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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.

Absolute cross sections for electron-impact single ionisation (EISI) of multiply charged tungsten ions (Wq+) with charge states in the range 11 ≤ q ≤ 18 in the electron-ion collision energy ranges from below the respective ionisation thresholds up to 1000 eV were measured employing the electron-ion crossed-beams method. In order to extend the results to higher energies cross section calculations were performed using the subconfiguration-averaged distorted-wave (SCADW) method for electron-ion collision energies up to 150~keV. From the combined experimental and scaled theoretical cross sections rate coefficients were derived which are compared with the ones contained in the ADAS database and which are based on the configuration-averaged distorted wave (CADW) calculations of Loch et al. [Phys. Rev. A 72, 052716 (2005)]. Significant discrepancies were found at the temperatures where the ions investigated here are expected to form in collisionally ionised plasmas. These discrepancies are attributed to the limitations of the CADW approach and also the more detailed SCADW treatment which do not allow for a sufficiently accurate description of the EISI cross sections particularly at the ionisation thresholds.

We have measured electron impact ionization (EII) for Fe8+ forming Fe9+ from below the ionization threshold to 1200 eV. These measurements were carried out at the TSR heavy ion storage ring. The objective of using a storage ring is to store the ion beam initially so that metastable levels decay, thereby allowing for measurements on a well-defined ground-level ion beam. In this case, however, some metastable levels were too long lived to be removed. We discuss several methods for quantifying the metastable fraction, which we estimate to be ~30%–40%. Although metastables remain problematic, the present storage ring work improves upon other experimental geometries by limiting the metastable contamination to only a few long-lived excited levels. We discuss some future prospects for obtaining improved measurements of Fe8+ and other ions with long-lived metastable levels.

We report measured rate coefficients for electron-ion recombination for Si10+ forming Si9+ and for Si9+ forming Si8+, respectively. The measurements were performed using the electron-ion merged-beams technique at a heavy-ion storage ring. Electron-ion collision energies ranged from 0 to 50 eV for Si9+ and from 0 to 2000 eV for Si10+, thus, extending previous measurements for Si10+ [Orban et al. 2010, Astrophys. J. 721, 1603] to much higher energies. Experimentally derived rate coefficients for the recombination of Si9+ and Si10+ ions in a plasma are presented along with simple parameterizations. These rate coefficients are useful for the modeling of the charge balance of silicon in photoionized plasmas (Si9+ and Si10+) and in collisionally ionized plasmas (Si10+ only). In the corresponding temperature ranges, the experimentally derived rate coefficients agree with the latest corresponding theoretical results within the experimental uncertainties., Comment: 17 pages, 7 figures, 3 tables, 66 references, submitted to the J. Phys. B special issue on atomic and molecular data for astrophysicists

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.

A crossed-beam setup was used to measure cross sections for electron-impact single and double ionization of W17 + ions. Absolute data and high-resolution scan spectra were obtained at collision energies ranging from threshold up to 1000eV. A comparison of the experimental results with theoretical calculations for direct ionization suggests substantial contributions of excitation–autoionization processes to electron-impact single ionization of W17 +.