## 3 EUV Spectra Analysis in the Plasma Recovery  
### Discharge Waveform  
The waveforms of EAST recovery discharge (#84601) and normal discharge (#84649) are shown in Figure 3. In the recovery discharge, the injected power from LHW1 and LHW2 was 0.8 MW each, from 0 to 8.6 s and 2.6 s to 8.4 s, respectively. The discharge had a plasma current of 300 kA and a line-averaged electron density of 2.2 \(\times\) 10\({}^{19}\) m\({}^{-3}\). In the normal discharge, the injected power of LHW1 and LHW2 was 0.7 MW and 1.8 MW, from 1.8 to 8.2 s and 2.2 s to 8.0 s, respectively. This discharge had a plasma current of 450 kA and a line-averaged electron density of 4.3\(\times\) 10\({}^{19}\) m\({}^{-3}\). In both discharges, energetic electrons were generated during the I\({}_{\rm p}\) start-up phase, which significantly contributed to the saturated ECE signal. In the normal discharge (#84649), the line intensity of D\({}_{\rm\alpha}\) and O VII (O\({}^{6+}\), Ionization Energy E\({}_{\rm i}\) = 739.3 eV) at 21.6 A was measured by the Filterscope system and the EUV_Short spectrometer, respectively. The line intensity slowly increased with n\({}_{\rm e}\), and then largely increased after 4.6 GHz LHW was superimposed due to the enhanced interaction between the plasma and wall (see the blue lines in Figure 3e,f). The spikes and dens in the time evolution of the signals were caused by the unstable power injection (see the blue line in Figure 3d). The energetic electrons started to slow down from t = 1.0 s despite a high loop voltage, and then the ECE signal was reduced. Meanwhile, the runaway signal remained at around 0 in the whole discharge (see the blue lines in Figure 3g,h). On the contrary, in the recovery discharge (#84601), the line intensity of O VII was very high at the plasma start-up phase of t \(\leq\) 0.1 s, i.e., 50-100 times higher than in the normal discharge, indicating a very high level of oxygen content. The emission lines disappeared at t = 133 ms and were replaced by random spike noises in the EUV spectra, indicating a quench of confined (collisional) plasma and the formation of low-temperature plasma (see the red line in Figure 3f). Other impurity lines, such as C VI 33.73 A and C V 40.27 A, had similar time traces as O VII. Therefore, the lifetime of confined (collisional) plasma in the recovery discharge, \(\tau_{\rm c}\), can be deduced from the duration of emission lines from H- and He-like oxygen and carbon ions in the discharge. Although the plasma current was maintained throughout the whole discharge of #84601, the confined (collisional) plasma  
Figure 3: Time-evolution of (**a**) plasma current, I\({}_{\rm p}\), (**b**) line averaged electron density, n\({}_{\rm e}\), (**c**) loop voltage, V\({}_{\rm loop}\), (**d**) total injection power of low hybrid wave (LHW), (**e**) line intensity of D\({}_{\rm\alpha}\), (**f**) line intensity of O VII (O\({}^{6+}\)) at 21.6 Å, (**g**) intensity of electron cyclotron emission (ECE) of HRS01, (**h**) runaway electron flux signal in recovery discharge #84601 (red lines) and normal discharge #84649 (blue lines). Vertical gray lines in (**g.h**) indicate the timings of corresponding jumps and spikes in ECE and Runaway signal, respectively. was only maintained for 133 ms. After that, the discharge changed to a runaway status filled with low-temperature plasma, and the plasma current was sustained by lower hybrid waves and runaway electrons. Lots of energetic electrons were continually accelerated from t = 133 ms and became runaway electron, leading to a gradually increasing runaway signal until the end of the I\({}_{\rm P}\) ramp-up phase (t = 1.8 s). During the I\({}_{\rm P}\) flattop phase, the step-like decrease in the ECE signal corresponded to spikes in the runaway signal, indicating sudden increases in thick target bremsstrahlung intensity caused by sudden losses of energetic electrons from plasma impinging on the first wall (see the red lines in Figure 3g,h).