Search

Since 2021, EAST tokamak has been operated with full tungsten divertors. Tungsten accumulation has been frequently observed in NBI-heated H-mode discharges, resulting in the degradation of plasma confinement performance. Control of tungsten impurity is thus critical for the maintenance of high-confinement plasmas. In this work the impact of the n = 1 resonant magnetic perturbation (RMP) on the behavior of intrinsic low- and high-Z impurities in EAST H-mode discharges are experimentally studied, utilizing high-performance extreme ultraviolet spectroscopic diagnostics. In the dedicated discharge, ELM mitigation, ELM suppression, H-L back transition, RMP penetration occurs in succession with increasing RMP current (IRMP). When IRMP is below the threshold for H-L back transition, IRMP_H-L = 2.29 kA, increasing influx of C2+ and C3+ ions and decreasing influx of C4+ and C5+ ions are observed simutaneously with enhancement of the RMP field. This opposite time behavior in the influx of C4+ and C3+ ion is then observed to be magnified during the RMP penetration phase. It indicates a impurity screening layer formed between the locations where C4+ and C3+ ions distribute during RMP application based on our previous analysis (W.M. Zhang et al 2024 Nucl. Fusion 64 086004). A large step of increase in C4+ influx after H-L back transition indicates C4+ ion mainly located at bottom of pedestal. A higher RMP coil currents threshold capable of impurity screening is found for high-Z impurity ions of Cu25+, Mo30+, W42+, i. e. 0.53–0.75 kA, than that for C4+ and C5+, i. e. 0.33 kA. Meanwhile, it is found that comparing to C4+ and C5+ ions the decontamination effect by this impurity screening layer is more efficient for these high-Z impurity ions in plasma core region, e.g. up to 70 % reduction in the impurity density, leading to a significant reduction of radiation power. Furthermore, the continuous reduction of core high-Z impurities level both in ELM mitigation and suppression phase proved that this impurity decontamination effect by RMP field is dominant over the impact of ELM activity to core high-Z impurities transport since tungsten is frequently observed to accumulate during original ELM-free phase. Experimental results from this work would contribute to further understanding of the underlying mechanism how the RMP field impacts the impurity transport.

Operation of EAST tokamak with full metal wall without any wall conditioning are attempted in 2023 and 2024 experimental campaign to address the issues related to ITER tungsten wall operation. It is found that H-mode plasma could be sustained even with a substantial increase in metallic impurity content caused by strong plasma-wall interaction under uncoated metal wall. Boronization wall conditioning is therefore performed to improve the plasma performance with higher injected power. This study aims to quantitatively assess the impact of boronization wall conditioning on metallic impurity concentration and behavior in the EAST tokamak. It is then proved to be an effective wall conditioning approach for significantly controlling high-Z impurity content. In this work, the impurity spectra at extreme ultraviolet (EUV) wavelength range measured by sets of fast-time-response and space-resolved EUV spectrometers are widely used in the data analysis. The variation in the boron content in plasma after boronization are investigated by monitoring the 2nd order of B V line at 48.59 Å. It is found that the persistence time of boron in EAST device after a boronization with 10 g of carborane (C2B10H12) is about 2000 s (∼150 shots) of discharge duration. The impact of different wall conditions (uncoated metal wall and boron coated wall) on metallic impurity content are then quantitatively studied. After boronization, the concentration of tungsten (CW) and molybdenum (CMo) dropped by 85 %, e. g., from 2.0 × 10-4 to 4.1 × 10-5 and from 4.6 × 10-5 to 6.3 × 10-6, respectively. While the concentration of copper (CCu) and iron (CFe) decreased by approximately 50 % and 65 %, respectively, e. g., from 4.3 × 10-5 to 2.1 × 10-5 and from 2.0 × 10-4 to 6.9 × 10-5. A comparison of the line emission profiles from tungsten ions of W26+ − W32+ and W43+ before and after boronization reveals an overall reduction in the intensity while without obvious change in the profile shape, which suggests a reduction in metallic impurities source after boronization instead of altering impurity transport in core plasma significantly.

Since the last IAEA-FEC in 2021, significant progress on the development of long pulse steady state scenario and its related key physics and technologies have been achieved, including the reproducible 403 s long-pulse steady-state H-mode plasma with pure radio frequency (RF) power heating. A thousand-second time scale (∼1056 s) fully non-inductive plasma with high injected energy up to 1.73 GJ has also been achieved. The EAST operational regime of high β _P has been significantly extended ( H _98y2 > 1.3, β _P ∼ 4.0, β _N ∼ 2.4 and n _e / n _GW ∼ 1.0) using RF and neutral beam injection (NBI). The full edge localized mode suppression using the n = 4 resonant magnetic perturbations has been achieved in ITER-like standard type-I ELMy H-mode plasmas with q _95 ≈ 3.1 on EAST, extrapolating favorably to the ITER baseline scenario. The sustained large ELM control and stable partial detachment have been achieved with Ne seeding. The underlying physics of plasma-beta effect for error field penetration, where toroidal effect dominates, is disclosed by comparing the results in cylindrical theory and MARS-Q simulation in EAST. Breakdown and plasma initiation at low toroidal electric fields (

High-Z impurity accumulation suppression and mitigation in core plasma is frequently observed in EAST edge localized mode mitigation experiments by using resonant magnetic perturbations (RMP) coils. To study the individual effects of the RMP field on impurity transport, based on high-performance extreme ultraviolet impurity spectroscopic diagnostics, the effect of the n = 1 ( n is the toroidal mode number) RMP field on the behavior of intrinsic impurity ions at the plasma edge, e.g. He ^+ , Li ^2+ , C ^2+ –C ^5+ , O ^5+ , Fe ^8+ , Fe ^15+ , Fe ^17+ , Fe ^22+ , Cu ^17+ , Mo ^12+ , Mo ^13+ and W ^27+ , is analyzed for the first time in L-mode discharges. Based on the evaluation of the location of these impurity ions, it is found that with the increase in RMP current ( I _RMP ), an impurity screening layer inside the last closed flux surface is formed, e.g. at ρ = 0.74–0.96, which is also the region that the RMP field affects. Outside this screening layer, the impurity ion flux of He ^+ , Li ^2+ , C ^2+ , C ^3+ , O ^5+ , Fe ^8+ , Mo ^12+ and Mo ^13+ ions increases gradually, while inside this screening layer, the impurity ion flux of C ^4+ , C ^5+ , Cu ^17+ , W ^27+ , Fe ^15+ , Fe ^17+ and Fe ^22+ ions decreases gradually. When I _RMP is higher than a threshold value, RMP field penetration occurs, accompanied with m / n = 2/1 mode locking, and the position of this screening layer moves to the plasma core region, i.e. ρ = 0.66–0.76, close to the q = 2 surface, and the opposite behavior of the impurity ion flux at two sides of the screening layer is strengthened dramatically. As a result, significant decontamination effects in the plasma core region, indicated by the factor of (( Γ _Imp ^Z+ ) _w/o –( Γ _Imp ^Z+ ))/( Γ _Imp ^Z+ ) _w/o (where ( Γ _Imp ^Z+ )/( Γ _Imp ^Z+ ) _w/o denotes the impurity ion flux ratio with and without RMP), is observed, i.e. 30%–60% for heavy impurity (Fe, Cu, Mo, W), and ∼27% for light impurity of C. In addition, the analysis of the decontamination effects of C and Fe impurities under four different RMP phase configurations shows that it may be related to the strength of the response of the plasma to RMP. These results enhance the understanding of impurity accumulation suppression by the n = 1 RMP field and demonstrate a candidate approach using RMP coils for W control in magnetic confinement devices.

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, τc, 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.

In the construction of shaft, the blockage of the mucking shaft may cause the mud-water inrush disaster. Oversized rock fragmentation is the main cause for the blockage of the mucking shaft in the raise boring machine (RBM) construction method. The influence degree of blasting parameters on rock fragmentation after blasting is quantified by adopting analytic hierarchy process (AHP). On this basis, the shaft blasting maximum rock fragmentation control model based on double hidden layer BP neural network is proposed. Results show that the maximum rock fragmentation discharged from the mucking shaft after blasting should not exceed 1/3 of the diameter of the slag chute. The influence weight of the minimum resistance line that accounts to 15.6%, when AHP is applied for the quantification of the blasting parameters, can be regarded as the most important blasting parameter. The average absolute errors between the predicted value and the actual value of the largest block size control model of the shaft blasting are only 2.6%. The inversion analysis of the model can rapidly obtain the required blasting parameters, which can be used to guide the construction of the tunnel ventilation shaft.