Charge-state Evolution from W5+ to W7+ at Energies below the Ionization Potentials

Experiments on an electron-beam ion trap (EBIT) and calculations using flexible atomic code (FAC) are carried out to study the charge-state evolution from W5+ to W7+. The W7+ line at 574.47 nm is observed with an electron-beam energy of about 48 eV, which is far below the ionization potentials of W5+ (65 eV) and W6+ (122 eV). Multicharge-state collisional-radiative (CR) calculations for W5+, W6+, and W7+ are performed with level-to-level processes with configuration interaction (CI), including direct ionization, collision excitation, radiative recombination, charge exchange, radiative transition, and autoionization. The CI strongly influences the calculated ionization cross sections for metastable levels. The CR-simulated spectra agree well with the experiments, and the calculated effective ionization cross section for W6+ has the same trend as the available experimental data [M. Stenke et al., J. Phys. B: At. Mol. Opt. Phys. 28, 2711 (1995)]. The metastable levels (similar to 40 eV for W5+; similar to 40 and similar to 85 eV for W6+) significantly contribute to the ionization through excitation-autoionization at rather low energies (<50 eV) in the EBIT plasma. These metastable levels could have a considerable influence on the charge-state evolution of tungsten ions in edge fusion plasma.