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

We present an experimental observation of the visible magnetic dipole transition between the fine structure levels in the ground term of ${\mathrm{Ba}}^{6+}$ ($5s{\phantom{\rule{0.16em}{0ex}}}^{2}5{p}^{2}$: $^{3}P_{0}\ensuremath{-}^{3}P_{1}$) with a compact electron beam ion trap. The wave number in vacuum of the transition is determined to be 15505.506(37) ${\mathrm{cm}}^{\ensuremath{-}1}$ by simultaneously observing the reference lines of neutral Ne. The present measurement reduces the uncertainty in the previously reported value obtained from observation of the electric dipole transitions to the ground terms in the extreme ultraviolet range [Phys. Scr. 46, 403 (1992)].

Direct determination of the energy of the first excited fine-structure level in Ba 6 +

To provide spectroscopic data for lowly charged tungsten ions relevant to fusion research, this work focuses on the ${\mathrm{W}}^{8+}$ ion. Six visible spectra lines from ${\mathrm{W}}^{8+}$ in the range of 420--660 nm are observed with a compact electron-beam ion trap in Shanghai. Furthermore, transition energies are calculated for the 30 lowest levels of the $4{f}^{14}5{s}^{2}5{p}^{4}, 4{f}^{13}5{s}^{2}5{p}^{5}$, and $4{f}^{12}5{s}^{2}5{p}^{6}$ configurations of ${\mathrm{W}}^{8+}$ by using the flexible atomic code (fac) and grasp package. Reasonably good agreement is found between our two independent atomic-structure calculations. The resulting atomic parameters are adopted to simulate the spectra based on the collisional-radiative model implemented in the fac code. This assists us with identification of six strong magnetic-dipole transitions in the $4{f}^{13}5{s}^{2}5{p}^{5}$ and $4{f}^{12}5{s}^{2}5{p}^{6}$ configurations from our experiments.

Visible spectra of W 8 + in an electron-beam ion trap

We study the level structures of palladium-like ${\mathrm{I}}^{7+}$ $(Z=53)$ and ${\mathrm{Ba}}^{10+}$ $(Z=56)$ and the behavior of their metastable states with an electron beam ion trap. The difference in the level structures between ${\mathrm{I}}^{7+}$ and ${\mathrm{Ba}}^{10+}$ arising from the configuration crossing between $4{d}^{\ensuremath{-}1}5p$ and $4{d}^{\ensuremath{-}1}4f$ around the atomic number $Z$ = 56 causes significant differences in the lifetimes of the metastable states. ${\mathrm{Ba}}^{10+}$ has several long-lived levels $(g{10}^{\ensuremath{-}3} \mathrm{s})$ in the electron configurations $4{d}^{\ensuremath{-}1}5s$ and $4{d}^{\ensuremath{-}1}4f$, whereas the lifetime of the same $4{d}^{\ensuremath{-}1}4f$ levels in ${\mathrm{I}}^{7+}$ is much shorter due to the appearance of deexcitation channels to a lower energy level in $4{d}^{\ensuremath{-}1}5p$. The extreme ultraviolet spectra identified through comparison with theoretical simulations based on a collisional-radiative model evince the level crossing between the $4{d}^{\ensuremath{-}1}5p$ and $4{d}^{\ensuremath{-}1}4f$ configurations in ${\mathrm{Ba}}^{10+}$. Additionally, we investigate the ionization processes from the metastable states in palladium-like ions by observing the electron energy dependence of the prominent visible line $(4{d}^{\ensuremath{-}1}:{^{2}\mathrm{D}}_{5/2}\text{\ensuremath{-}}{^{2}\mathrm{D}}_{3/2})$ of rhodium-like ${\mathrm{I}}^{8+}$ and ${\mathrm{Ba}}^{11+}$. The electron energy dependencies reflecting the properties of the metastable states show a difference between iodine and barium, which can be explained by the populations of the metastable states calculated with theoretical simulations.

5 p − 4 f level crossing in palladium-like ions and its effect on metastable states

We present the first measurements of visible radiation from W10+. The ions are produced and confined in an electron-beam ion trap (EBIT) at Shanghai EBIT Laboratory, and the spectra are recorded with a Czerny-Turner spectrometer in the wavelength range 300–700 nm. The observed five new spectral lines are identified using detailed collisional-radiative (CR) modeling of the EBIT spectra. In addition, we perform large-scale theoretical calculations for the lowest 50 energy levels of W10+ using FAC and GRASP codes. The excitation energies obtained by the two independent calculations fit well with each other, with ~0.5% difference on average, and the resulting values are adopted to check our line identifications, with wavelength deviations from experimental results ~1.5% on average.

Measurement and identification of visible lines from W10

Naoki KIMURA1,2), Ryunosuke KODAMA3), Kento SUZUKI3), Shimpei OISHI3), Michiharu WADA4), Kunihiro OKADA1), Noriaki OHMAE5,6), Hidetoshi KATORI5,6,7) and Nobuyuki NAKAMURA3) 1)Department of Physics, Sophia University, Tokyo 102-8554, Japan 2)RIKEN Nishina Center, Saitama 351-0198, Japan 3)Institute for Laser Science, The University of Electro-Communications, Tokyo 182-8585, Japan 4)Wako Nuclear Science Center, Institute of Particle and Nuclear Studies, High Energy Accelerator Research Organization (KEK), Saitama 351-0198, Japan 5)Quantum Metrology Laboratory, RIKEN, Saitama 351-0198, Japan 6)RIKEN Center for Advanced Photonics, Saitama 351-0198, Japan 7)Department of Applied Physics, The University of Tokyo, Tokyo 113-8656, Japan

Kento Suzuki

Papers

Direct determination of the energy of the first excited fine-structure level in Ba 6 +

5 p − 4 f level crossing in palladium-like ions and its effect on metastable states

Direct Wavelength Measurement of the Visible M1 Transition in Ba 7+ with a Novel Calibration Method