We present experimentally measured and theoretically calculated rate coefficients for the electron-ion recombination of W 18+ ([Kr]4d 10 4f 10) forming W 17+. At low electron-ion collision energies, the merged-beam rate coefficient is dominated by strong, mutually overlapping recombination resonances. In the temperature range where the fractional abundance of W 18+ is expected to peak in a fusion plasma, the experimentally derived Maxwellian recombination rate coefficient is 5 to 10 times larger than that which is currently recommended for plasma modeling. The complexity of the atomic structure of the open-4f system under study makes the theoretical calculations extremely demanding. Nevertheless, the results of the present Breit-Wigner partitioned dielectronic recombination calculations agree reasonably well with the experimental findings. This also gives confidence in the ability of the theory to generate sufficiently accurate atomic data for the plasma modeling of other complex ions.

Abstract.A crossed-beams setup was used to measure cross sections for electron-impact single and double ionization of W 17+ ions. Absolute data and high-resolution scan spectra were obtained at collision energies ranging from threshold up to 1000 eV. Comparison of the experimental results with theoretical calculations for direct ionization suggests substantial contributions of excitation-autoionization processes to electron-impact single ionization of W 17+ .

We have measured dissociative recombination of HCl + with electrons using a merged beams configuration at the heavy-ion storage ring TSR located at the Max Planck Institute for Nuclear Physics in Heidelberg, Germany. We present the measured absolute merged beams recombination rate coefficient for collision energies from 0 to 4.5 eV. We have also developed a new method for deriving the cross section from the measurements. Our approach does not suffer from approximations made by previously used methods. The cross section was transformed to a plasma rate coefficient for the electron temperature range from T = 10 to 5000 K. We show that the previously used HCl + DR data underestimate the plasma rate coefficient by a factor of 1.5 at T = 10 K and overestimate it by a factor of 3.0 at T = 300 K. We also find that the new data may partly explain existing discrepancies between observed abundances of chlorine-bearing molecules and their astrochemical models.

An electrostatic cryogenic storage ring, CSR, for beams of anions and cations with up to 300 keV kinetic energy per unit charge has been designed, constructed, and put into operation. With a circumference of 35 m, the ion-beam vacuum chambers and all beam optics are in a cryostat and cooled by a closed-cycle liquid helium system. At temperatures as low as (5.5 ± 1) K inside the ring, storage time constants of several minutes up to almost an hour were observed for atomic and molecular, anion and cation beams at an energy of 60 keV. The ion-beam intensity, energy-dependent closed-orbit shifts (dispersion), and the focusing properties of the machine were studied by a system of capacitive pickups. The Schottky-noise spectrum of the stored ions revealed a broadening of the momentum distribution on a time scale of 1000 s. Photodetachment of stored anions was used in the beam lifetime measurements. The detachment rate by anion collisions with residual-gas molecules was found to be extremely low. A residual-gas density below 140 cm(-3) is derived, equivalent to a room-temperature pressure below 10(-14) mbar. Fast atomic, molecular, and cluster ion beams stored for long periods of time in a cryogenic environment will allow experiments on collision- and radiation-induced fragmentation processes of ions in known internal quantum states with merged and crossed photon and particle beams.

Absolute cross sections for electron-impact single ionisation (EISI) of multiply charged tungsten ions (W q+ ) 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.