuration averaged distorted wave calculations. Derived plasma rate coefficients are provided for convenient use in plasma modelling.

30th International Conference on Photonic, Electronic and Atomic Collisions, ICPEAC XXX, Cairns, Australien, 26 Jul 2017 - 1 Aug 2017; Journal of physics / Conference Series 875, 052005 (2017). doi:10.1088/1742-6596/875/6/052005, Absolute rate coefficients for electron-ion recombination of astrophysically relevant Si9+ and Si10+ ions were measured at a heavy-ion storage ring. Hyperfine quenching of long-lived metastable levels was used for preparing practically pure ground-state beams in the storage ring. From the measured results rate coefficients for recombination of Si9+ and Si10+ ions in a plasma are derived and represented by simple parameterizations., Published by IOP Publ., Bristol

The cryogenic storage ring (CSR) for positive and negative atomic, molecular and cluster ions is starting operation at the Max Planck Institute for Nuclear Physics in Heidelberg.

Electron-ion recombination of W19+ forming W18+ has been studied both experimentally and theoretically. An exceptionally large recombination rate coefficient was observed by the storage-ring experiment at zero electron-ion collision energy. It exceeds the expectation for nonresonant radiative recombination (RR) by more than two orders of magnitude. This follows the pattern seen previously for the neighboring charge states of tungsten. The results of our present theoretical calculations, which were carried-out within the Breit-Wigner partitioned framework for resonant electron-ion recombination of complex ions, agree with this observation.

Electron-impact ionization of tungsten ions has been measured in the energy range from threshold up to 1000 eV. Configuration-averaged distorted wave calculations were used to explain the experimental data.

Cross sections for photorecombination (PR) and for electron-impact ionization (EII) of astrophysically relevant Mg-like Fe14+ ions have been measured by employing the electron-ion merged-beams technique at the Heidelberg heavy-ion storage ring TSR. Rate coefficients for PR and EII of Fe14+ ions in a plasma are derived from the measurements. Both agree with the most recent theoretical results.

Absolute recombination rate coefficients of Be-like Si10+ have been measured employing the electron-ion merged-beams method at the storage ring TSR. The experimental center-of-mass energy range 0-2000 eV covers dielectronic recombination (DR) resonances associated with K- and L-shell excitations.

Electron-ion recombination of W18+ forming W17+ has been studied both experimentally and theoretically. The W18+ recombination rate coefficient is dominated by strong recombination resonances even at the lowest experimental energies. According to our calculations these resonances are mainly associated with 4f → 5l excitations of the W18+(4f10) core. For the purpose of fusion plasma modeling, the total W18+ recombination rate coefficient in a plasma is derived from our results.

Absolute cross sections for electron-impact ionization of Wq+ ions (q ≤ 17) are measured using a well established electron-ion crossed beams technique. A special problem with many-electron tungsten ions is the existence of large numbers of long-lived excited states and their presence in the primary ion beams employed in the experiments.

We have carried out direct measurements of the absolute recombination rate coefficients of four charge states of tungsten in the range from W18+ to W21+ in a heavy ion storage ring. We find that the rich atomic fine structure of the open f-shell leads to very high resonant enhancement of the recombination rate at energies below ~50 eV. Even in the higher energy domain relevant to fusion plasma this leads to a recombination rate coefficient that is more than four times higher than predicted by the commonly used ADAS database of recombination rates. In addition to the experimental measurements we have carried out theoretical calculations using Autostructure. For W20+ these predict a plasma recombination rate coefficient that agrees much better with the measurement than the ADAS model but still fail to reproduce the experimental data in detail.

We report on dissociative recombination measurements of several halogen-containing molecular ions using a merged electron and ion beams geometry.

Electron-impact single-ionization cross sections of Ar5+ ions have been measured employing the crossed-beams technique. In the comparison with former experimental data sets significant discrepancies became apparent. In the energy region from 240 eV up to 300 eV strong contributions from indirect-ionization processes involving excitation of the 2p subshell were observed.

Cross sections for electron impact single and double ionization of W17+ ions have been measured using the electron ion crossed-beams technique. Absolute data and fine-step spectra of the W18+ yield have been obtained at collision energies from threshold up to 1000 eV. The measured cross sections for net single ionization exceed calculations of contributions from direct knock-off ionization by 50 to 100 %.

Electron-impact single-ionization cross sections of Ar6+ ions have been measured in the energy region from below 14 eV up to 1000 eV. Strong contributions of indirect-ionization processes involving excitation of the 2p subshell have been observed. Ions in long-lived excited states were present in the primary ion beam and resulted in ionization onsets below the ground-state ionization threshold.