G. Merkelis

Energy levels and emission spectra of $W^{25+}$ ion have been studied by performing the large-scale relativistic configuration interaction calculations. Configuration interaction strength is used to determine the configurations exhibiting the largest influence on the $4f^{3}$, $4d^{9}4f^{4}$, $4f^{2}5s$, $4f^{2}5p$, $4f^{2}5d$, $4f^{2}5f$, $4f^{2}5g$, and $4f^{2}6g$ configuration energies. It is shown that correlation effects are crucial for the $4f^{2}5s \rightarrow 4f^{3}$ transition which in single-configuration approach occurs due to the weak electric octupole transitions. As well, the correlation effects affect the $4f^{2}5d \rightarrow 4f^{3}$ transitions by increasing transition probabilities by an order. Corona model has been used to estimate the contribution of various transitions to the emission in a low-density electron beam ion trap (EBIT) plasma. Modeling in 10--30 nm wavelength range produces lines which do not form emission bands and can be observed in EBIT plasma.

Spectra of the W$^{25+}$ ion are studied using the collisional-radiative model (CRM) with an ensuing cascade emission. It is determined that the cascade emission boosts intensities only of a few lines in the $10 - 3$ nm range. The cascade emission is responsible for the disappearance of structure of lines at about 6 nm in the electron beam ion trap plasma. Emission band at 4.5 to 5.3 nm is also affected by the cascade emission. The strongest lines in the CRM spectrum correspond to $4d^{9} 4f^{4} \rightarrow 4f^{3}$ transitions, while $4f^{2} 5d \rightarrow 4f^{3}$ transitions arise after the cascade emission is taken into account.

Energy levels, radiative transition wavelengths and probabilities have been studied for the W$^{26+}$ ion using multiconfiguration Dirac-Fock and Dirac-Fock-Slater methods. Corona and collisional-radiative models have been applied to determine lines and corresponding configurations in a low-density electron beam ion trap (EBIT) plasma. Correlation effects for the $4f^{2}$, $4d^{9}4f^{3}$, $4f5l$ ($l=0,...,4$), $4fng$ ($n=5, 6, 7$) configurations have been estimated by presenting configuration interaction strengths. It was determined that correlation effects are important for the $4f5s \rightarrow 4f^{2}$ transitions corresponding to weak electric octupole transitions in a single-configuration approach. Correlation effects influence the $4f5d \rightarrow 4f^{2}$ transitions by increasing transition probabilities by an order of magnitude. Identification of some lines observed in fusion plasma has been proposed. Spectra modeling shows strong increase of lines originating from the $4f5s \rightarrow 4f^{2}$ transitions. Other transitions from the $10-30$ nm region can be of interest for the EBIT plasma.