Observation of indirect ionization of W7+ in an electron-beam ion-trap plasma

In this work, visible and extreme ultraviolet spectra of ${\mathrm{W}}^{7+}$ are measured using the high-temperature superconducting electron-beam ion trap (EBIT) at the Shanghai EBIT Laboratory under extremely low-energy conditions (lower than the nominal electron-beam energy of 130 eV). The relevant atomic structure is calculated using the flexible atomic code package based on the relativistic configuration interaction method. The grasp2k code, in the framework of the multiconfiguration Dirac-Hartree-Fock method, is employed as well for calculating the wavelength of the M1 transition in the ground configuration of ${\mathrm{W}}^{7+}$. A line from the ${\mathrm{W}}^{7+}$ ions is observed at a little higher electron-beam energy than the ionization potential for ${\mathrm{W}}^{4+}$, making this line appear to be from ${\mathrm{W}}^{5+}$. A hypothesis for the charge-state evolution of ${\mathrm{W}}^{7+}$ is proposed based on our experimental and theoretical results; that is, the occurrence of ${\mathrm{W}}^{7+}$ 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.