Tungsten Ions Research Articles

Visible spectroscopy of highly charged tungsten ions with the JET charge exchange diagnostic

Since the installation of the ITER-like wall on the JET tokamak, visible emission from a broad range of tungsten charge states has complicated plasma ion temperature and toroidal rotation measurements. A plethora of charge exchange transitions, from ions up to W56+, and 21 suspected magnetic dipole transitions have been observed. In particular W39+ and W46+ charge exchange emission contaminates the long-established carbon measurement at 529 nm. Fortunately the wavelength and relative intensity of the tungsten charge exchange lines is predictable and their influence can be mitigated when they are included in the spectral fit. Neon based charge exchange measurements at 525 nm are now preferred on JET since the ITER-like wall, however in this case an unidentified tungsten magnetic dipole transition contaminates the spectrum when the electron temperature is low.

Heavy impurity transport in tokamaks subject to plasma rotation, NTV and the influence of saturated ideal MHD perturbations

Strongly peaked tungsten accumulation is a common feature of high performance plasma scenarios in JET with the ITER-like wall, particularly during MHD activity induced by continuous modes. This study investigates the effect of long living internal kink modes on heavy impurity transport in the presence of strong flows and NTV ambipolar electric field. A novel formulation which includes these effects is presented and applied in the VENUS-LEVIS code in order to follow tungsten ions in a saturated JET-like internal kinked toroidally rotating plasma configuration. The synergy between 3D magnetic fields, strong flows and NTV is seen to cause tungsten accumulation in contrast to what is observed in similar axisymmetric configurations. Rapid inward transport of impurities in JET plasmas following the triggering of continuous modes is explained by the work presented here, and we use the same theory to postulate why outward transport can occur in kinked ASDEX-U plasmas.

Theoretical level energies and transition data for 4p64d8, 4p54d9 and 4p64d74f configurations of W[formula omitted] ion

The ab initio quasirelativistic approach developed specifically for the calculation of spectral parameters of highly charged ions was used to derive transition data for the Ru-like tungsten ion W30+. The configuration interaction method was applied to include electron correlation effects. The relativistic effects were taken into account in the Breit–Pauli approximation. The level energies, radiative lifetimes τ, Landé g-factors were determined for the ground configuration 4p64d8 and two excited configurations 4p54d9 and 4p64d74f. The radiative transition wavelengths λ and emission transition probabilities A for the electric dipole, electric quadrupole, electric octupole, magnetic dipole, and magnetic quadrupole transitions among the levels of these configurations were generated.

Turbulent transport of the W ions in tokamak plasmas: properties derived from a test particle approach

A detailed analysis of a special type of turbulent pinch mechanism for tungsten (W) ions is presented. It is based on the numerical simulation of ion trajectories in a complex test-particle model that contains realistic representations of the turbulence and of the confining magnetic field, plasma rotation, ion collisions, the polarization drift and the parallel stochastic acceleration. The origin of the pinch is a special type of order generated by an average poloidal velocity V p superposed on the E × B stochastic drift. It consists of a pair of radial symmetric drifts, the hidden drifts (HDs), that exactly compensate. The polarization drift and the parallel acceleration drive radial pinch velocities V x by destroying the perfect anti-symmetry of the HDs. These phenomena that were found in simplified models are theoretically validated here in a realistic model of W ion transport. The main conclusion of the paper is that this turbulent mechanism can be experimentally relevant for a large domain of parameters, which cover both existing plasmas and ITER conditions, and it can be responsible for an important amount of the W ion accumulation.

Charge exchange recombination spectroscopy of W (q = 61–66) for application to ITER neutral hydrogen beam diagnostics

In this paper, we present a detailed theoretical analysis of charge exchange recombination spectroscopy based on interactions of the planned ITER neutral beams (diagnostic beam of 100 keV u−1 and heating beam of ≈1 MeV u−1) with highly-charged ions of tungsten. The results of the present spectral synthesis are based on the new set of nl-resolved charge exchange (CX) cross sections for recombination of the W ions (q = 61–66) with atomic hydrogen calculated using the classical trajectory Monte Carlo method. A large-scale collisional-radiative model describing the population kinetics of the high-n atomic states of Si-like through O-like W ions has been developed using the NOMAD code for typical conditions of the ITER core plasma, and the resulting spectra have been generated for wavelengths in the x-ray to visible range (0.1–1000 nm). A detailed analysis of the plasma emission predicts a significant effect of CX recombination on the W line intensity ratios that can be used for more advanced diagnostics of the ITER plasma.

Simultaneous Observation of Tungsten Spectra of W0 to W46+ Ions in Visible, VUV and EUV Wavelength Ranges in the Large Helical Device

Spectroscopic studies for emissions released from tungsten ions have been conducted in the Large Helical Device (LHD) for contribution to the tungsten transport study in tungsten divertor fusion devices and for expansion of the experimental database of tungsten line emissions. Tungsten ions are distributed in the LHD plasma by injecting a pellet consisting of a small piece of tungsten metal wire enclosed by a carbon tube. Line emissions from W0, W5+, W6+, W24+–W28+, W37+, W38+, and W41+–W46+ are observed simultaneously in the visible (3200–3550 Å), vacuum ultraviolet (250–1050 Å), and extreme ultraviolet (5–300 Å) wavelength ranges and the wavelengths are summarized. Temporal evolutions of line emissions from these charge states are compared for comprehensive understanding of tungsten impurity behavior in a single discharge. The charge distribution of tungsten ions strongly depends on the electron temperature. Measurements of emissions from W10+ to W20+ are still insufficient, which is addressed as a future task.

Emission Lines in 290–360 nm of Highly Charged Tungsten Ions W20+–W29+

Forbidden transitions in the near-UV and visible wavelength of highly charged tungsten (W) ions are potentially useful as novel tungsten diagnostics means of fusion plasmas. Emission lines in 290–360 nm from Wq+ ions interacting with an electron beam of 540–1370 eV are measured, using a compact electron-beam-ion-trap. The charge states of 64 lines are identified as W20+–W29+. A magnetic-dipole (M1) line of W29+ between the excited states (4d84f)[(4d5/2−2)44f7/2]13/2→[(4d5/2−2)44f5/2]13/2 is newly identified; the wavelength is determined as 351.03(10) nm in air. The theoretical wavelength calculated using the multiconfiguration Dirac–Hartree–Fock method is in a good agreement with the measurement.

Determination of coordination form of tungsten ion in molten alkali chloride by electrochemical method

The results of the coordination form of low-concentration ions in molten salt have great reference significance for exploring the electrolysis reaction mechanism. Given the low concentration of tungsten ions after the electrochemical dissolution of tungsten carbide, this can be difficult to judge the coordination form of tungsten ions through spectral characterization. This paper will struggle to tackle the problem by comparing the electrochemical behaviors of tungsten ions from the electrochemical dissolution of tungsten carbide and NaCl–KCl–2 wt% Na2WO4 melt. The electrochemical method had been investigated as a mean to understand the fundamental thermodynamic and mass transport properties of tungsten ions. The apparent standard potentials of oxidation and reduction couple at several temperatures were calculated by using a proposed new method. The diffusion coefficients of tungsten ions were determined. It can be concluded that the tungsten ions in the molten salt after tungsten carbide electrolysis were simple ions without coordinating anions by comparing the dynamics and thermodynamic calculation results of the two systems. The developed simulation boxes can serve as a verification for further theoretical investigations of the coordination form of tungsten ion groups at molecular scale. The results of this work provide necessary knowledge for the development methods to determine the dynamics and thermodynamic properties and coordination forms of metal ions.

Preconcentration and ultrasensitive spectrophotometric estimation of tungsten in soils using polyurethane foam in the presence of rhodamine B: Kinetic and thermodynamic studies, and designing a simple automated preconcentration system

This work describes designing and using a simple automated preconcentration system for estimation of tungsten in soil spectrophotometrically. The preconcentration of tungsten is based upon the retention of ion associate of [RB+.HWO4-] inside a minicolumn packed with polyurethane foam. The retained ion associate is then automatically and quantitatively eluted by acetone. The tungsten concentration was estimated by measuring the absorbance of elution at 584 nm. The absorbance of elution follows Beer's law in concentration range of 0.5–100 µgL−1 with a detection limit of 0.09 µgL−1. The enhancement factor and relative standard deviation of proposed methodology were 50 and 1.7%, respectively. The system developed in this research is characterized by its low cost and high efficiency, as it uses low-cost PUF as a stationary phase and a few number of pumps and valves. On the other hand, the designed system can be used, in the future, with many detection techniques easily. The composition of the ion association formed between W(VI) ions and RB was calculated by continuous variation method (Job plot) to be 1:1 (W6+:RB). On the other hand, the ion associate in its solid state was synthesized and characterized using elemental analysis and ATR–FTIR technique. The retention of tungsten (VI) ions in the form of [RB+.HWO4-] ion association on the surface of PUFs was confirmed by SEM images and XRF spectra. Moreover, the kinetic and thermodynamic behaviors of the retention of [RB+.HWO4-] on the polyurethane foams were studied in detail. The kinetic models revealed that the retention of [RB+.HWO4-] on PUF surface obeys the pseudo-second-order reaction. The thermodynamic investigations confirmed that the adsorption of [RB+.HWO4-] on PUF surface was spontaneous at 298 K and the interaction between [RB+.HWO4-] and active sites available on PUF surface was physical and electrostatic in nature.

Photocatalytic degradation of indigo carmine by using tungsten doped bismuth vanadate

The photocatalytic degradation of indigo carmine dye was studied using tungsten doped BiVO4. Bismuth vanadate powder has been synthesized by sol-gel technique. The incorporation of W ions into BiVO4 was confirmed by EDX. The photocatalytic activity was studied using different samples of BiVO4 by variation of pH, concentration of dye, dose of semiconductor and light intensity. It has been found that degradation of dye is more effective in presence of W doped BiVO4, It shows the role of tungsten ions for enhancing degradation of indigo carmine by reducing the recombination of photogenerated electrons and holes.

An extreme ultraviolet spectrometer working at 10–130 Å for tungsten spectra observation with high spectral resolution and fast-time response in Experimental Advanced Superconducting Tokamak

A newly developed flat-field extreme ultraviolet (EUV) spectrometer working at 10–130 Å wavelength range with high spectral resolution and fast-time response has been developed to observe line emissions from highly ionized tungsten ions in the Experimental Advanced Superconducting Tokamak (EAST). This flat-field EUV spectrometer is equipped with a laminar-type varied-line-spacing (VLS) concave holographic grating (2400 grooves/mm at grating center) fixed at an angle of incidence of 88.6°, and a back-illuminated charge-coupled device (CCD) with pixel numbers of 1024 × 255 (26 × 26 μm2/pixel). The CCD is routinely operated at 5 ms per frame with full vertical binning (FVB) mode. An excellently high spectral resolution of Δλ0=4–6 pixels (full width at foot position of spectral line profile), which corresponds to full width of half maximum (FWHM) of 0.1 Å at 20 Å, has been obtained by carefully adjusting angles and positions of the grating and CCD. Thus, the line identification of complicated tungsten spectra became possible, when a precise wavelength calibration was done. Typical tungsten spectra are recorded at low (Te=1.0 keV) and high (Te=2.6 keV) electron temperatures. Tungsten unresolved transition arrays (W-UTA) existing at wavelength ranges of 15–35 and 45–65 Å were preliminarily identified as line emissions from W24+–W33+ and W27+–W45+ ions, respectively, by referring to previous experimental results and National Institute of Standards and Technology (NIST) database. Developing a set of analytical computer programs, line spectra observed with the EUV spectrometer have been automatically identified after data acquisition basically for all discharges during daily EAST experiment.

Oxo versus Sulfido Coordination at Tungsten: A Spectroscopic and Correlated Ab Initio Electronic Structure Study.

The tungsten ion that resides at the active site of a unique class of enzymes only found in esoteric hyperthermophilic archaea bacteria is known to possess at least one terminal chalcogenide ligand. The identity of this as either an oxo or sulfido (or both) is difficult to ascertain from structural studies; therefore, small-molecule analogues are developed to calibrate and substantiate spectroscopic signatures obtained from native proteins. The electronic structures of Tp*WECl2 (E = O, S; Tp* = hydrotris(3,5-dimethylpyrazol-1-yl)borate) have been scrutinized using electronic, electron paramagnetic resonance (EPR), and X-ray absorption spectroscopy to assess the impact of terminal chalcogen on the adjacent cis chloride ligands. Examination at the Cl K-edge provides a direct probe of the bonding and therein lability of these chloride ligands, and in conjunction with density functional theoretical and multireference calculations reveals greater bond covalency in Tp*WOCl2 compared to Tp*WSCl2. The computational model and electronic structure assignment are corroborated by the reproduction of spin-Hamiltonian parameters, whose magnitude is dominated by the sizeable spin-orbit coupling of tungsten.

Hierarchical W18O49/NiWO4/NF heterojunction with tuned composition and charge transfer for efficient water splitting

Designing and synthesizing heterojunction electrocatalyst with abundant active sites is critical to optimize the activity of electrochemical water splitting. Herein, a typical hierarchical W18O49/NiWO4 heterojunction grown on nickel foam was successfully synthesized via a multistep synthesis strategy. The electrochemical oxidation and reduction process triggers the amorphization of W18O49 and significantly enhances the interfacial interaction, thereby endowing the composites with enriched active sites. The W18O49/NiWO4/NF is completely reconstructed to NiWO4/NF and NiWO4/NiOOH/NF with amorphous-crystalline interface (denoted as H-AM/NiWO4/NF for HER and O-AM/NiWO4/NiOOH/NF for OER). The valence of tungsten ions is well modulated to achieve a high proportion of tetravalent W (IV) active sites accompanied by abundant oxygen vacancies. The increased Ni (III) ions as OER active sites have strong orbital overlap with reaction intermediate, which stabilize the low-valent W (IV) by producing more NiOOH species during the oxygen evolution reaction. Benefiting from the amorphous–crystalline composites, unsaturated W (IV) sites, abundant oxygen vacancies, and highly active NiOOH species, the activated electrocatalyst exhibits excellent electrocatalytic activity, achieving a small overpotential of 1.53 V at 10 mA·cm−2 for water-splitting and maintaining long-term stability at least 100 h.

Interactions in Electrodeposited Poly-3,4-Ethylenedioxythiophene-Tungsten Oxide Composite Films Studied with Spectroelectrochemistry.

Cyclic voltammograms and optical absorption spectra of PEDOT/WO3 composite films were recorded in order to identify possible interactions and modes of improved performance of the composite as compared to the single materials. Changes in the shape of redox peaks related to the W(VI)/W(V) couple in the CVs of WO3 and the composite PEDOT/WO3 films indicate electrostatic interactions between the negatively charged tungsten oxide species and the positively charged conducting polymer. Smaller peak separation suggests a more reversible redox process due to the presence of the conducting polymer matrix, accelerating electron transfer between tungsten ions. Electronic absorption spectra of the materials were analyzed with respect to changes of the shapes of the spectra and characteristic band positions. There are no noticeable changes in the position of the electronic absorption bands of the main chromophores in the electronic spectra of the composite film. Obviously, the interactions accelerating the redox performance do not show up in the optical spectra. This suggests that the existing electrostatic interactions in the composite do not significantly change the opto-electronic properties of components of the composite but resulted in the redistribution of fractions of polaron and bipolaron forms in the polymer.

Electrochemical preparation of W coatings in Na2WO4-WO3-ZrO2 molten salt

In this paper, the flat tungsten coatings with uniform grains were prepared via electrolytic tungsten at 1173 K in ternary molten salt system formed by tungstate, tungsten oxide and different mass fractions of ZrO2. Dense tungsten coatings containing the second phase of ZrO2 were obtained by constant direct current electrodeposition in molten salt, and the coatings have preferential growth orientation along the (2 1 1) crystal plane. These coatings were characterized via X–ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive spectrometry (EDS) and laser scanning confocal microscope (LSCM). The results show that W-ZrO2 coating obtained at Na2WO4-WO3-ZrO2. In addition, the existence of zirconia in molten salt reduced the coating roughness from 13.567 μm to 3.017 μm, the current efficiency was lost from 84.15% to 70.76%, and the grains became fine and uniform. What’s more, the mechanical properties and thermal cycle performance of the coating have been effectively improved. To further illustrate the electrochemical process, the electrochemical behavior of tungsten ions and zirconium ions during the electrodeposition was investigated by cyclic voltammetry (CV) and square wave voltammetry (SWV). The results indicated that the reduction reactions of tungsten ions and zirconium ions are divided into two steps, and the reaction processes are as follows: W(VI) → W(IV) → W, Zr(IV) → Zr(II) → Zr.

Design of EAST lower divertor by considering target erosion and tungsten ion transport during the external impurity seeding

To demonstrate the performance of tungsten (W) as the divertor target material and to solve the power handling problem during high power long-pulse discharge, the upgrade of EAST lower divertor is planned. In this work, the physical design of the W divertor is presented by using 2D edge plasma code SOLPS and Monte Carlo impurity transport code DIVIMP. The optimized divertor geometry is proposed after systematic examination of target shapes, target slant angles and the pump opening locations. The performance of the designed divertor is further assessed by impurity seeding. By comparing the medium and high power discharges with argon (Ar) seeding, the differences on the divertor power radiation and impurity core accumulation are distinguished. The simulated effective ion charge Z eff fits well the scaling law, which is based on multi-machine database. Ar seeding and neon (Ne) seeding scans are carried out separately. The simulation results indicate Ar has higher power radiation efficiency than that of Ne, thus promoting the achievement of plasma detachment. However, the core compatibility with Ar is worse than with Ne. The W target erosion and W impurity transport during impurity seeding are simulated by the DIVIMP–SOLPS coupled modeling. It illustrates that under the similar divertor plasma conditions, Ar seeding causes more serious W erosion and more severe core contamination by W impurity, than Ne seeding. Finally, the divertor in–out asymmetry is studied by considering electromagnetic drifts. The simulation results manifest that the designed open vertical inner target reduces in–out asymmetry due to that its weak power radiation capability is offset by the ion flow driven by the drifts. In addition, the designed divertor is compatible with the quasi snowflake magnetic configuration. These studies will improve the understanding of W target sputtering and W impurity transport control during the radiative divertor discharges for CFETR/DEMO.

Relativistic R-matrix calculations of the photoionization of W63+ ions

The direct and resonant single-photon photoionization of W63+ ions from their ground state 1s 22s 22p 63s 2 S 1/2 and four lowly-excited states 1s 22s 22p 63p 2 P 1/2,3/2 and 1s 22s 22p 63d 2 D 3/2,5/2 has been studied within the framework of the relativistic R-matrix method and the multiconfigurational Dirac–Fock method. Special attention has been paid to obtaining direct and resonant ionization limits and to identifying possible resonant symmetries of W63+ ions. To this end, the photoionization cross-sections have been calculated using the Dirac Atomic R-matrix Code package. Direct and resonant ionization limits were obtained for the ground-state photoionization of W63+ ions, and the resonant peaks identified were found to be associated with the resonances 1s 22s 22p 53lnl′ and 1s 22s2p 63lnl′ (l, l′ = s, p, d) for the fine-structured energy level or configuration at the level of fine structure or configuration. For excited-state photoionization, however, the resonant ionization limits and resonant peaks were hardly distinguishable due to much more densely spaced resonant peaks and a complicated resonant structure, although the respective direct ionization limits are obtained. We expect that this work will remedy the current lack of fundamental studies of the photoionization of W63+ ions and be helpful in the diagnosis and simulation of fusion plasmas in conjunction with the available photoionization data for tungsten ions in other charge states.

Benchmarking calculations with spectroscopic accuracy of level energies and wavelengths in W LVII–W LXII tungsten ions

Atomic properties of n=3 states of the W56+ - W61+ ions are systematically investigated through two state-of-the-art methods, namely, the second-order many-body perturbation theory, and the multi-configuration Dirac–Hartree–Fock method combined with the relativistic configuration interaction approach. The contributions of valence-valence and core-valence electron correlations, the Breit interaction, the higher-order retardation correction beyond the Breit interaction through the transverse photon interaction, and the quantum electrodynamical corrections to the excitation energies are studied in detail. The excitation energies and wavelengths obtained with the two methods agree with each other within ≈0.01%. The present results achieve spectroscopic accuracy and provide a benchmark test for various applications and other theoretical calculations of W56+ - W61+ ions. They will assist spectroscopists in their assignment and direct identification of observed lines in complex spectra.

Experiments and modelling of multiple sequential MeV ion irradiations and deuterium exposures in tungsten

Bulk tungsten samples were irradiated sequentially with 20 MeV tungsten ions and exposed to deuterium plasma. The experiments were performed in order to simulate the displacement damage that fusion neutrons will cause in a tungsten plasma-facing component of a future fusion device. To study the influence of the presence of hydrogen isotopes during the creation of displacement damage on the final defect density, tungsten irradiation and deuterium decoration cycles were performed up to three times. Deuterium depth profiling with 3He Nuclear Reaction Analysis and Thermal Desorption Spectroscopy showed that the deuterium concentration increased after each additional tungsten irradiation and deuterium exposure. After the third cycle, the deuterium concentration reached a maximum of 3.6 at.% at the given plasma exposure temperature of 370 K. We attribute this increase in retention to the stabilization of the displacement damage during the tungsten irradiation by the presence of deuterium. The experimental results were simulated using the MHIMS-R macroscopic rate-equation code, which was recently upgraded with a damage stabilization term to describe experiments where tungsten was irradiated with MeV tungsten ions and simultaneously exposed to low-energy deuterium ions. Using this novel model, it was possible to quantitatively describe also the present results for the sequential irradiation/exposure scheme, with model parameters that were congruent with parameters derived from the simultaneous experiment. Modelling shows that kinetic de-trapping of trapped deuterium takes place during irradiation However, it is not the dominant process that explains defect stabilization. In addition, the model facilitates the extrapolation of present experimental results to an even larger number of sequential tungsten irradiation and deuterium exposure cycles. The model predicts that after about five sequential irradiation and plasma exposure cycles, a stationary state is reached with an associated maximum trapped D concentration of 4.2 at.% for the given exposure temperature of 370 K.

Visible spectra of W8+ in an electron-beam ion trap

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.