## 3 EUV Spectra Analysis in the Plasma Recovery
### Impurity Behavior Analysis
At this wavelength range, low-Z impurity lines from highly ionized lithium, carbon, nitrogen, and oxygen ions, e.g., C V-C VI (C\({}^{4+}\)-C\({}^{5+}\)), N VII, and O VI-O VIII (O\({}^{5+}\)-O\({}^{7+}\)), as well as tungsten unresolved transition array (W-UTA, W\({}^{24+}\)-W\({}^{32+}\)) appeared in normal discharges with a core electron temperature of \(\sim\)1.5 keV. While only spike noises caused by high-energy runaway electrons were recorded in recovery discharges, as indicated by the red vertical arrows. Figure 6c,d show the comparison of spectra at 190-495 A. In addition to the spike noises, many impurity lines from lowly ionized carbon, oxygen, and iron \begin{table}
\begin{tabular}{c c c c c} \hline
**Status** & **Shot No.** & **I\({}_{\rm p}\) (kA)** & **n\({}_{\rm e}\) (10\({}^{19}\) m\({}^{-3}\))** & **\textless{}\textless{}\textgreater{} (Å)** \\  & & **t1/t2** & **t1/t2** & **\textless{}\textless{}\textgreater{}**\textgreater{}**\textgreater{}**\textgreater{}**\textgreater{}**\textgreater{}**\textgreater{}**\textgreater{}**\textgreater{}**\textgreater{}**\textgreater{}**\textgreater{}**\textgreater{}** \\ \hline \multirow{4}{*}{Recovery} & \#84601 & 50/300 & 0.3/2.2 & 8–65 \\  & \#84599 & 50/300 & 0.3/2.3 & 65–190 \\ \cline{2-5}  & \#84600 & 50/350 & 0.3/3.4 & 190–320 \\ \hline \multirow{4}{*}{Normal} & \#84649 & 70/450 & 0.1/4.3 & 8–65 \\  & \#84678 & 70/400 & 0.2/2.1 & 65–190 \\ \cline{1-1} \cline{2-5}  & \#85126 & 70/500 & 0.1/3.3 & 190–320 \\ \hline \end{tabular}
\end{table} 
Table 1: Plasma parameters at initial phase (t1 = 0.08 s) and current platform (t2 = 5.0 s) and wavelength interval of the spectrometers in recovery and normal discharges. ions, e.g., C IV (C\({}^{3+}\)), O II-O III (O\({}^{+}\)-O\({}^{2+}\)), and Fe V (Fe\({}^{4+}\)) appeared, especially O III and Fe V lines with strong intensity dominating the spectra, indicating a very low electron temperature of the bulk plasma, e.g., 55-75 eV. Comparison of EUV spectra during the current platform reveals the differences in impurity behavior between normal collisional plasma and runaway plasma in recovery discharges. In normal discharges, highly ionized low-Z impurity ions and tungsten UTA existed in the bulk plasma due to a higher core electron temperature of \(\sim\)1.5 keV. In contrast, weakly ionized low-Z impurity, particularly O\({}^{2+}\) and Fe\({}^{4+}\) ions, appeared in the recovery plasma due to much lower electron temperature and poor plasma confinement. Additionally, the EUV spectra, especially in the wide wavelength range of 8-320 A, suffered from numerous spike noises in the recovery discharges. The spike noises, which were caused by X-ray emission resulting from thick target bremsstrahlung, randomly appeared in time and were characterized by existing at a single pixel with no broadening and lasting only one frame in the spectrum. Figure 5: EUV spectra comparison during I\({}_{\rm P}\) ramp-up phase (t = 0.08 s) of recovery (red lines) and normal (blue lines) discharges. (**a**) 8–65 Å, (**b**) 65–190 Å, (**c**) 190–320 Å, and (**d**) 320–495 Å. 4 The Effect of Lithium Granule Injection on Impurity Suppression and Plasma Performance Improvement  
In order to investigate the effect of lithium coating on impurity suppression and improvement of plasma confinement, a small amount of lithium granule (10-20 mg/s) was injected in real time during several successive discharges (#84603-#84609). The plasma parameters, e.g., plasma current (I\({}_{\rm p}\)), average electron density (n\({}_{\rm e}\)), loop voltage (V\({}_{\rm loop}\)), and the accumulated amount of lithium granule injection are listed in Table 2. Note that #84604, #84608, and #84610 are not included in the table due to disruption occurring at 1.18, 1.58, and 1.24 s, respectively. In addition, a late disruption occurred at 6.6 s in discharge #84606.