4.17.4.1 Thermal Shock Resistance

Tungsten-armored PFCs will be subjected to differ­ent types of heat fluxes dependent on their field of application (see Section 4.17.2). Among others, this includes thermal transient loads (e. g., ELMs and disruptions). The behavior of the material under these conditions, that is, the combination of cyclic steady state and transient heat loads, is a key factor that has to be considered for the selection of a suitable grade of W.

The machines simulating these operational conditions are electron and ion beam facilities, quasi­stationary plasma accelerators, plasma guns, and high-energy lasers. A most critical issue is the compa­rability of such simulations. Therefore, a round robin test involving some representative facilities was made for investigating the influence of the different time regimes and different power density levels. The results showed that when compared on the basis of a heat flux parameter P (MW m~2 s1/2), which is directly propor­tional to the temperature increase, the cracking and melting thresholds are almost identical. This permits a direct transfer of the qualitative results obtained in any of these facilities.161 In contrast, quantitative results representative of the operational conditions in large fusion devices can only be obtained when the loads are applied in the desired time range. The reason for this is the heat penetration depth and the related stress field that is produced, which influences crack and melting depth.

There are several parameters influencing the thermal shock behavior of tungsten that will be dis­cussed in the following sections for the different materials under disruption and ELM-like loads.