Electron-impact excitation cross sections from the ground state to the individual magnetic sublevels of the excited state $1s2{s}^{2}2{p}_{1/2}J=1$ for highly charged berylliumlike ions have been calculated by using a fully relativistic distorted-wave (RDW) method. The degrees of linear polarization of the corresponding radiations are obtained. It is found that the Breit interaction makes the linear polarization of radiations decrease, and this character becomes more evident with increasing of incident electron energy. For a given energy in threshold units, the linear polarization without the Breit interaction included increases very slowly as the atomic number increases, however, the linear polarization with the Breit interaction included decreases rapidly. These characteristics are very different from the conclusions for the linear polarization of the same radiations but formed by the dielectronic recombination process [Phys. Rev. Lett. 103, 113001 (2009)], in which the Breit interaction makes the linear polarization increase, and the linear polarization with the Breit interaction included increases with increasing of the atomic number.

Influence of Breit interaction on the polarization of radiation following inner-shell electron-impact excitation of highly charged berylliumlike ions

Highly charged ions play a crucial role in magnetic fusion plasmas. These plasmas are excellent sources for producing highly charged ions and copious amounts of radiation for studying their atomic properties. These studies include calibration of density diagnostics, x-ray production by charge exchange, line identifications and accurate wavelength measurements, and benchmark data for ionization balance calculations. Studies of magnetic fusion plasmas also consume a large amount of atomic data, especially in order to develop new spectral diagnostics. Examples we give are the need for highly accurate wavelengths as references for measurements of bulk plasma motion, the need for accurate line excitation rates that encompass both electron-impact excitation and indirect line formation processes, for accurate position and resonance strength information of dielectronic recombination satellite lines that may broaden or shift diagnostic lines or that may provide electron temperature information, and the need for accurate ionization balance calculations. We show that the highly charged ions of several elements are of special current interest to magnetic fusion, notably highly charged ions of argon, iron, krypton, xenon, and foremost of tungsten. The electron temperatures thought to be achievable in the near future may produce W70+ ions and possibly ions with even higher charge states. This means that all but a few of the most highly charged ions are of potential interest as plasma diagnostics or are available for basic research.

Highly Charged Ions in Magnetic Fusion Plasmas: Research Opportunities and Diagnostic Necessities

Cross sections for electron-impact excitation and capture to individual magnetic sublevels of Cu-like to Se-like gold ions have been calculated by using a fully relativistic distorted-wave (RDW) method. These magnetic cross sections have further been used to obtain the degrees of linear polarization for the first two strongest $5f\ensuremath{\rightarrow}3d$ radiative lines. A detailed comparison has been made between polarizations of these lines from the electron-impact excitation process and those of the same lines but from the dielectronic recombination process. It is found that for a Cu-like gold ion the polarization in the electron-impact excitation process increases sharply before starting to decrease at higher incident electron energies, and it reaches the maximum at around twice the threshold energy. The situations for other gold ions are very similar. However, as for the dielectronic recombination process, the polarization changes among a very large range with different gold ions. Therefore, the obvious differences between the polarizations from different formation processes can be employed in diagnosing the formation mechanism of the corresponding lines.

Degrees of polarization of the two strongest 5 f → 3 d lines following electron-impact excitation and dielectronic recombination processes of Cu-like to Se-like gold ions

Excitation energies for $3l\ensuremath{-}{4l}^{\ensuremath{'}}$ particle-hole states of Ni-like ions are determined to second order in relativistic many body perturbation theory. The calculations start from a closed-shell Dirac-Fock potential, and include second-order Coulomb and Breit-Coulomb interactions. Retarded electric-dipole matrix elements (in length and velocity forms) are calculated in second order for transitions from excited $3l\ensuremath{-}{4l}^{\ensuremath{'}}$ states to the closed-shell ground state. Wavelengths for 3-4 and 4-4 transitions are compared with experimental data, and with other high-precision calculations. Trends of oscillator strengths as functions of nuclear charge Z are shown graphically for selected transitions.

Relativistic many-body calculations of excitation energies and oscillator strengths in Ni-like ions

Photoionized plasma was studied experimentally under laboratory conditions by means of high intensity short pulse lasers. The experiment consists of a gold cavity filled with nitrogen gas. Six laser beams were focused on the inner surface of the gold cavity, thereby generating an almost black-body radiation having temperature of 80 eV inside the cavity. This radiation heats the nitrogen gas mainly by means of photoionization. L-shell emissions from N V to N VII have been observed in the wavelength range between 90 and 200 A. A time-dependent Detailed Configuration Accounting computer program has been developed to analyze the experimental spectra. In contrast to standard analysis of astrophysical observations, the evidence for photoionization is inferred from the spectral lines ratios. Comparison between the experimental and simulated line spectra indicates that the radiation heated nitrogen attains temperature of 20-30 eV, much lower than the source radiation temperature. Paradoxically, it is also shown that similar line emissions can be reproduced computationally also when the radiation and plasma temperatures both equal approximately 60 eV. This misleading result indicates that experimental simulation in laboratory is sometimes necessary to avoid misinterpretation of astrophysical spectra. (C) 2008 American Institute of Physics.

http://absimage.aps.org/image/APR08/MWS_APR08-2008-000647.pdf

Experimental evidence and theoretical analysis of photoionized plasma under x-ray radiation produced by intense laser

K-shell absorption spectroscopy of radiatively heated Al sample is reported. The experiment was conducted on Shenguang-Ⅱ Laser facility. Eight main beams of the facitliy are injected into a conical gold cavity to produce high-temperature X-ray heating source. This X-ray source is then used to irradiate an Al foil located in the center of the cavity, and heat the foil to temperature of tens of electronvolt. After a time delay, a ninth short-pulsed laser beam is focused on a gold disk to generated an X-ray point-backlighter. By measuring the spectra both directly from backlighter and attenuated by the heated sample in the wavelength range of 075—085nm, the K-shell absorption spectrum of Al is obtained. By fitting the experimental spectrum with theoretical calculation, electron temperature of the heated sample is determined.

Absorption experiments on radiatively heated Al plasma

The KLL dielectronic recombination processes of highly charged He-like to C-like Kr ions have been studied experimentally The measurement was performed on the newly developed Shanghai electron beam ion trap (Shanghai-EBIT) facility Characteristic x-rays from both dielectronic recombination and radiative recombination are detected as the electron beam energy is scanned through the resonances The KLL resonant strengths obtained are 541 × 10−19, 433 × 10−19, 359 × 10−19, 205 × 10−19 and 098 × 10−19 cm2 eV for He-like to C-like Kr ions, respectively

https://iopscience.iop.org/article/10.1088/0256-307X/26/3/033401/pdf

Measurement of the KLL Dielectronic Recombination Resonances in He-like to C-like Kr Ions

A collisional?radiative model is developed for population calculations of plasmas in non-local thermodynamic equilibrium. The rate equations in detailed configuration accounting are solved to obtain ion populations. The configuration averaged rate coefficients are used in the rate equations. The cross sections are calculated based on the first perturbation theory. Wavefunctions required in cross section calculations are obtained by the Hartree?Fock?Slater self-consistent-field model. This kinetic model is applied to low- and medium-Z as well as high-Z plasmas. The results are compared with those of other theoretical models and experiments. The comparisons show that the present results of the mean charge state for low- and medium-Z elements agree well with other theoretical ones, while for high-Z elements, the present mean ionization stages are about two stages lower than the experimental ones.

Ionization Balance in Non-Local-Thermodynamic-Equilibrium Plasmas

The He 2+ ＋H Impact Ionization process is investigated using a continu um distorted wave method with the eikonal initial-state(CDW-EIS) approximation.The total,single- and double-differential cross sections are obtained in the projectile energy ranges from 30keV/u to 2000keV/u.The total cross sections are compared with the existing theoretical and experimental data.When the projectile energy is larger than 100keV/u,our results are in good agreement with the experimental measurements,while about 50% less than experimental data in the lower energy range.The ionization mechanism of soft-collisions,electron captured to continuum states and binary-encounter collisions are also displayed.

CDW-EIS calculation in He 2+ +H impact ionization

A programme is developed to calculate the polarizations of the radiation emitted after electron impact excitation. The fully relativistic distorted-wave method is used in cross-section calculations. The programme is applied to He- and Li-like ions. The calculated values of line polarization are compared with other theoretical results and experimental values. For He-like U, at lower incident energy, the present polarization agrees with the other theoretical ones within 1%, while at higher energy, the differences increase up to about 10%. For He-like Fe and Ti, the present results of polarization degree for most of the lines agree with the experimental data within the experimental error bars. For the Li-like Ti line q (1s2s2p2P3/2 to 1s22s), the present value of the polarization agree excellently with another theoretical one, and both the values are consistent with the measured data within the experimental error bar.

Polarization of Radiation Emitted after Electron Impact Excitation

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