## 5 Summary and Discussion  
In the plasma of a superconducting tokamak after a vacuum failure, during the I\({}_{\rm P}\) ramp-up phase of recovery discharge, energetic electrons can be easily further accelerated and turn into runaway electrons, leading to frequent occurrence of runaway discharges or disruptions. Several wall conditioning techniques are then required to remove residual impurity gas and water on the wall and suppress the energetic runaway electrons.  
In this paper, the impurity behavior in plasma recovery of EAST after a vacuum failure was studied using two EUV spectrometers with a fast-time-response of 5 ms/frame. The study revealed that during the recovery discharges, oxygen and carbon content was dozens of times higher than normal discharges, probably causing high V\({}_{\rm loop}\) and plasma runaway. The lifetime of highly ionized impurity ions of C\({}^{5+}\) and O\({}^{6+}\) was used to deduce the lifetime of sustained collisional plasma. Meanwhile, the EUV spectra showed that O\({}^{2+}\)-O\({}^{6+}\) lines and O\({}^{2+}\)-O\({}^{3+}\) lines dominated the spectra in the collisional plasma and the runaway status, respectively, from which the electron temperature of less than 75 eV was preliminarily evaluated for the runaway status in the discharge. Unfortunately, the metal impurities of Fe and Mo were found in both plasma statuses of the recovery discharges.  
Real-time injection of a small amount of lithium granule in successive discharges effectively suppressed the runaway electrons and their impingement frequency on the inner wall. Meanwhile, plasma-wall interaction may be weakened by a gradually formed thin lithium film, leading to reduced impurity influx of oxygen and carbon. As a result, the lifetime of collisional plasma was extended.  
Disruptions of tokamak plasma can lead to high thermal loads on the first wall, introduce impurities into the vacuum chamber, and make it challenging to re-establish normal collisional plasma in subsequent discharges. Although lithium injection has a positive effect on impurity control and plasma recovery, the coating of lithium has several drawbacks and is not likely to be applied in fusion reactors. The role of boron pellet injection into the plasma is currently being investigated in EAST experiments. The research of plasma recovery of EAST provides an important reference for establishing the first plasma in future superconducting fusion devices such as ITER and CFETR.  
Conceptualization, L.Z. and Z.Z.; methodology, L.Z.; software, Y.C. ([PERSON]); validation, R.Z. and A.T.; formal analysis, Z.Z. and L.Z.; investigation, L.Z. and W.Z. ([PERSON]); resources, A.H., Z.S., G.Z. and H.L.; data curation, F.Z., W.Z. ([PERSON]), Z.L. and Y.C. ([PERSON]); writing--original draft preparation, Z.Z.; writing--review and editing, L.Z., S.M. and Y.Y.; visualization, Z.Z.; supervision, L.Z.; project administration, L.Z.; funding acquisition, L.Z. and Y.Y. All authors have read and agreed to the published version of the manuscript.  
Figure 12: Lifetime of confined plasma during lithium granule injection (#84601–#84612). The orange circles indicate the discharges in which an early disruption occurred.  
**Funding:** This work is supported by the National Magnetic Confined Fusion (MCF) Energy Research & Development Program (No. 2022 YFE03180400), National Natural Science Foundation of China (Grant Nos. 11704076, 11911530229, U1732140, 2021 AMF01002, 1916321 TS00103201), the National Key Research & Development Project (No. 2020 YFB1902100), the Youth Innovation Promotion Association of Chinese Academy of Sciences (Grant No. 2021445) and Chinese Academy of Sciences President's International Fellowship Initiative (PIFI) (No. 2020 VMA0001).  
**Institutional Review Board Statement:** Not applicable.  
**Informed Consent Statement:** Not applicable.  
**Data Availability Statement:** The data presented in this study are available on request from the corresponding author.  
**Conflicts of Interest:** The authors declare no conflict of interest.