Jaan K. Lepson

Emission-Line Spectra of Ar IX-Ar XVI in the Soft X-Ray Region 20-50 Å

As part of a larger project to complete a comprehensive catalog of astrophysically relevant emission lines in support of new-generation X-ray observatories using the Lawrence Livermore electron beam ion traps EBIT-I and EBIT-II, we present observations of argon lines in the extreme-ultraviolet region. Our database includes wavelength measurements with standard errors, relative intensities, and line assignments for Ar IX-Ar XVI between 20 and 50 A. The experimental data are complemented with a full set of calculations using the Hebrew University Lawrence Livermore Atomic Code (HULLAC). Despite differences in calculated and measured wavelengths, we find the calculated lines to be of great utility in analyzing our laboratory spectra. The calculated line intensities are generally sufficient to identify the strongest transitions in each charge state. We note, however, an underestimation by theory of the strength of the 3s → 2p lines relative to the 3d → 2p lines in Ar IX, Ar X, and Ar XI. The laboratory data are compared with Chandra observations of Procyon, resulting in the identification of an Ar IX line that was previously thought to be from S IX.

Emission Lines of Fe VII-Fe X in the Extreme Ultraviolet Region, 60-140 Å

We report on emission spectra of iron in the extreme ultraviolet recorded at an electron density of ~5 × 1011 cm-3 at the Lawrence Livermore electron beam ion trap EBIT-II. We present a summary of observed emission lines, including wavelengths and emission intensities. We also illustrate our technique for isolating pure charge states of the desired ion and present spectra of pure Fe VII-Fe X. Our measurements add a large number of newly identified lines to existing line lists in the extreme-ultraviolet region, 60-140 A. While many of these lines are quite weak, they add up to a significant flux that can seriously affect interpretations of global fitting models, especially when applied to stars with material at the appropriate temperatures, such as Procyon, α Cen, and the Sun.

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.

Observation of Quasi-Continuum Line Emission from Fe VII to Fe X in the Extreme-Ultraviolet Region below 140 Å

The line emission of Fe VII-Fe X ions in the extreme-ultraviolet region below 140 A was measured in controlled laboratory experiments under conditions representative of stellar coronae. The observations are compared with predictions from standard spectral models using the CHIANTI and MEKAL atomic databases. We find that the atomic databases miss most of the line flux in this region. While some of the missing lines form isolated features, most add up to form a quasi continuum in the 60-120 A region. This incompleteness can explain the poor fit when applying global-fitting techniques to spectra from cool stars measured by the Extreme-Ultraviolet Explorer satellite, the origin of which has been a source of controversy since the original observations were made.

EMISSION-LINE SPECTRA OF S VII-S XIV IN THE 20-75 A WAVELENGTH REGION

As part of a larger project to complete a comprehensive catalogue of astrophysically relevant emission lines in support of new-generation X-ray observatories using the Lawrence Livermore electron beam ion traps EBIT-I and EBIT-II, the authors present observations of sulfur lines in the soft X-ray and extreme ultraviolet regions. The database includes wavelength measurements with standard errors, relative intensities, and line assignments for 127 transitions of S VII through S XIV between 20 and 75 {angstrom}. The experimental data are complemented with a full set of calculations using the Hebrew University Lawrence Livermore Atomic Code (HULLAC). A comparison of the laboratory data with Chandra measurements of Procyon allows them to identify S VII-S XI lines.

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.

Grazing-incidence spectrometer for soft x-ray and extreme ultraviolet spectroscopy on the National Spherical Torus Experiment

A compact grazing-incidence spectrometer has been implemented on the National Spherical Torus Experiment for spectral measurements in the 6–65A spectral region. The spectrometer employed a 2400l∕mm grating designed for flat-field focusing and a cryogenically cooled charge-coupled device camera for readout. The instrument was tested by recording the K-shell lines of boron, carbon, nitrogen, and oxygen, as well the L-shell lines from argon, iron, and nickel that fall into this spectral band. The observed linewidth was about 0.1A, which corresponds to a resolving power of 400 for the CV lines. A temporal resolution as fast as 50ms was obtained.

HIGH-RESOLUTION LABORATORY MEASUREMENTS OF CORONAL LINES IN THE 198-218 Å REGION

We present high-resolution laboratory measurements of the emission from various ions of C, N, O, F, Ne, S, Ar, Fe, and Ni in the extreme ultraviolet wavelength band centered around the λ211 Fe XIV channel of the Atmospheric Imaging Assembly on the Solar Dynamics Observatory. While all of the strong iron lines in this region are well known, we note many weaker lines of iron that are not yet identified. The high resolution of our measurements also allows us to resolve several lines in Fe XI, Fe XII, and Fe XIII between 200 and 205 A, whose identities were in question based on a disagreement between different databases. The spectra of the elements other than iron are much less known, and we find a multitude of lines that are not yet in the databases. For example, the CHIANTI database clearly disagrees with the NIST data listings on several of the argon lines we observe and also it contains only about half of the observed sulfur lines.

Measurement and Modeling of Density-Sensitive Lines of Fe XIII in the Extreme Ultraviolet

We present an analysis of the spectral emission of Fe XIII near 200 A. High-resolution spectra were recorded at two densities (~2 × 1011 and ~1013 cm−3) in the laboratory and compared to collisional radiative model calculations based on the CHIANTI database, and to models using atomic data from distorted-wave and R-matrix calculations. The Fe XIII lines in this wavelength range are sensitive indicators of plasma density below ~ 1011 cm–3. The laboratory data thus test calculations in the astrophysical high-density limit. Significant differences between the measurements and models were found for several line ratios. Differences in the wavelengths employed in the different models also changed the agreement with the measurements. The best agreement was found for comparisons with CHIANTI.

Time-resolved x-ray and extreme ultraviolate spectrometer for use on the National Spherical Torus Experiment

We describe upgrades to a compact grazing-incidence spectrometer utilized on the National Spherical Torus Experiment for monitoring light and heavy impurities. A fast-readout charge couple device camera has been implemented that allows the recording of spectra with up to 25 ms time integration. This capability is used to study the time evolution of the K-shell emission of hydrogenlike and heliumlike boron, carbon, nitrogen, and oxygen between 10 and 65 A. Different camera positioning pieces have been employed to extend the possible spectral range to as high as 140. Several lines that cannot be ascribed to the usual elements found in the plasma have been observed in this spectral range, although often only in a few isolated discharges.