Polarization measurements on a magnetic quadrupole line in Ne-like barium

Background and Blended Spectral Line Reduction in Precision Spectroscopy of EUV and X-ray Transitions in Highly Charged Ions

Extreme ultraviolet spectra of Na-like and Mg-like Os and Ir were recorded at the National Institute of Standards and Technology using a grazing incidence spectrometer. We report a method in EBIT spectral analysis that reduces signals from contaminant lines of known or unknown origin. We utilize similar ion charge distributions of heavy highly charged ions that create similar potentials for lighter contaminating background elements. First-order approximations to ion distributions are presented to demonstrate differences between impurity elements with and without heavy ions present.

Contemporary x-ray wavelength metrology and traceability

We report recent advances in absolute x-ray wavelength metrology in the context of producing modern standard reference data. Primary x-ray wavelength standards are produced today using diffraction spectrometers using crystal optics arranged to be operated in dispersive and non-dispersive geometries, giving natural-line-width limited profiles with high resolution and accuracy. With current developments, measurement results can be made traceable to the Système internationale definition of the meter by using diffraction crystals that have absolute lattice-spacing provenance through x-ray-optical interferometry. Recent advances in goniometry, innovation of electronic x-ray area detectors, and new in situ alignment and measurement methods now permit robust measurement and quantification of previously-elusive systematic uncertainties. This capability supports infrastructures like the NIST Standard Reference Data programs and the International Initiative on X-ray Fundamental Parameters and their contributions to science and industry. Such data projects are further served by employing complementary wavelength-and energy-dispersive spectroscopic techniques. This combination can provide, among other things, new tabulations of less-intense x-ray lines that need to be identified in x-ray fluorescence investigation of uncharacterized analytes. After delineating the traceability chain for primary x-ray wavelength standards, and NIST efforts to produce standard reference data and materials in particular, this paper posits the new opportunities for x-ray reference data tabulation that modern methods now afford.

EBIT Observation of Ar Dielectronic Recombination Lines near the Unknown Faint X-Ray Feature Found in the Stacked Spectrum of Galaxy Clusters

Motivated by possible atomic origins of the unidentified emission line detected at 3.55-3.57 keV in a stacked spectrum of galaxy clusters, an electron beam ion trap (EBIT) was used to investigate the resonant dielectronic recombination (DR) process in highly charged argon ions as a possible contributor to the emission feature. The He-like Ar DR-induced transition 1s22l-1s2l3l' was suggested to produce a 3.62 keV photon near the unidentified line at 3.57 keV and was the starting point of our investigation. The collisional-radiative model NOMAD was used to create synthetic spectra for comparison with both our EBIT measurements and with spectra produced with the AtomDB database/Astrophysical Plasma Emission Code (APEC) used in the Bulbul et al. work. Excellent agreement was found between the NOMAD and EBIT spectra, providing a high level of confidence in the atomic data used. Comparison of the NOMAD and APEC spectra revealed a number of missing features in the AtomDB database near the unidentified line. At an electron temperature of T e = 1.72 keV, the inclusion of the missing lines in AtomDB increases the total flux in the 3.5-3.66 keV energy band by a factor of 2. While important, this extra emission is not enough to explain the unidentified line found in the galaxy cluster spectra.

Imaging crystal spectrometer for high-resolution x-ray measurements on electron beam ion traps and tokamaks

We describe a crystal spectrometer implemented on the Livermore electron beam ion traps that employ two spherically bent quartz crystals and a cryogenically cooled back-illuminated charge-coupled device detector to measure x rays with a nominal resolving power of λ/Δλ ≥ 10 000. Its focusing properties allow us to record x rays either with the plane of dispersion perpendicular or parallel to the electron beam and, thus, to preferentially select one of the two linear x-ray polarization components. Moreover, by choice of dispersion plane and focussing conditions, we use the instrument either to image the distribution of the ions within the 2 cm long trap region, or to concentrate x rays of a given energy to a point on the detector, which optimizes the signal-to-noise ratio. We demonstrate the operation and utility of the new instrument by presenting spectra of Mo³⁴⁺, which prepares the instrument for use as a core impurity diagnostic on the NSTX-U spherical torus and other magnetic fusion devices that employ molybdenum as plasma facing components.

Strong higher-order resonant contributions to x-ray line polarization in hot plasmas

We studied angular distributions of x rays emitted in resonant recombination of highly charged iron and krypton ions, resolving dielectronic, trielectronic, and quadruelectronic channels. A tunable electron beam drove these processes, inducing x rays registered by two detectors mounted along and perpendicular to the beam axis. The measured emission asymmetries comprehensively benchmarked full-order atomic calculations. We conclude that accurate polarization diagnostics of hot plasmas can only be obtained under the premise of inclusion of higher-order processes that were neglected in earlier work.

Rotating effects on the Landau quantization for an atom with a magnetic quadrupole moment

Based on the single particle approximation [Dmitriev et al., Phys. Rev. C 50, 2358 (1994) and C.-C. Chen, Phys. Rev. A 51, 2611 (1995)], the Landau quantization associated with an atom with a magnetic quadrupole moment is introduced, and then, rotating effects on this analogue of the Landau quantization is investigated. It is shown that rotating effects can modify the cyclotron frequency and breaks the degeneracy of the analogue of the Landau levels.

On an atom with a magnetic quadrupole moment subject to harmonic and linear confining potentials

The quantum dynamics of an atom with a magnetic quadrupole moment that interacts with an external field subject to harmonic and linear confining potentials is investigated. It is shown that the interaction between the magnetic quadrupole moment and an electric field gives rise to an analogue of the Coulomb potential and, by confining this atom to harmonic and linear confining potentials, a quantum effect characterized by the dependence of the angular frequency on the quantum numbers of the system is obtained. In particular, it is shown that the possible values of the angular frequency associated with the ground state of the system are determined by a third-degree algebraic equation.

Observation of coherence in the time-reversed relativistic photoelectric effect

The photoelectric effect has been studied in the regime of hard x rays and strong Coulomb fields via its time-reversed process of radiative recombination (RR). In the experiment, the relativistic electrons recombined into the 2p_{3/2} excited state of hydrogenlike uranium ions, and both the RR x rays and the subsequently emitted characteristic x rays were detected in coincidence. This allowed us to observe the coherence between the magnetic substates in a highly charged ion and to identify the contribution of the spin-orbit interaction to the RR process.

Experimental demonstration of the Breit interaction which dominates the angular distribution of x-ray emission in dielectronic recombination

We report the experimentally determined angular distribution of the [1s2s(2)2p(1/2)](1)→[1s(2)2s(2)](0) transition in dielectronic recombination of Li-like Au. Recently, Fritzsche et al. [Phys. Rev. Lett. 103, 113001 (2009)] predicted that the Breit interaction plays a dominant role in the angular distribution of this transition. However, the predicted phenomenon has not yet been observed experimentally due to technical difficulties in conventional methods. To overcome the difficulties, we combine two different measurements with an electron beam ion trap (EBIT) to obtain the x-ray angular distribution. One is the x-ray measurement at 90° and another is the integral resonant strength measurement through the ion charge abundance in the EBIT. Our measurements agree well with the theoretical prediction and confirm the dominance of the Breit interaction.

X-ray line polarization of He-like Si satellite spectra in plasmas driven by high-intensity ultrashort pulsed lasers

We present a modeling study of x-ray line polarization in plasmas driven by high-intensity, ultrashort duration pulsed lasers. Electron kinetics simulations of these transient and nonequilibrium plasmas predict non-Maxwellian and anisotropic electron distribution functions. Under these conditions, the magnetic sublevels within fine structure levels can be unequally populated which leads to the emission of polarized lines. We have developed a time-dependent, collisional-radiative atomic kinetics model of magnetic sublevels to understand the underlying processes and mechanisms leading to the formation of polarized x-ray line emission in plasmas with anisotropic electron distribution functions. The electron distribution function consists of a thermal component extracted from hydrodynamic calculations and a beam component determined by PIC simulations of the laser-plasma interaction. We focus on the polarization properties of the He-like Si satellites of the L y(alpha) line, discuss the time evolution of polarized satellite spectra, and identify suitable polarization markers that are sensitive to the anisotropy of the electron distribution function and can be used for diagnostic applications.

Measurement of the electron cyclotron energy component of the electron beam of an electron beam ion trap

The energy component associated with the cyclotron motion of the beam electrons in the Livermore EBIT-II electron beam ion trap was inferred from measurements of the linear polarization of the K-shell x-ray lines emitted from helium-like Mg(10+) ions. The average line polarization was found to be reduced by about 20% from its nominal value. From this it was inferred that the electron cyclotron motion accounted for 190+/-30 eV of the total electron beam energy. The measured value is in good agreement with the predictions of optical electron beam propagation. It does not agree with the estimates derived from other model assumptions, such as the rigid-rotator model, for determining the size of the energy stored in the electron motion perpendicular to the beam propagation direction.