## 4 Results and Discussion  
The identification of four weak lines was based on results from CRM simulations combined with previous measurements [9; 16]. As shown in Figure 1, the experimental and theoretical spectra at 5.5 keV match well. Among the four weaker spectral lines, Na1 and Mg1 represent two-electron-one-photon transitions (TEOPT). TEOPT lines typically exhibit low intensity, attributed to their minimal transition probabilities induced by configuration interaction. Nevertheless, when a level in the TEOPT undergoes strong mixing with a correlated level that possesses the property for a strong one-electron electric-dipole transition channel, the transition probability of the TEOPT can be significantly enhanced, thus facilitating its observation. In the present case, the upper level of Mg1 (2p\({}^{5}\)\({}_{3/2}\)3s\({}_{1/2}\)3p\({}^{2}\)\({}_{J=1}\) is mixed with the upper level of 3D\({}_{\text{Mg}}\) (2p\({}^{5}\)\({}_{3/2}\)3s\({}^{2}\)3d\({}_{5/2}\))\({}_{J=1}\), resulting in the line-intensity of Mg1 reaching 30% of the latter. The upper level of the Na1 (2p\({}^{5}\)\({}_{3/2}\)3p\({}^{2}\)\({}_{J=1/2}\) is mixed with (2p\({}^{5}\)\({}_{3/2}\)3s\({}_{1/2}\)3d\({}_{5/2}\))\({}_{J=1/2}\), whose transition to the ground state has a line-intensity 1.45 times that of Na1 in the simulation. However, it cannot be observed due to blending with 3D\({}_{\text{Na}}\) in the spectrum. The Na1 line is very close to the 3D line used for ion temperature diagnostic in tokamaks (within 3.7 mA), making it indistinguishable in the spectrometer with insufficient resolution. This is disadvantageous for ion temperature diagnostics [24], especially at low electron temperatures. In our measurement, the Na1 line is only prominent at an electron-beam energy of 5.5 keV. The transition Na2 \(\{(2\)p\({}^{6}\)3p\({}_{1/2}\))\({}_{J=1/2}\) - \((2\)p\({}^{5}\)\({}_{3/2}\)3p\({}_{1/2}\)3d\({}_{5/2}\))\({}_{J=1/2}\)}, observed for the first time, is another satellite of 3D but with 3p as the spectator electron, unlike 3D\({}_{\text{Na}}\) with 3s as the spectator electron. Its emission intensity is only 10.8% of the latter due to the absence of collisional pumping from its lower level and ground level. Thus, Na2 is almost blended with the background in the spectrum but can still be identified through careful comparison with the theoretical spectrum. In Maxwellian plasma, it may be enhanced due to resonant excitation through dielectronic capture, potentially serving as a means for electron temperature diagnostics. That is similar to well-known dielectronic satellites \(k\) (1s\({}^{2}\)2p-1s2p\({}^{2}\)) in Li-like ions with 2p as the spectator electron [83; 84]. Voigt profile is employed in the fitting of 3D and 3F lines at various electron-beam energies, which is crucial for line-intensity measurement according to Ref. [46]. In EBIT spectroscopy, Doppler broadening due to ion temperature and instrumental broadening typically exhibit Gaussian profiles. Additionally, excited levels have the energy distributions characterized by Lorentzian profile, with a natural width \(\Gamma\) related to the level lifetime \(\tau\) according to the [PERSON]'s uncertainty principle [85]. Decoupling the broadening of the Voigt profile shape arising from the convolution of Lorentzian and Gaussian profile is a method for measuring level lifetimes [86]. According to calculations by [PERSON] et al. [65], the lifetimes of the upper levels of 3D and 3F are 2.10 fs and 5.44 fs, respectively, corresponding to natural widths of 0.187 mA and 0.073 mA in terms of wavelength. In the fitting of 3D and 3F, these values are considered as fixed Lorentzian broadening within the Voigt function. Figure 2 illustrates the Voigt fitting curve for the 3D line at the spectrum under the electron-beam energy of 7.2 keV compared with the Gaussian one. It can be observed that the Voigt fitting curve closely follows the Lorentzian wings on both sides of the spectral peak, which is more consistent with the trend of the experimental data.