The x-ray transitions in Cu- and Ni-like tungsten ions in the 5.19–5.26 Å wavelength range that are relevant as a high-temperature tokamak diagnostic, in particular for JET in the ITER-like wall configuration, have been studied. Tungsten spectra were measured at the upgraded Shanghai- Electron Beam Ion Trap operated with electron-beam energies from 3.16 to 4.55 keV. High-resolution measurements were performed by means of a flat Si 111 crystal spectrometer equipped by a CCD camera. The experimental wavelengths were determined with an accuracy of 0.3–0.4 mÅ. The wavelength of the ground-state transition in Cu-like tungsten from the $3p^{5}3d^{10}4s4d[{(3/2,{(1/2,5/2)}_2]}_{1/2}$ level was measured. All measured wavelengths were compared with those measured from JET ITER-like wall plasmas and with other experiments and various theoretical predictions including cowan, relac, multiconfigurational Dirac-Fock (MCDF), and fac calculations. To obtain a higher accuracy from theoretical predictions, the MCDF calculations were extended by taking into account correlation effects (configuration-interaction approach). It was found that such an extension brings the calculations closer to the experimental values in comparison with other calculations.

We demonstrate that the $4f^{14}6s6p\left({}^{3}P_2\right)-4f^{13}5d6s^2\left({}^{3}P_2^{*}\right)$ transition in neutral ytterbium (Yb) can serve as an additional clock transition with the highest fine-structure constant (${\alpha }_e$) varying sensitivity coefficient $[q=-46165(3000\left)\right]$. We demonstrate a scheme to attain simultaneous magic trapping conditions for this clock transition with other two proposed clock transitions $4f^{14}6s^2\left({}^{1}S_0\right)-{}^{3}P_2$ and ${}^{1}S_0-{}^{3}P_2^{*}$, which also possess very large $q$ values. These conditions can be realized by subjecting Yb atoms to a bias magnetic field with tuning the particular polarization angles of the trapping laser. Upon realization, it can serve as the most potential optical lattice clock to probe ${\alpha }_e$ variation.

The magnetic field is extremely important for understanding the properties of the solar corona. However, there are still difficulties in the direct measurement of the coronal magnetic field. The magnetic-field-induced transition (MIT) in Fe x, appearing in coronal spectra, was discovered to have prospective applications in coronal magnetic field measurements. In this work, we obtained the extreme ultraviolet spectra of Fe x in the wavelength range of 174–267 Å in the Shanghai High-temperature Superconducting Electron Beam Ion Trap, and examined the effect of MIT in Fe x by measuring the line ratios between 257.262 Å and the reference line of 226.31 Å (257/226) at different magnetic field strengths for the first time. The electron density that may affect the 257/226 value was also obtained experimentally and verified by comparing the density-sensitive line ratio (175.266 Å/174.534 Å) measurements with the theoretical predictions, and there was good agreement between them. The energy separation between the two levels of 3s²3p⁴3d ⁴ D _5/2 and 3s²3p⁴3d ⁴ D _7/2, one of the most critical parameters for determining the MIT rate, was obtained by analyzing the simulated line ratios of 257/226 with the experimental values at the given electron densities and magnetic fields. Possible reasons that may have led to the difference between the obtained energy splitting and the recommended value in previous works are discussed. Magnetic field response curves for the 257/226 value were calculated and compared to the experimental results, which is necessary for future MIT diagnostics.

We report theoretical results for magnetic-field-induced $2{p}^{5}3s{\phantom{\rule{0.16em}{0ex}}}^{3}{P}_{0,2}$-${}2{p}^{6}\phantom{\rule{0.16em}{0ex}}{}^{1}{S}_{0}$ $E1$ transitions in Ne-like ions with zero nuclear spin ($I=0$) between Mg iii and Zn xxi as well as in Ne i. We demonstrate that it is important to include both ``perturber'' states $2{p}^{5}3s{\phantom{\rule{0.16em}{0ex}}}^{1}{P}_{1}$ and $2{p}^{5}3s{\phantom{\rule{0.16em}{0ex}}}^{3}{P}_{1}$ in order to produce reliable transition rates. Furthermore, we investigate the trends of the rates along the isoelectronic sequence of the $2{p}^{5}3s{\phantom{\rule{0.16em}{0ex}}}^{3}{P}_{0,2}$-${}2{p}^{6}\phantom{\rule{0.16em}{0ex}}{}^{1}{S}_{0}$ transitions and their competition with the $2{p}^{5}3s{\phantom{\rule{0.16em}{0ex}}}^{3}{P}_{0}$-${}2{p}^{5}3s{\phantom{\rule{0.16em}{0ex}}}^{3}{P}_{1}$ $M1$ and the $2{p}^{5}3s{\phantom{\rule{0.16em}{0ex}}}^{3}{P}_{2}$-${}2{p}^{6}\phantom{\rule{0.16em}{0ex}}{}^{1}{S}_{0}$ $M2$ decays. For the $2{p}^{5}3s{\phantom{\rule{0.16em}{0ex}}}^{3}{P}_{0}$ state the magnetic-field-induced transition becomes the dominant decay channel for the light elements even in a relatively weak magnetic field, and it will therefore prove useful in diagnostics of the strength of magnetic fields in different plasmas. The influence of an external magnetic field on the lifetime of the $2{p}^{5}3s{\phantom{\rule{0.16em}{0ex}}}^{3}{P}_{2}$ state is much smaller but still observable for the ions near the neutral end of the sequence. As a special case, the magnetic field effect on the lifetimes of $2{p}^{5}3s{\phantom{\rule{0.16em}{0ex}}}^{3}{P}_{0,2}$ states of neutral ${}^{20}$Ne is discussed. It is found that the lifetimes are drastically reduced by a magnetic field, which may be an underlying reason for the discrepancies in the lifetime of the $2{p}^{5}3s{\phantom{\rule{0.16em}{0ex}}}^{3}{P}_{2}$ state between experiment [14.73(14) s] and theory (17.63 s).

After a vacuum failure in a tokamak, plasma runaway or plasma disruptions frequently occur during plasma recovery, causing difficulties in rebuilding a well-confined collisional plasma. In this work, the impurity behavior during plasma recovery after a vacuum failure in the 2019 spring campaign of the Experimental Advanced Superconducting Tokamak (EAST) was studied by analyzing the spectra recorded by fast-time-response extreme ultraviolet (EUV) spectrometers with 5 ms/frame. During the plasma current ramp-up in recovery discharges, a high content of the low-Z impurities of oxygen and carbon was found, i.e., dozens of times higher than that of normal discharges, which may have caused the subsequent runaway discharges. The electron temperature in the recovery discharge may have dropped to less than 75 eV when the collisional plasma quenched to the runaway status, based on the observable impurity ions in the two cases. Therefore, the lifetime of collisional plasma in the recovery discharge, τ_c, was deduced from the lifetime of H- and He-like oxygen and carbon ions identified from EUV spectra. It was found that, after several discharges with real-time lithium granule injection, the runaway electron flux and O⁺ influx reduced to 45% and 20%, respectively. Meanwhile, the lifetime of confined plasma was extended from 113 ms to 588 ms, indicating the effective suppression of impurities and runaway electrons and improvement in plasma performance by real-time lithium granule injection. The results in this work provide valuable references for the achievement of first plasma in future superconducting fusion devices such as ITER and CFETR.