In situ melting phenomena on W plasma-facing components for lower divertor during long-pulse plasma operations in EAST

Abstract

Tungsten (W) is one of the most promising plasma-facing materials (PFMs) for future fusion device. Although its melting point is the highest among all the metals, it still has great risk of melting under extremely high plasma heat fluxes, which is a big concern for the ITER and future reactors. Actively-cooled W plasma-facing components (PFCs) with both monoblocks and flat-type structure have been successfully installed in the lower divertor of the EAST tokamak since 2021, which provide a good opportunity for direct comparison of damage mechanism for the two types of PFCs. Various in-situ melting phenomena on the lower divertor have been observed by CCD cameras, which are further verified by Post-mortem inspections. Severe melting even exfoliation of the edge beveled W plates on some W/Cu flat-type components at horizontal outer targets were observed. A large number of droplets were ejected during long-pulse operations, which induced the significant increase of W impurity and total irradiation in the core plasma, and thus greatly deteriorated the plasma performance and even cause disruptions. Two different shaping structures of flat-type PFCs show different position of melting and corresponding mechanisms. Few slight melting has been found on the sharp leading edges of W/Cu monoblocks at the inter-cassette modules for horizontal targets with small droplets ejection, which is much improved compared to that occurred on the upper W divertor, illustrating that the application of large-sized bevel chamfer at inter-cassette modules was generally effective. In addition, an unexpected melting phenomenon on the dome plate was attributed to the extreme transient heat flux during disruption with runaway electrons. The application of both types of W/Cu PFCs for divertor provides important experiences and lessons for engineering design and optimization of divertor PFCs in future fusion devices.

Long-term plasma exposure of ITER-like W/Cu monoblocks with pre-damaged surfaces in EAST experiments

“…The timeline of REs intensity evolution is synchronous with the occurrence of the hot spot and droplet spraying phenomenon on the limiter, as detected by CCD cameras. Similar phenomena were also observed on the dome plate of the lower divertor of EAST [9], confirmed by CCD monitoring system and relevant analysis. During the plasma disruption, REs with huge energy up to 10 MeV will be deposited on some local regions of Plasma-Facing Components (PFCs), such as limiter or divertor.…”

“…The tested main limiter was composed of W/Cu flat-type components or titanium-zirconium-molybdenum (TZM) tiles. This melting is attributed to the steady-state heat load in the normal plasma discharge process, closely resembling what occurred on the upper and lower divertor targets [9,10]. The migration and ejection of droplets significantly impact plasma discharges and can even directly lead to plasma quenching, thereby restricting high-performance plasma operation.…”

Melting of W/Cu flat type component for main limiter and its impact on plasma operation in EAST experiments

“…As the droplets dispersed, they entered the core plasma, ultimately causing plasma disruption. Similar phenomena were observed in the upper divertor area with CCD camera which were confirmed to be tungsten melting after post-mortem inspections [29,31]. Consequently, it was deduced that the W melting of the main limiter occurred.…”

“…W/Cu flat-type PFCs are widely used in the baffle and dome regions due to their low cost, lightweight, good machinability, and the optimized flat-type PFCs are promising candidate for the use in high heat flux regions of the CFETR divertor [24,28]. Flat-type PFCs were previously installed and tested in the lower diverter area [29]. To further evaluate the performance of flat-type PFCs in tokamaks, a new W/Cu flattype limiter, as shown in figure 2, was designed and tested in the 2022 EAST spring campaign with the aim of achieving high thermal exhaust capacity.…”

Isotope engineering achieved by local coordination design in Ti-Pd co-doped ZrCo-based alloys

“…The deuterium-tritium (D-T) fusion reaction that can produce substantial net energy is currently one of the most promising ways to tackle the energy crisis [1][2][3][4] . However, the development of fusion energy suffers from low fuel burn efficiency, due to the decoupled distributions and distinct transport behaviors of D/T particles [5][6][7][8][9] . The key to achieve self-sustaining and long-term D-T fusion reaction, thus, lies in fueling with precise D/T ratio.…”