Characterization of the in situ leading-edge-induced melting on the ITER-like tungsten divertor in EAST

Abstract

Understanding tungsten melting behavior and its influence on plasma operation is one of the main concerns for ITER, which will be operated with a full tungsten divertor. Similar to ITER, actively cooled tungsten cassette modules have been successfully installed for the upper divertor in EAST for a stronger heat exhaust ability. However, an unexpected in situ tungsten melting phenomenon has been observed around the strike point area on both the inner and outer targets during recent EAST plasma campaigns. It was identified that such tungsten melting was caused by the leading edges, which leads to much high heat load on the protruded edges of the cassette modules. All tungsten melting occurred only at the edges of the cassette modules where larger misalignment up to millimeter scale was formed. The melted layer, which was mainly driven by the electromagnetic force, was moved either up or down under different conditions. Only a few bridge connections along the vertical direction could be formed. Such tungsten melting ejected a large number of droplets into the core plasma, and resulted in a sharp increase of tungsten impurity and power radiation and could eventually lead to disruptions. With droplet ejection and melted layer removal, the melted corner seems to form a moderate chamfer structure and thus may mitigate the temperature rise. Under current operation conditions, EAST can tolerate such melting to some extent. Several solutions to help avoid this kind of melting have been proposed according to the thermal analysis for future EAST operations.

In situ leading-edge-induced damages of melting and cracking W/Cu monoblocks as divertor target during long-term operations in EAST

“…Indeed, serious melting phenomena covering a large poloidal range of 7 cm were identified at the leading edges of W/Cu monoblocks, which may be due to the wide range of strike point in different discharges. The distribution of melted monoblocks as well as the molten layer movement have been characterized in previous work [21]. Metallic inspection of a molten W/Cu monoblock shows the recrystallized equiaxed crystal in the vicinity of the molten leading edge, as shown in figure 5(c), illustrating that the recrystallization occurred during plasma operation and the temperature exceeded the recrystallization temperature of W material (∼1200 • C).…”

“…Figure 4(c) shows the morphology of the damaged targets where the droplets were ejected from. The duration of droplet emission is about 20 ms. With ejection of W droplets, the impurity and radiation in the core plasma increased significantly and finally resulted in disruption [21]. All the shots with this kind of melting disrupted finally in EAST.…”

“…In order to mitigate the leading-edge-induced high thermal effects, each W/Cu monoblock was chamfered with dual chamfering shaping in toroidal and radial directions (1 mm × 1 mm) for both leading edges [20] because of EAST operations in both normal and reverse magnetic field directions. After manufacture, the misalignment at poloidal gaps between neighboring monoblocks within each CM (intra-CM) is usually below 0.3 mm [21]. Moreover, it is also relatively stable during operation because of the strong connection of all monoblocks in every individal CM.…”

The Effect of Vanadium Doping on Carbon Behavior in Tungsten: A First‐Principle Study

“…[ 11,12 ] Tungsten itself subjects to various elements’ irradiation, [ 13 ] which results in its decline of mechanical properties and the structural strength, [ 14,15 ] such as cracking and droplet ejection. [ 16,17 ]…”

“…[11,12] Tungsten itself subjects to various elements' irradiation, [13] which results in its decline of mechanical properties and the structural strength, [14,15] such as cracking and droplet ejection. [16,17] As is known to all that alloying can get a new kind of material which possesses better properties. [18][19][20] W and W alloys usually possess the properties of low tritium inventory and low erosion rate under the extreme service environment.…”