Dielectronic recombination (DR) of xenonlike W20+ forming W19+ has been studied experimentally at a heavy-ion storage-ring. A merged-beams method has been employed for obtaining absolute rate coefficients for electron-ion recombination in the collision energy range 0-140 eV. The measured rate coefficient is dominated by strong DR resonances even at the lowest experimental energies. At plasma temperatures where the fractional abundance of W20+ is expected to peak in a fusion plasma, the experimentally derived plasma recombination rate coefficient is over a factor of 4 larger than the theoretically-calculated rate coefficient which is currently used in fusion plasma modeling. The largest part of this discrepancy stems most probably from the neglect in the theoretical calculations of DR associated with fine-structure excitations of the W20+([Kr] 4d10 4f8) ion core.Comment: 7 pagers, 4 figures, accepted for publication in Physical Review

We report the first direct laboratory measurement of the spontaneous emission due to the hyperfine splitting of the ground state of a highly charged hydrogenlike ion excited by electron collisions. The transition between the F 4 and F 3 levels of the 1s 2 S 1͞2 configuration of hydrogenlike 165 Ho 651 was observed and its wavelength was determined to 5726.4 6 1.5 Å. After taking into account relativistic, nuclear charge distribution, Bohr-Weisskopf, and QED corrections, we observe a significant deviation from commonly tabulated values of the nuclear dipole magnetic moment of this nucleus. [S0031-9007(96)00712-0] PACS numbers: 32.10. Fn, 12.20.Fv, 21.10.Ky, 32.30.Jc Measurements of the hyperfine splitting of the ground state of hydrogen provide a very sensitive tool to explore QED and nuclear contributions to the electron energy. Hydrogenlike ions allow an extension into a region in which these contributions scale to larger values. The 1s electron of a highly charged ion probes the structure of the nucleus more deeply, due to its Coulomb field, enabling a very sensitive test of theory in strong fields. This method complements measurements of muonic atoms, where the muon probes the magnetic field distribution of the nucleus (Bohr-Weisskopf effect). While the spontaneous 1s hyperfine transition in H, D, and He 1 has been observed in radio astronomy, laboratory measurements in hydrogen and low-Z hydrogenlike ions have to rely on stimulated emission using a maser setup due to the extremely long lifetime of the upper hyperfine level (1.1 3 10 7 yr for H) [1,2]. This technique was successful in measuring the splitting in H, D, T, and He 1 . Laser pumping has recently been applied to H-like Bi 821 circulating at nearly 60% the speed of light in a heavy-ion storage ring [3] resulting in the first measurement of the hyperfine splitting in multiply charged ions. Here laser fluorescence was detected as the laser frequency was tuned in and out of resonance with the Doppler-shifted transition. Accurate a priori knowledge of the hyperfine transition is required, lest a prohibitively large scan in the laser frequency be performed.We measure the F 4 to F 3 hyperfine transition of the 1s ground level of H-like 165 Ho 661 using passive emission spectroscopy. The measured wavelength of 5726.4 Å differs 89 Å from a recent calculation [4]. The fact that the transition is excited by electron collisions and the relative ease of the present technique, in principle, opens the upper half of the periodic table to scientific scrutiny.Hydrogenlike holmium ions are produced and stored in a high energy electron beam ion trap (SuperEBIT) [15] by an electron beam of variable energy axially compressed by a high magnetic field. The SuperEBIT has been used to perform very accurate spectroscopic measurements of highly charged ions in the x-ray regime [6], while no such measurements have been made in the optical. The possibility of performing spectroscopy of highly charged ions in the visible on a low-energy EBIT was demonstrated only recently by M...

We report ionization cross section measurements for electron impact single ionization (EISI) of Fe 11+ forming Fe 12+ and electron impact double ionization (EIDI) of Fe 11+ forming Fe 13+ . The measurements cover the center-of-mass energy range from approximately 230 eV to 2300 eV. The experiment was performed using the heavy-ion storage ring TSR located at the Max-Planck-Institut für Kernphysik in Heidelberg, Germany. The storage ring approach allows nearly all metastable levels to relax to the ground state before data collection begins. We find that the cross section for single ionization is 30% smaller than was previously measured in a single-pass experiment using an ion beam with an unknown metastable fraction. We also find some significant differences between our experimental cross section for single ionization and recent distorted wave (DW) calculations. The DW Maxwellian EISI rate coefficient for Fe 11+ forming Fe 12+ may be underestimated by as much as 25% at temperatures for which Fe 11+ is abundant in collisional ionization equilibrium. This is likely due to the absence of 3s excitation-autoionization (EA) in the calculations. However, a precise measurement of the cross section due to this EA channel was not possible because this process is not distinguishable experimentally from electron impact excitation of an n = 3 electron to levels of n 44 followed by field ionization in the charge state analyzer after the interaction region. Our experimental results also indicate that the EIDI cross section is dominated by the indirect process in which direct single ionization of an inner shell 2l electron is followed by autoionization, resulting in a net double ionization.

Many of the fundamental questions in astrophysics can be addressed using spectroscopic observations of photoionized cosmic plasmas. However, the reliability of the inferred astrophysics depends on the accuracy of the underlying atomic data used to interpret the collected spectra. In this paper, we review some of the most glaring atomic data needs for better understanding photoionized plasmas.

We have performed a differential emission measure (DEM) analysis for a polar coronal hole observed during solar minimum in 2007. Five observations are analyzed spanning the coronal hole from the central meridian to the boundary with the quiet-Sun corona. The observed heights ranged from 1.05 to 1.20 R . The analysis shows that the plasma is not strictly isothermal anywhere, but rather has a high-temperature component that extends up to log T (K) = 6.2-6.3. The size and importance of this component depend on location, and its evolving magnitude with height marks the boundary between the coronal hole and the quiet corona, where it becomes dominant. The DEM of the coronal hole plasma below log T (K) = 6.0 decreases faster with height than that of the high-temperature component. We discuss the possible nature of the high-temperature component. Our results highlight the potential limitations of isothermal analyses. Such methods actually measure a DEM-weighted average temperature and as a result can infer artificial temperature gradients. Assuming the gas is isothermal along the line of sight can also yield incorrect electron densities. By revealing structures along the line of sight, a DEM analysis can also be used to more reliably interpret electron temperature and density measurements.