## 3 EUV Spectra Analysis in the Plasma Recovery
### Impurity Behavior Analysis  
As mentioned earlier, the observable wavelength range of the EUV_Short spectrometer was fixed to be 8-65 A in the experiment, and wavelength scans (i.e., 65-190, 190-320, and 320-495 A) were performed for the EUV_Long spectrometer in three discharges during plasma recovery. The EUV spectra covering 5-495 A observed at the I\({}_{\rm P}\) ramp-up phase and platform phase in recovery discharges were then compared with those observed in the normal discharges to analyze the differences in impurity behavior. The parameters of the discharges, including plasma current (I\({}_{\rm P}\)), average electron density (n\({}_{\rm e}\)), and the wavelength interval of the spectrometers, are listed in Table 1.  
Figure 4: Image frames recorded by high-speed CCD camera at (**a**,**c**) t = 0.08 s, and (**b**,**d**) t = 5.0 s in normal discharge (#84649) and recovery discharge (#84601), respectively. Figure 5 shows the typical EUV spectra during the I\({}_{\rm p}\) ramp-up phase (t = 0.08 s, before plasma quenching) in both recovery discharges (#84599, #84600, and #84601) and normal discharges after lithium coating (#84649, #84678, and #85126), covering a wavelength range of 8-495 A. Figure 5a-c show a comparison of spectra in the 8-320 A range. Several emission lines from H-like and He-like low-Z impurity of carbon (C V-C VI, C\({}^{4+}\)-C\({}^{5+}\)), nitrogen (N VI-N VII, N\({}^{5+}\)-N\({}^{6+}\)), and oxygen (O VII, O\({}^{6+}\)) with strong intensity appeared in the short wavelength range of 8-65 A (see the red line in Figure 5a). The intensities of C V-C VI, N VI-N VII, and O VII in the recovery discharge are 20-40, 10-20, and 40-50 times higher than those in the normal discharge (see the blue line in Figure 5a), respectively, indicating a high content of low-Z impurities in plasma recovery since the electron density and temperature of the plasma are similar in both cases. At the wavelength of 65-190 A, the most commonly observed emission lines in normal discharges were several O VI lines at 150.1, 173.079, and 184.117 A, as shown in Figure 5b. Li III (Li\({}^{2+}\)) at 135 A and 113.9 A also appeared with strong intensity due to wall conditioning of lithium coating. In contrast, in the recovery discharges, many oxygen lines in the ionization state down to O\({}^{2+}\) appeared at 110-380 A, probably due to a high content of oxygen, potentially from residual water inside the EAST vacuum vessel, among which O V and O VI lines mainly appeared at 110-230 A, while O IV and O III lines mainly appeared at 200-380 A. Emission lines from lowly ionized iron and molybdenum ions also appeared at the long wavelength range of 340-460 A in the recovery discharges, such as Fe V (Fe\({}^{4+}\)) and Mo V (Mo\({}^{4+}\)), probably due to residual metal dust from runaway electrons hitting the walls inside the EAST vacuum vessel, which was not observed in the case of normal discharges. Figure 5d shows a comparison of line intensity at 320-495 A, where impurity lines of carbon, oxygen, and iron (mainly Fe V) in recovery discharge (#84601) were observed. The EUV spectra recorded during recovery discharges are invaluable data that were rarely obtained and analyzed in previous research. The higher content of low-Z impurities in the current ramp-up phase, and the presence of residual metal dust inside the EAST vacuum chamber after a vacuum failure, may explain why the collisional plasma cannot be maintained. Figure 6 presents the EUV spectra obtained at 8-495 A during current platform (t = 5.0 s) in recovery (#84599, #84600, and #84601) and normal (#84649, #84678, and #85126) discharges after lithium coating. Figure 6a,b show the comparison of spectra at 8-190 A.