The thermal fatigue resistance of tungsten is strongly related to its performance as part of current inertially, and future actively cooled components for applica­tion in magnetic fusion devices. The functional requirements these components have to fulfill are listed in Section 4.17.2.

State of the art inertially cooled components include W coatings on graphite, CFC, and TZM.12,13,46,47,54,140,207,208 These concepts are used or are going to be used in the large operating toka — maks, for example, AUG and JET. During thermal loading, they mainly suffer from the problem of high interfacial stresses as a result of the CTE difference between the W coating and the substrate. Further­more, interfacial reaction products and their poten­tial reduced power handling capability have to be taken into account.

Besides coatings, recent development of an iner­tially cooled bulk tungsten divertor for JET20,123 showed that under thermal fatigue loads the W quality is of minor importance for the integrity of the compo­nent. The major issue for the tungsten PFM was found to be the necessary shadowing of the plasma-loaded surface to avoid overheating and melting at tile edges as a result of the shallow angle of the incident plasma. This was realized by surface shaping.209

In the design of actively cooled components, tungsten is joined to a water-cooled Cu-based heat sink (ITER) or He-cooled steel or W-based heat sinks (e. g., DEMO, ARIES-CS). Direct cooling of the tungsten armor should be avoided, particularly without castellation, as the induced stresses might cause catastrophic material failure with subsequent water or He-leakage.124 Therefore, the only perfor­mance requirements are a sufficiently good surface quality to reduce possible crack initiation points and therefore suitable fabrication and surface finish­ing technologies,1 , , 1 the chemical compatibility

with the heat sink and, if present, the joining interface material, and the cyclic stability of the joint(s). The latter is influenced by the temperature gradient applied during steady state heat loads, the difference of the CTEs, the quality of the joining process and, perhaps most important for reducing induced stres­ses, the tile size, or the dimensions of the castellated segments (see Section 4.17.3.2.4).

Smaller tile sizes significantly improve the stress situation at the interface, and also at the top surface of the PFM. This has to be taken into account when comparing the thermal fatigue results of various kinds of components and the response of different grades and alloys of W, as shown by Makhankov et at., where smaller tile sizes resulted in little or no crack formation. Furthermore, variations in the size of the component investigated can often explain the contradictory results presented in the literature that show good behavior of a material in one test while it fails in another. However, there are limitations to the minimum size of tiles and a compromise between operational and economical needs has to be made.

Despite the fact that design and manufacturing technique seems to be more important than the mechanical properties and the microstructure of the particular W grade or alloy, the latter should still be considered. Similar to thermal shock results, the risk of delamination parallel to the loaded surface at the interface90 or anywhere in the bulk material has to be minimized. Therefore, the grain orientation of the PFM microstructure should be perpendicular to the loaded surface, although this still bears the risk of crack formation toward the cooling structure.108 To avoid subsequent water or He-leakage in case of crack propagation into the heat sink material, particularly in the He-cooled divertor design (see Section 4.17.4.2.2), suitable material and design solutions still have to be found. Furthermore, sha­dowing of adjacent tiles similar to the JET bulk W divertor has not yet been included in the design of the actively cooled components.