M. F. Gu

Flat-field grating spectrometer for high-resolution soft x-ray and extreme ultraviolet measurements on an electron beam ion trap

A R=44.3 m grazing-incidence grating spectrometer has been implemented on the Livermore electron beam ion traps for high-resolution measurements in the soft x-ray and extreme ultraviolet region spanning from below 10 up to 50 A. The instrument uses a grating with variable line spacing (about 2400 l/mm for a flat field of view. Spectra are recorded with a back-illuminated charge-coupled device detector. The new instrument greatly improves upon the resolution achieved with existing grating spectrometers and complements crystal spectrometers at the shorter wavelengths both in terms of wavelength coverage and polarization independent reflectivity response.

Atomic Data for High-Energy Configurations in Fe XVII-XXIII

In the present work we calculate energy levels, oscillator strengths, A-values, and electron-ion excitation collision strengths for high-energy configurations for the iron ions Fe XVII to Fe XXIII. Collision rate coefficients are calculated using the distorted-wave approximation; the contribution of resonant excitation is taken into account by using the isolated-resonance approximation for configurations with principal quantum numbers n = 2 and 3. Results are compared with R-matrix calculations available in the literature for the lowest configurations; our results are the first ever published for highly excited configurations with n ≥ 4 for most ions. Synthetic spectra are calculated from the resulting rate coefficients and compared to available spectral codes. Some applications of the results of the present calculation to solar physics are discussed. These data will be part of the next version of the CHIANTI database.

Experimental and theoretical evaluation of density-sensitive N VI, AR XIV, and FE XXII line ratios

The line ratios of the 2p-3d transitions in the B-like spectra Ar XIV and Fe XXII have been measured using the electron beam ion traps at Livermore. Radiative-collisional model calculations show these line ratios to be sensitive to the electron density in the ranges ne = 10{sup 10} to 10{sup 12} cm{sup -3} and ne = 10{sup 13} to 10{sup 15} cm{sup -3}, respectively. In our experiment, the electron beam density of about 10{sup 11} cm{sup -3} was varied by about a factor of 5. Our data show a density effect for the line doublet in Ar XIV, and good agreement with theory is found. The relative intensity of the Fe XXII doublet shows good agreement with our predicted low density limit. The N VI K-shell spectrum was used to infer the actual electron density in the overlap region of ion cloud and electron beam, and systematic measurements and calculations of this spectrum are presented as well. The Ar XIV and Fe XXII spectra promise to be reliable density diagnostics for stellar coronae, complementing the K-shell diagnostics of helium-like ions.

Laboratory Measurement and Theoretical Modeling of K-shell X-ray Lines from Inner-shell Excited and Ionized Ions of Oxygen

We present high resolution laboratory spectra of K-shell X-ray lines from inner-shell excited and ionized ions of oxygen, obtained with a reflection grating spectrometer on the electron beam ion trap (EBIT-I) at the Lawrence Livermore National Laboratory. Only with a multi-ion model including all major atomic collisional and radiative processes, are we able to identify the observed K-shell transitions of oxygen ions from O III to O VI. The wavelengths and associated errors for some of the strongest transitions are given, taking into account both the experimental and modeling uncertainties. The present data should be useful in identifying the absorption features present in astrophysical sources, such as active galactic nuclei and X-ray binaries. They are also useful in providing benchmarks for the testing of theoretical atomic structure calculations.

Many-Body Perturbation Theory Wavelengths of High-n X-Ray Transitions of Fe and Ni L-shell Ions

Using a combined configuration interaction and many-body perturbation theory method, we compute the wavelengths of n → 2 (3 ≤ n ≤ 7) transitions of Fe and Ni L-shell ions. The uncertainties of the results are determined to be a few mA through extensive comparisons with existing laboratory measurements. We provide a complete line list of these X-ray transitions of Fe and Ni and incorporate them in the emissivity tables of the Astrophysical Plasma Emission Database (APED).

Capella Corona Revisited: A Combined View from XMM-Newton RGS and Chandra HETGS and LETGS

We present a combined analysis of the X-ray emission of the Capella corona obtained with XMM-Newton RGS and Chandra HETGS and LETGS. An improved atomic line database and a new differential emission measure (DEM) deconvolution method are developed for this purpose. Our new atomic database is based on the Astrophysical Plasma Emission Database and incorporates improved calculations of ionization equilibrium and line emissivities for L-shell ions of abundant elements using the Flexible Atomic Code. The new DEM deconvolution method uses a Markov Chain Monte Carlo (MCMC) technique that differs from existing MCMC or χ2-fitting-based methods. We analyze the advantages and disadvantages of each individual instrument in determining the DEM and elemental abundances. We conclude that results from either RGS or HETGS data alone are not robust enough due to their failure to constrain the DEM in some temperature region or the lack of significant continuum emission in the wavelength band of the spectrometers, and that the combination of HETGS and RGS produces more stringent constraints on the DEM and abundance determinations. Using the LETGS data, we show that the recently discovered inconsistencies between the EUV and X-ray lines of Fe XVIII and XIX also exist in more highly charged iron ions, up to Fe XXIII, and that enhanced interstellar absorption due to partially ionized plasma along the Capella line of sight may explain some, but not all, of these discrepancies.

Electron Impact Excitation Cross Section Measurement for n=3 to n=2 Line Emission in Fe17+ to Fe23+

We have measured the electron impact excitation cross sections for the strong iron L-shell 3 {yields} 2 lines of Fe XVIII to Fe XXIV at the EBIT-I electron beam ion trap using a crystal spectrometer and NASA-Goddard Space Flight Centers 6 x 6 pixel array microcalorimeter. The cross sections were determined by direct normalization to the well established cross section of radiative electron capture through a sophisticated model analysis which results in the excitation cross section for the strong Fe L-shell lines at multiple electron energies. This measurement is part of a laboratory X-ray astrophysics program utilizing the Livermore electron beam ion traps EBIT-I and EBIT-II.

Dielectronic Recombination Rate Coefficients of Na-like Ions from Mg II to Zn XX Forming Mg-like Systems

Dielectronic recombination (DR) rate coefficients for Na-like ions of elements with 12 ≤ Z ≤ 30 forming Mg-like systems are calculated in the independent-process, isolated-resonance approximation. Both ΔN = 0 and ΔN > 0 core excitation DR channels are included, as well as the inner-shell excitation channels. Convenient analytic fits are given to the calculated rate coefficients, and comparison is made with the existing theoretical and experimental results.

Laboratory Measurements of High-n Iron L-Shell X-Ray Lines

We present a comprehensive wavelength survey of Fe L-shell X-ray lines between 7 and 11 {angstrom} measured using flat crystal spectrometers and the EBIT-I and EBIT-II electron beam ion traps at the Lawrence Livermore National Laboratory. This survey includes all significant emission lines produced by over 200 n {yields} 2 transitions in Fe XIX-XXIV, with n = 4-10. The identification and assignment of transitions are made with the help of detailed theoretical modeling using the Flexible Atomic Code (FAC).

Wavelength Measurements of Ni L-Shell Lines between 9 and 15 Å

We present accurate wavelength measurements of nickel L-shell X-ray lines resulting from Δn ≥ 1 transitions between 9 and 15 A. We have used the electron beam ion trap, SuperEBIT, at the Lawrence Livermore National Laboratory and a flat-field grating spectrometer to record the spectra. The most significant emission lines of Ni XIX-XXVI in our spectral coverage are identified, and their relative intensities are determined. The resulting data set provides valuable input for the analyses of high-resolution X-ray spectra of stellar coronae sources, including the Sun.