Two pairs of space-resolved extreme ultraviolet (EUV) spectrometers working at 5–138 Å with different vertical observation ranges of −7 ≤ Z ≤ 19 and −18 ≤ Z ≤ 8 cm have been newly developed to observe the radial profile of impurity line emissions and to study the transport of high-Z impurity ions intrinsically existing in EAST tokamak plasmas. Both spectrometers are equipped with a complementary metal-oxide semiconductor (CMOS) detector (Andor Marana-X 4.2B-6, Oxford Instruments) with sensitive area of 13.3 × 13.3 mm² and number of pixels equal to 2048 × 2048 (6.5 × 6.5 µm²/pixels). Compared to the currently operating space-resolved EUV spectrometers with a charge-coupled detector (CCD: 1024 × 255 pixels, 26 × 26 µm²) working at 30–520 Å, this spectrometer’s performance was substantially improved by using the CMOS detector. First, the spectral resolution measured at full width at half maximum was improved in the whole wavelength range, e.g., Δλ_{1/2_CMOS} = 0.092 Å and Δλ_{1/2_CCD} = 0.124 Å at C VI 33.73 Å and Δλ_{1/2_CMOS} = 0.104 Å and Δλ_{1/2_CCD} = 0.228 Å at Mo XXXI 115.999 Å, thus enabling a more accurate analysis of spectra with complicated structure such as tungsten unresolved transition array in the range 45–65 Å. Second, the temporal resolution was largely improved due to the high-speed data acquisition system of the CMOS detector, e.g., Δt_{_CMOS} = 15 ms/frame and Δt_{_CCD} = 200 ms/frame at routine operation in the radial profile measurement. Third, signal saturation issues that occurred when using the old CCD sensor during impurity accumulation now disappeared entirely using the CMOS detector due to lower exposure time at high readout rates, which largely improved the observation performance in similar impurity burst events. The above-mentioned performance improvements of the space-resolved EUV spectrometer led to a rapid change in the W XXXIII (52.22 Å) radial profile during a single cycle of low-frequency sawtooth oscillation with f_st = 5–6 Hz at a sufficient detector count rate.

After a vacuum failure in a tokamak, plasma runaway or plasma disruptions frequently occur during plasma recovery, causing difficulties in rebuilding a well-confined collisional plasma. In this work, the impurity behavior during plasma recovery after a vacuum failure in the 2019 spring campaign of the Experimental Advanced Superconducting Tokamak (EAST) was studied by analyzing the spectra recorded by fast-time-response extreme ultraviolet (EUV) spectrometers with 5 ms/frame. During the plasma current ramp-up in recovery discharges, a high content of the low-Z impurities of oxygen and carbon was found, i.e., dozens of times higher than that of normal discharges, which may have caused the subsequent runaway discharges. The electron temperature in the recovery discharge may have dropped to less than 75 eV when the collisional plasma quenched to the runaway status, based on the observable impurity ions in the two cases. Therefore, the lifetime of collisional plasma in the recovery discharge, τ_c, was deduced from the lifetime of H- and He-like oxygen and carbon ions identified from EUV spectra. It was found that, after several discharges with real-time lithium granule injection, the runaway electron flux and O⁺ influx reduced to 45% and 20%, respectively. Meanwhile, the lifetime of confined plasma was extended from 113 ms to 588 ms, indicating the effective suppression of impurities and runaway electrons and improvement in plasma performance by real-time lithium granule injection. The results in this work provide valuable references for the achievement of first plasma in future superconducting fusion devices such as ITER and CFETR.