The spectrum of seven times ionized tungsten (W VIII) relevant to tokamak divertor plasmas

Abstract

Tungsten spectra excited in the vacuum sparks of different setups and electric circuits were recorded on two high-resolution vacuum spectrographs. For the region 190-500 Å, a 10 m normal incidence spectrograph with a 3600 lines/mm grating was used in the Meudon Observatory. In the region below 350 Å, a set of spectra was produced using a 3 m grazing incidence spectrograph with a 3600 lines/mm grating in the Institute of Spectroscopy in Troitsk. A total of 187 lines in the region 160-271 Å were identified as transitions to the low-lying odd configurations 4f 13 5s 2 5p 6 and 4f 14 5s 2 5p 5 from the interacting excited even 4f 12 5s 2 5p 6 5d + 4f 13 5s 2 5p 5 (5d + 6s) + 4f 14 5s 2 5p 4 (5d + 6s) + 4f 14 5s5p 6 configurations. It was found that 4f 13 5s 2 5p 6 2 F 7/2 is the ground level of W VIII. The fine structure splitting and relative positions of the odd terms were established, and 98 levels of the excited even configurations were found.

Detailed Analysis of Configuration Interaction and Calculation of Radiative Transition Rates in Seven Times Ionized Tungsten (W VIII)

“…These comparisons are summarized in Figure 1 showing the ratios τ(B)/τ(A), τ(C)/τ(B) and τ(D)/τ(C), respectively. When looking at Figure 1a, it is clear that the numerous missing odd-parity configurations in the HFR(A) model, similar to the one used by Ryabtsev et al [13], make without doubt this latter model insufficient to provide a reliable set of transition probabilities, the mean deviation between the HFR(B) and HFR(A) lifetimes being found to be within about 30%, with notable discrepancies reaching a factor of 2-3 in some cases. Moreover, Figure 1b shows that the HFR(B) model does not either include enough configuration interaction to give a reasonable accuracy of the radiative lifetime calculations, the differences between the data computed with models HFR(C) and HFR(B) still reaching a factor of 1.5-2 in many cases.…”

“…However, using the weighted average energies of sub-configurations based on the 4f predicted by Kramida and Shirai [14]. The level identifications reported in [13] were supported by calculations performed using the pseudo-relativistic Hartree-Fock (HFR) method of Cowan [18] combined with a semi-empirical adjustment of the energy parameters. In the HFR model considered by these latter authors, also used for providing transition probabilities corresponding to the experimentally observed spectral lines, the 4f 5f even-parity configurations were included.…”

“…As mentioned in the previous section, the only available radiative rates in W VIII were reported by Ryabtsev et al [13] who used the HFR method with a rather unbalanced physical model since it included only 2 odd-parity configurations (4f 5f) in the even parity. In order to estimate the effects of configuration interaction on the radiative parameters of W VIII, different physical models based on the pseudo-relativistic Hartree-Fock method were considered in the present work.…”

“…In all of them, the electrostatic interaction Slater integrals, F k , G k and R k were scaled down by a factor 0.85, as suggested by Cowan [18] while the spin-orbit parameters were kept to their ab initio values. The first model used, HFR(A), was the same as the one considered in [13] for computing their transition probabilities. The second model, HFR(B), was built to rebalance configuration interaction within both parities by adding to the HFR(A) model the 4f (5d + 6s + 6d + 7s) odd-parity configurations, leading to 12 and 13 configurations in each parity, respectively.…”

“…The main goal of the present work is to extend all our previous studies related to tungsten ions to the seven times ionized species (W VIII) for which 187 spectral lines were very recently observed leading to the first experimental identification of energy levels in this ion [13]. As we did for the first W ions, we also used here the pseudo-relativistic Hartree-Fock method putting the emphasis on the sensitivity of the radiative rates to electronic correlation effects in this particularly complex atomic system characterized by interacting configurations of the type 4f nl (nl = 5d, 6s).…”