On the interpretation of the intense emission of tungsten ions at about 5 nm

Abstract

The origin of the intense emission band at about 5 nm, dominating the emission spectra of tungsten ions in the ASDEX Upgrade tokamak and EBIT, is discussed. It is shown that the emission spectra of various ions calculated taking into account only the excitations from the ground level agree fairly well with the results obtained in the collisional-radiative model; thus, the contribution of the excitations from the other levels is small. Though the excitation spectrum for all sequence of ions W 29+ -W 37+ corresponds to the same transitions 4p 6 4d N → 4p 5 4d N+1 + 4p 6 4d N−1 4f, its energetic width essentially changes going on from the charge of ion q = 34 to q = 35. It is caused by the appearance of the excitations 4p 1/2 -4d 3/2 to the open 4d N 3/2 subshell, which are not quenched by configuration mixing. The satellite line at about 4.5 nm is explained by the transitions of the same type, although between configurations with one spectator 5s electron. The existence of one more group of intense lines in the region of 2 nm, corresponding to 5s-4p transitions, is predicted.

Intensity enhancement in the emission spectra of Sb, Sn, and W ions due to the mixing of configurations with symmetric exchange of symmetry

“…From the comparison of calculated transition probabilities for Sn 12+ with the semiempirical ones, which take into account the interaction of many excited configurations, it follows that the extension of configuration set only weakly influences the width and shape of the intense group of lines. It agrees with the conclusion made in [14] from the comparison of the emission spectra for tungsten ions. In this work the emission spectra obtained after dipole excitation from the ground level were calculated for tungsten ions using the following sets of configurations for the initial and final states:…”

“…In the two-configuration approximation the emission spectra of tungsten ions W 29+ -W 34+ , excited from the ground level, were obtained to be very similar to the photoexcitation spectra from this level [14] and both types of spectra were concentrated in a very narrow interval of wavelengths. Consequently, the transitions from or to the ground level are more influenced by SEOS correlations than the other transitions.…”

“…However, for the ions W 35+ -W 37+ with the number of 4d electrons N ≤ 3 the intense lines are dispersed in a considerably wider interval. As explained in [14], the transitions to the open 4d N 3/2 subshell account for it. However, the splitting of shells into subshells does not manifest itself at medium ionization degrees.…”

“…Such emission of the separate tungsten ions was investigated experimentally in an EBIT (Electron Beam Ion Trap) facility and interpreted theoretically using the collisional-radiative model [12,13]. It was shown in [14] that in the case of a low density plasma obtained in EBIT and the contemporary tokamaks all the main features of these spectra can be reproduced using the assumption that the excitations originate only from the ground level. It enabled one to explain some main regularities of the quasicontinuum band and its satellite.…”

“…The aim of this work is to continue the theoretical investigation of these important spectra by considering the influence of some relativistic and configuration mixing effects not taken into account in [14], and to compare the main features of spectra for highly and moderately charged ions. Calculations have been performed in a relativistic CI approximation using DiracFock wave functions [15] and in quasirelativistic approximation [16] as well.…”