## 2 Experimental Setup  
EAST is a fully superconducting diverted tokamak with a D-shaped poloidal cross section, featuring major parameters such as toroidal magnetic field \(\mathrm{B_{T}}\) in the range of 2.0-3.5 T, plasma current \(\mathrm{I_{p}}\) up to 1 MA, and plasma elongation k between 1.2 and 2.0. It can be operated in upper single null (USN), lower single null (LSN), and double null (DN) divertor configurations, with Deuterium (\(\mathrm{D_{2}}\)) mostly used as working gas [15]. In order to improve the machine's capability towards high-performance, steady-state discharge operation, the upper and lower graphite divertors of EAST were upgraded using ITER-like W/Cu monoblock in 2014 and 2021, respectively. EAST tokamak was operated with lower graphite divertor, molybdenum first wall, and upper tungsten divertor during 2014-2021 [16]. In the middle of the EAST 2019 spring campaign, a vacuum failure occurred. EAST vacuum vessel was exposed to air for 161 h to fix the leak point. Subsequently, the vacuum vessel underwent a series of wall conditioning, including baking at 150 \({}^{\circ}\)C for 70 h, baking at 180 \({}^{\circ}\)C for 130 h, D\({}_{2}\)-GDC for 47 h, and He-GDC for 64 h, with ICRF-DC assistance for approximately 21h, in order to recover the vacuum condition [6]. As mentioned earlier, during plasma recovery before routine lithium coating carried out, real-time lithium granule injection was performed to study the effect of lithium on the improvement of plasma performance. The plasma parameters for the discharges studied in this work were Bt = 2.5 T, USN configuration, I\({}_{\text{P}}\) = 300-500 kA, and n\({}_{\text{e}}\) = 2.1-4.3 \(\times\) 10\({}^{19}\) m\({}^{-3}\). Lower hybrid wave (LHW) at a frequency of 2.45 GHz (LHW1) and 4.6 GHz (LHW2) were injected for plasma current driving and heating, and the total power was in the range of 1.0-2.5 MW. During plasma recovery, the confined plasma would only last for tens or hundreds of milliseconds in one discharge. Two fast-time-response EUV spectrometers, namely, 'EUV_Short' [17] and 'EUV_Long' [18], working at 8-130 A and 20-500 A, respectively, with acquisition rate of 5 ms/frame were used to observe the EUV spectra emitted from impurity ions, while many other diagnostic systems in EAST are not available, e.g., T\({}_{\text{e}}\) and its profile measurement due to poor plasma performance, and visible spectra acquisition due to a much longer cycle time required. The EUV_Short and EUV_Long spectrometers were installed to EAST horizontal C-port and D-port, respectively, in the EAST 2019 experimental campaign [17; 18]. The spectrometers were developed to measure spectra from highly ionized tungsten in EAST with a tungsten divertor, as well as spectra lines of low-Z and mid-Z impurities [19; 20]. Both spectrometers are grazing-incidence, flat-field spectrometers consisting of entrance slits, varied line spacing (VLS) concave holographic gratings, and back-illuminated CCDs with effective areas of 26.6 \(\times\) 6.6 mm\({}^{2}\) and 1024 \(\times\) 256 pixels, respectively. The incidence angles of the gratings are 88.6\({}^{\circ}\) and 87.0\({}^{\circ}\) for EUV_Short and EUV_Long, respectively. The groove spacing at the grating center are 2400 grooves/mm and 1200 grooves/mm, respectively. The top view of the optical layout of the EUV_Short and EUV_Long spectrometers are shown in Figure 1a,b), respectively. During inter-discharges, a remotely controlled pulse motor is used to move the CCD along the focal plane to change the wavelength interval. In this work, the EUV_Short spectrometer was set to observe a fixed wavelength range of 8-65 A, while wavelength scans were performed three times to observe spectra at the full wavelength range for the EUV_Long spectrometer, i.e., 65-190, 190-320, and 320-495 A. The intensity of the spectrometers was absolutely calibrated by comparing experimental and theoretical intensities of the bremsstrahlung continuum [18].