Visible spectra of ${\mathrm{W}}^{8+}$ 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 $4f^{14}5s^{2}5p^4, 4f^{13}5s^{2}5p^5$, and $4f^{12}5s^{2}5p^6$ 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 $4f^{13}5s^{2}5p^5$ and $4f^{12}5s^{2}5p^6$ configurations from our experiments.

Figure 1

Comparisons of the excitation energies of our grasp calculations (black squares), Kozioł and Rzadkiewicz's [50] calculations (red circles), and Berengut et al. [49] (blue triangles) with our fac results.

Figure 2

Spectra from lowly charged W ions obtained by SH-HtscEBIT at corrected electron-beam energy of 79.5, 104.5, 127.1, and 149.5 eV in the range of 420–660 nm. Accumulation time of each spectrum is 3 h. Six arrowed lines $A1$–$A6$ are assigned to come from the ${\mathrm{W}}^{8+}$ ion, while the line at 574.45(3) nm is the $M1$ transition between the fine structure levels in the $4f^{13}5s^{2}5p^6{}^{2}F$ ground term of ${\mathrm{W}}^{7+}$.

Figure 3

Comparison between the intensities of six lines $A1$–$A6$ from experiments and our CRM simulation. Note that the experimental intensities are normalized to $A5$, while the CRM intensities are normalized to $A1$.

Figure 4

Comparison between the experimental and calculated spectra of the ${\mathrm{W}}^{8+}$ ion in the range of 420–660 nm. The top panel is the synthetic spectrum based on the wavelength and intensity measured by SH-HtscEBIT at electron-beam energy 104.5 eV. The middle panel is the simulated spectrum with wavelength from our grasp calculations and intensity from our CRM simulation (electron density $5\times {10}^{10}\mathrm{c}{\mathrm{m}}^{-3}$ and electron-beam energy 104.5 eV). The bottom panel is the simulated spectrum with wavelength from our large-scale fac calculations and intensity from our CRM simulation. Note that each line is assumed to have the Gaussian profile with the full width at half maximum (FWHM) 0.14 nm, which corresponds to our experimental resolution.