Observation of indirect ionization of W7+ in EBIT plasma

In this work, visible and EUV spectra of W7+ have been measured using the high-temperature superconducting electron-beam ion trap at the Shanghai EBIT laboratory under extremely low-energy conditions. The relevant atomic structure has been calculated by using the flexible atomic code package and the GRASP2K code. A hypothesis for the charge-state evolution of W7+ is proposed based on our results. The occurrence of W7+ ions results from indirect ionization caused by stepwise excitation between some metastable states of lower-charge-state W ions, at the nominal electron beam energy of 59 eV.