This paper reports x-ray crystal spectroscopy (XCS) on highly charged ions of tungsten impurity within the Large Helical Device (LHD) under plasma conditions with a central electron temperature around 4 keV. By combining measured spectra with Flexible Atomic Code (FAC) calculations and a collisional radiative model weighted by ADAS ionization and recombination rates, emission lines from W43+ to W47+ charge states were systematically identified in the 3.7-4.0 & Aring; band. The dataset was constructed by integrating 10 discharges to cover the entire wavelength range, using tungsten introduced reproducibly via coaxial W pellet injection. Notably, the W47+ line at 3.7691 & Aring; extends the highest charge state observed to date in the LHD. Leveraging the high dispersion characteristics of XCS, three diagnostics directly relevant to ITER-class edge/pedestal plasmas with Te of several keV were demonstrated: (i) simultaneous measurement of multiple charge states via EUV-x-ray co-observation. This enables monitoring the evolution of tungsten charge state distribution in response to electron temperature variations, supporting impurity transport studies and operational monitoring. (ii) Ion temperature estimation from the Doppler width of the W46+ 3.8784 & Aring; line showing a core peak, including error propagation via determination of instrument functions and cross-comparison with the charge exchange recombination spectroscopy. (iii) An electron temperature indicator based on the W46+ 3.8784/3.8956 & Aring; intensity ratio. This exhibits strong sensitivity to Te, and FAC calculations reproduced the observed Te dependence of the line intensity ratio in the Te range of 3-5 keV.

Lanthanide elements play an important role in several astrophysical phenomena, such as opacities in neutron star mergers. However, their atomic data, particularly transition probabilities, remain incomplete in databases such as the atomic spectral database (ASD) of the National Institute of Standards and Technology (NIST). This lack of data hinders the accurate analysis of radiative transfer in kilonovae. In this study, we report the transition probabilities of Eu I and Eu II measured using laser-induced breakdown spectroscopy (LIBS). By analyzing the emission spectra of europium in the 200-1200 nm wavelength range recorded with the LIBS technique, we determine the transition probabilities for emission lines that are not available in the NIST ASD. We report transition probabilities for 16 Eu I and 44 Eu II lines and compare our results with the available literature data.

We have constructed collisional-radiative (CR) models for tungsten ions, Wq+, with q = 18–25, including ionizing and recombining processes as well as collisional excitation/deexcitation and radiative decay, and applied the models to analyze extreme ultraviolet (EUV) spectra measured in magnetically confined fusion plasmas with a tungsten pellet injection into the Large Helical Device (LHD). The CR models are constructed as hybrids with fine-structure levels for lower states and relativistic configuration-averaged levels for higher states to include autoionizing states. The dielectronic recombination process is treated as dielectronic capture to the autoionizing states and radiative decay from the autoionizing states in the CR model. We calculated EUV spectra of the tungsten ions for ionizing and recombining plasmas. Measured spectra at the 2–4 nm region were used to estimate the charge state distribution of tungsten ions in LHD plasmas by comparing calculated emission peaks of n = 4–5 transitions of Wq+ with q = 22–25. Using the estimated ion distribution, the synthesized spectral profile at 4.5–7 nm shows characteristic profiles similar to the measured so-called unresolved transition array (UTA) due to n = 4–4 transitions. We also synthesized the spectra at 10–35 nm, where the UTA due to n = 5–5 transitions appeared, and we partly reproduced the UTA with synthesized spectra. The CR models for the tungsten ions were validated with the measured spectra of LHD plasmas.

We calculated adiabatic states of atomic hydrogen in strong magnetic fields treating the radial distance of the spherical coordinates as the adiabatic parameter. The results show a transition from diamagnetic Kepler motion in the inner region to gyromotion along the magnetic field at large distances. At the boundary, avoided crossings of adiabatic potential curves occur along the ridge of the diamagnetic potential, indicating strong local non-adiabatic coupling. These findings clarify the mixed Coulomb-magnetic dynamics of hydrogen and contribute to more accurate modeling of atomic processes in strongly magnetized astrophysical environments. (c) 2026 The Japan Society of Plasma Science andNuclear Fusion Research

Kilonova spectra provide us with direct information about rapid neutron-capture nucleosynthesis (r-process) in neutron star (NS) mergers. In this paper, we study the signatures of elements beyond the third r-process peak expected to be produced in neutron-rich ejecta in the photospheric spectra of kilonova. We select Ra ii, Ac iii, and Th iii as our candidates because they have a small number of valence electrons and low-lying energy levels, which tend to result in strong absorption features. We systematically calculate the strength of bound-bound transitions of these candidates by constructing a line list based on the available atomic database. We find that Th iii is the most promising species, showing strong transitions at near-infrared wavelengths. By performing radiative transfer simulations, we find that Th iii produces broad absorption features at similar to 18000 & Aring; in the spectra when the mass ratio of actinides to lanthanides is larger than the solar r-process ratio and the mass fraction of lanthanides is less than or similar to 6 x 10-4. Our models demonstrate that the Th feature may be detectable if the bulk of the ejecta in the line-forming region is dominated by relatively light r-process elements with the mixture of a small fraction of very neutron-rich material. Such conditions may be realized in the mergers of unequal-mass NSs or black hole-NS binaries. To detect the Th absorption features, observations from space (such as with the JWST) or high-altitude sites are important as the wavelength region of the Th features overlaps with that affected by strong telluric absorption.

In this study, we present experimentally determined transition probabilities of 86 singly ionized cerium emission lines. We used a widely employed spectroscopic technique, laser-induced breakdown spectroscopy (LIBS), to measure the transition probabilities considering the great demand for experimentally determined atomic data. In astrophysics, cerium is a significant element of interest, especially when examining kilonova emissions that are observed following the merging of two neutron stars. The experimental data are compared with previously reported data.

This article reviews various achievements in spectroscopy of highly charged ions of a variety of heavy elements injected into the Large Helical Device (LHD) plasmas. We focus on discrete and quasi-continuum spectra observed in extreme ultraviolet (EUV) and soft X-ray wavelength ranges using multiple grazing incidence spectrometers. In particular, the atomic number dependence and temperature dependence of the spectral features have been investigated more comprehensively than ever before over extremely wide ranges based on comparisons with theoretical models and other experimental data. Consequently, the series of studies could provide an experimental database valuable for investigations of basic atomic physics issues specific to highly charged heavy ions, as well as the applications to industrial light source developments.

In recent years, the development of soft X-ray (SXR) light sources has progressed rapidly, particularly for applications within the water window region (23-44 & Aring;), where high-resolution, high-contrast imaging of biological cells and macromolecules is possible. In this study, we analyze water window emission lines of samarium-like (Sm-like) Pb20+ ions observed using a compact electron beam ion trap at an electron beam energy of 565 eV. The analysis is carried out using a collisional-radiative model based on atomic data from HULLAC. Three distinct meta-stable states of Pb20+, each with a fractional population exceeding 1%, are identified. To explore the conditions under which metastable states are formed or suppressed across the isoelectronic sequence, we performed systematic atomic structure and transition rate calculations for Sm-like ions from Hg to Po (Z = 80-84). These results provide new insights into the role of energy level crossings and decay pathways in the formation of long-lived states, offering implications for optimizing soft X-ray emission in plasma-based light sources. (c) 2025 The Japan Society of Plasma Science andNuclear Fusion Research

Lanthanides play essential roles in opacities for the kilonova, the ultraviolet-optical-infrared emission from the neutron star merger detected by the gravitational wave (GW170817). In this paper, we examined suitable statistical representations for quasi-continuous spectra of singly ionized lanthanides (Z = 59-70) based on our previous ab-initio calculations using the multi-configuration Dirac-Hartree-Fock and parametric potential methods. Using higher order Gram-Charlier expansions, an improved representation was obtained for the statistical distribution of the level energies. Systematic features that appear in mean, variance, and skewness of the distribution are explained in terms of atomic structure theories. We found that the effective line strength, that is defined as a product of the line strength and the Boltzmann factor at a give temperature, follows the log-logistic distribution. The scaling parameter of the log-logistic distribution gives an indicator down to which transitions should be included in opacity calculations for neutron star mergers.

We report a high-precision determination of the natural-abundance-averaged nuclear charge-radius difference between Yb and Lu using extreme ultraviolet (EUV) spectroscopy of highly charged ions (HCIs). By measuring the $D_1$ transition energies in Na- and Mg-like charge states of Lu and Yb confined in the Tokyo electron-beam ion trap, we extract meV-level energy shifts that are directly sensitive to nuclear-size effects. Transition-energy differences obtained from these spectra are compared with state-of-the-art relativistic many-body perturbation theory, including a new treatment of Mg-like ions. We develop a generalized framework to propagate uncertainties arising from nuclear deformation and surface diffuseness and evaluate corresponding nuclear-sensitivity coefficients. Combining Na- and Mg-like results yields mutually consistent radius differences, demonstrating the robustness of both the experimental calibration and the theoretical predictions. To determine absolute isotopic radii, we perform a generalized least-squares optimization incorporating our HCI constraints together with optical-isotope-shift data and muonic-atom results. This analysis establishes that the $^{175}$Lu charge radius is smaller than that of $^{174}$Yb, restoring the expected odd-even staggering across the $N=94$ isotonic chain. Our recommended value, $R(^{175}\text{Lu}) = 5.291(11)$ fm, reduces the uncertainty of the Lu radius by a factor of three compared with the previous electron-scattering result and resolves a long-standing anomaly in rare-earth nuclear systematics. This work demonstrates that EUV spectroscopy of HCIs provides a powerful and broadly applicable method for precision nuclear-structure studies in heavy, deformed nuclei. The techniques developed here enable future investigations of isotonic and isoelectronic sequences, including radioactive nuclides and higher-$Z$ systems.

A highly charged muonic ion is a unique few-body atomic system where a negatively charged muon and a few electrons are simultaneously bound to a single nucleus. We report the first state-selective observation of highly charged muonic Ar (mu Ar) by electronic K x-ray spectroscopy using an array of transition-edge sensor microcalorimeters. The high-precision K x-ray spectra provide a clear signature of the presence of muonic atoms with one, two, and three electrons, i.e., H-like, He-like, and Li-like mu Ar. With the aid of theoretical calculations, we confirmed that the peak positions are consistent with the x-ray energies from highly charged Cl ions, and the intensities reflect deexcitation dynamics of highly charged mu Ar.

Understanding the structures of the unresolved transition array (UTA) observed in extreme ultraviolet (EUV) spectra from many-electron atoms is crucial for various applications, including fusion science and nanolithography. To measure the fine structure of the UTA from tungsten and tin at around 5 and 13.5 nm at the Tokyo electron beam ion trap, we developed a high-resolution EUV spectrometer. The designed spectrometer achieves a resolving power of λ/dλ > 5000 at 5 and 13.5 nm. The fabricated large-area grating was experimentally examined at beamline BL5B of the UVSOR synchrotron facility to evaluate the diffraction efficiencies and their variation across the ruled area. The measured diffraction efficiencies are 0.65% ± 0.07% at 5 nm (second order) and 7.9% ± 0.2% at 13.5 nm (first order). The variation in the diffraction efficiency across the ruled area is 2.2%, 13.6%, and 10.0% in zeroth, first, and second order diffractions, respectively. The discrepancies in diffraction efficiencies between the experiments and the calculations were 5.2%, 29%, and 35% for the zeroth, first, and second diffraction orders, respectively.

The water window emission lines (20-40 & Aring;) of highly charged bismuth (Bi) ions in laser produced plasmas are useful as light sources for biological microscopy of living cells. However, broad distributions of charge states and overlapping of transition arrays in the laser produced high density plasmas make precise line identification difficult. In this paper, we identified strong emission lines in the water window range from Bi ions decomposing to each charge state with a compact electron beam ion trap (CoBIT). To this end, we constructed a collisional radiative model accounting for detail atomic processes of the Bi ions interacting with mono-energetic electron beams. Each transition array observed in the experimental spectra was carefully identified with the present calculations. Distributions of the observed peaks show distinct features depending on charge state abundance in the CoBIT. We found that line emissions mediated via meta-stable excited states play an important role.

Spectroscopic studies of emissions released from tungsten ions combined with a pellet injection technique have been conducted in the Large Helical Device for contribution to the tungsten transport study in tungsten divertor fusion devices and for expansion of the experimental database of tungsten line emissions. Emission lines were explored for the observation of low to intermediate charge states in the range of W10 + to W20+, and the line spectra of W13 + were observed for the first time in fusion plasma experiments. The wavelengths of the observed W13 + lines were 243.1 & Aring;, 247.6 & Aring;, 248.3 & Aring;, and 249.1 & Aring; in the extreme ultraviolet wavelength range, and all of them were emission from the 4f(13)5s(2)- 4f(13)5s5p transitions.

We present spectral measurements performed to identify the highly-forbidden electric-octupole (E 3) 4 f7/2,5/2-5s1/2 transitions in Ag-like highly charged ions and to study the Z dependence of their intensities. Theoretical predictions indicate that these transitions can be observed in a narrow range of elements around tungsten (Z = 74), as the strong configuration interaction leads to level crossings and to an increased population of the 5s level. Here we probe the atomic structure and population dynamics in Ag-like ions using extreme-ultraviolet spectral measurements in several elements (Yb, Lu, W, Re, Os, Ir, Au) in the range Z = 70 to 79. The spectra were recorded independently in the compact electron-beam ion trap at the National Institute for Fusion Science and in the electron-beam ion trap at the National Institute of Standards and Technology. The measured wavelengths and the advanced theoretical calculations confirm observations of the E3 transitions in Ag-like ions in the predicted narrow element range.

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The atomic data of heavy elements, especially rare-earth metals, plays a crucial role in enhancing our understanding and interpreting kilonova spectra and underlying astrophysical processes. Among these elements, Erbium (Er) is particularly intriguing because it is important for opacities of the kilonova observed in 2017 (GW170817). In order to assess the atomic data, optical spectra of Er ions were precisely measured in $$385-400\,\hbox {nm}$$ at Large Helical Device (LHD). In the present experiment, Er was injected into the core plasma of LHD through carbon pellets containing Er powders. The electron density and temperature of the Er-contained C pellet ablation cloud were obtained to be $$1.6 \times 10^{22}\,\hbox {m}^{-3}$$ and 1.4 eV using the Stark broadening of a C II line and the Boltzmann plot of Er II lines, respectively. Transition probabilities of observed Er II lines were assessed using the Boltzmann plot analysis. Recent measurements with laser-induced breakdown spectroscopy (LIBS) of an Er II line at 393.86 nm were confirmed by the present work.

The observation of the kilonova AT2017gfo and investigations of its light curves and spectra confirmed that neutron star mergers are sites of r -process nucleosynthesis. However, the identification of elements responsible for the spectral features is still challenging, particularly at the near-infrared wavelengths. In this study, we systematically searched for all possible near-infrared transitions of heavy elements using experimentally calibrated energy levels. Our analysis reveals that most candidate elements with strong absorption lines are lanthanides ( Z = 57–71) and actinides ( Z = 89–103). This is due to their complex structures leading to many low-lying energy levels, which results in strong transitions in the near-infrared range. N. Domoto et al. (2022) have shown that La iii and Ce iii can explain the absorption features at λ ∼ 12000–15000 Å. While our results confirm that these two elements show strong infrared features, we additionally identify Gd iii as the next most promising species. Due to its unique atomic structure involving the half-filled 4 f and the outer 5 d orbitals, Gd iii has one of the lowest-lying energy levels, between which relatively strong transitions occur. We also find absorption lines caused by Gd iii in the near-infrared spectrum of a chemically peculiar star HR 465, which supports their emergence in kilonova spectra. By performing radiative transfer simulations, we confirm that Gd iii lines affect the feature at ∼12000 Å previously attributed to La iii . Future space-based time-series observations of kilonova spectra will allow the identification of Gd iii lines.

The nuclear charge radius is a fundamental observable that encodes key aspects of nuclear structure, deformation, and pairing. Isotonic (constant neutron number) systematics in the deformed rare-earth region have long suggested that odd-Z nuclei are more compact than their even-Z neighbors - except for Lu, whose recommended radius appeared anomalously large relative to Yb and Hf. We report a high-precision determination of the natural-abundance-averaged Lu-Yb charge-radius difference using extreme-ultraviolet spectroscopy of highly charged Na-like and Mg-like ions, supported by high-accuracy relativistic atomic-structure calculations - a recently introduced method with the unique ability to measure inter-element charge radius differences. Combined with muonic-atom and optical isotope-shift data, our result resolves the longstanding Lu inversion anomaly and reestablishes a pronounced odd-even staggering along the N=94 isotonic chain. The magnitude of this staggering is unexpectedly large, far exceeding that observed in semi-magic nuclei and in deformed isotopic sequences. State-of-the-art nuclear density functional theory calculations, including quantified uncertainties, fail to reproduce this enhancement, possibly indicating missing structural effects in current models. Our work demonstrates the power of highly charged ions for precise, element-crossing charge-radius measurements and provides stringent new constraints for future theoretical and experimental studies of nuclear-size systematics.

Simultaneous-crossing of three energy-levels has been found for the first time in the atomic number Z-dependent wavelength curves at Z = 66 of Ge-like highly charged atomic ions. Large Helical Device (LHD) and EBIT are used for obtaining the wavelength of EUV emissions from the transitions of 4s or 4p excited states into their ground state ([Ni]4s24p2-)0. Multi-Configuration Dirac-Fock calculation has been carried out for analysis. Interactions between the three excited-state configurations ([Ni]4s24p-4d-)1, ([Ni]4s4p-(4p2+)0)1, and ([Ni]4s4p-(4p2+)2)1 lead to the three-dimensional rotation of atomic state-vectors in the configuration space on the way for Z to pass the crossing point. Z-evolution of the atomic states has been analyzed employing the quaternion algebra. The spectral anomalies near the level-crossing are simulated by the atomic state-vector rotation. The optically invisible states are thus become visible by sharing the intensities of optically allowed configuration ([Ni]4s24p-4d-)1.