V. Decaux

High-Resolution Measurement of the Kα Spectrum of Fe XXV-XVIII: New Spectral Diagnostics of Nonequilibrium Astrophysical Plasmas

We present laboratory measurements of high-resolution spectra of iron Kα emission under transient ionization conditions similar to those that are believed to exist in stellar flares and young supernova remnants. Taking advantage of our high spectral resolution (λ/Δλ ≥ 2000), we identify a number of transitions that can serve as diagnostics of ionizing plasmas. By varying the excitation energy in the experiments, we constrain the effects of the electron distribution on these diagnostic lines. Using our measured line ratios, we deduce values for the ionization time, η = Net, in the plasma, which agree with the actual values to ~20% accuracy. This result gives us confidence to our ability to derive similar constraints on astrophysical plasmas from remote X-ray spectroscopic observations.

High-resolution measurements of the K-alpha spectra of low-ionizationm species of iron: A new spectral signature of nonequilibrium ionization conditions in young supernova remnants

We present the first systematic laboratory measurements of high-resolution K-alpha spectra of intermediate ions of iron, Fe X-XVII. These lines are not produced in collisional equilibrium plasmas because of the relevant charge states cannot exist at the high electron temperatures required for appreciable excitation of the K-alpha transitions. However, they can provide excellent spectral diagnostics for nonequilibrium ionization conditions, such the ionizing plasmas of young supernova remnants. To facilitate the line identifications, we compare our spectra with theoretical atomic calculations performed using multiconfiguration parametric potential and Dirac-Fock atomic codes. Our measurements also allow direct comparison with time-dependent ionization balance calculations for ionizing plasmas, and good agreement is found.

Temperature of the ions produced and trapped in an electron‐beam ion trap

Spectrometers were constructed with nominal resolving power λ/Δλ≤22 000 at a Bragg angle of 45° and were used to measure the temperature of ions produced and trapped in the Livermore electron‐beam ion trap facility. A temperature in excess of 500 eV was found for heliumlike Ti20+ ions produced under standard operating conditions for spectroscopy. The temperature was reduced to 130 eV by lowering the trapping potential and electron‐beam current. A description of the spectrometers and representative measurements are given.

Laboratory Measurements of Fe XXIV L-Shell Line Emission

Recent ASCA spectra exhibit discrepancies with the relative line intensities of various Fe XXIII and XXIV L-shell emission lines predicted by standard plasma emission codes. To address this issue, we have carried out a series of high-resolution, broadband measurements of Fe XXIV line emission using an electron beam ion trap facility. X-ray lines produced in the trap are detected and resolved using Bragg crystal spectrometers. We report measurements of 3 → 2 and 4 → 2 transitions, which result primarily from electron impact excitation. Overall, good agreement is found with distorted wave calculations.

Polarization spectroscopy of x-ray transitions from beam-excited highly charged ions

Polarization spectroscopy of x-ray lines represents a diagnostic tool to ascertain the presence of electron beams in high-temperature plasmas. Making use of the Livermore electron beam ion trap, which optimizes the linear x-ray line polarization by exciting highly charged ions with a monoenergetic electron beam, we have begun to develop polarization diagnostics and test theoretical models. Our measurement relies on the sensitivity of crystal spectrometers to the linear polarization of x-ray lines which depends on the value of the Bragg angle. We employed two spectrometers with differing analyzing crystals and simultaneously recorded the K-shell emission from heliumlike Fe24+ and lithiumlike Fe23+ ions at two different Bragg angles. A clear difference in the relative intensities of the dominant transitions is observed, which is attributed to the amount of linear polarization of the individual lines.

Measurement of the temperature of cold highly charged ions produced in an electron beam ion trap

Abstract The temperature of highly charged titanium ions produced and trapped in an electron beam ion trap was determined by precisely measuring the broadening of K-shell emission lines in Ti19+ caused by the thermal Doppler motion. While interacting with a 3330 V electron beam the measured temperature of the titanium ions ranges from about 700 eV when the ions are confined in a deep trap to about 70 eV for ions in a shallow trap. The latter value represents the lowest temperature at which the K-shell X-ray emission from highly charged titanium ions has ever been recorded.

On the mechanism of formation of hollow atoms below a surface

Abstract The interaction of Ar17+, Fe25+ and Kr35+ ions on surfaces has been studied by looking at the X-rays emitted in flight by the ions with a SiLi detector. A model has been developed to describe the behavior of the ions inside the solid and the formation of “hollow atoms” below the surface.

Studies of He‐like krypton for use in determining electron and ion temperatures in very‐high‐temperature plasmas

High‐resolution measurements of the He‐like Kr K‐shell radiation are presented showing the direct excitation lines as well as Li‐ and Be‐like dielectronic recombination lines appearing in the KLL resonance. The measurements were performed on the Electron Beam Ion Trap (EBIT) facility and motivated by the need for accurate atomic data of high‐Z He‐like K‐shell transitions for diagnostic applications in determining the central ion and electron temperature of very‐high‐temperature plasmas produced in next‐generation tokamaks, such as the International Thermonuclear Experimental Reactor (ITER).

Energies of neon-like n = 4 to n = 2 resonance lines

A multi-configuration Dirac-Fock method is used to calculate the energies of the n = 4 → n = 2 resonance lines for neon-like ions with nuclear charge Z from 18 to 92. These results are compared with calculations done by the Z-expansion method and the model potential method. High resolution measurements done at the EBIT facility are presented for several high-Z ions and provide an important benchmark against which the various code results are compared.

Overview of the current spectroscopy effort on the Livermore electron beam ion traps

An overview is given of the current spectroscopic effort on the Livermore electron beam ion trap facilities. The effort focuses on four aspects: spectral line position, line intensity, temporal evolution, and line shape. Examples of line position measurements include studies of the K-shell transitions in heliumlike Kr34+ and the 2s-2p intrashell transitions in lithiumlike Th87+ and U89+, which provide benchmark values for testing the theory of relativistic and quantum electrodynamical contributions in high-Z ions. Examples of line intensity measurements are provided by measurements of the electron-impact excitation and dielectronic recombination cross sections of the heliumlike transition-metal ions Ti20+ through Co25+. A discussion of radiative lifetime measurements of metastable levels in heliumlike ions is given to illustrate our time-resolved spectroscopy techniques in the microsecond range. We also present a measurement of the spectral lineshape that illustrates the very low ion temperatures that can be achieved in an EBIT.