As the high atomic number is an intrinsic material property that cannot be changed, the only possibility to avoid plasma contamination by tungsten is to adapt to the loading realities, that is, thermal loads and plasma wall interaction conditions, and the energy of the incident plasma particles. In particular, surface crack formation, loosening of particles, and particle ejection or melting are addressed (see Section 4.17.4). Concerning the latter, the addition of suitable alloying elements or dispersoids (see Section 4.17.3.3) reduces the material’s thermal conductivity causing a reduction of allowed applied heat fluxes. From this point of view only low-alloyed grades should be considered and the best grade is tungsten of high purity.

4.17.3.2.1 Recrystallization

Recrystallization is a thermally activated process. Therefore, it is expected that the activation energy of nucleation is dominated by small angle grain boundaries. The activation energy of grain growth is dominated by large angle grain boundaries.64 The temperature of recrystallization depends mainly on the deformation history, that is, the higher the degree of deformation, the lower the recrystallization tem — perature,65,66 and the chemical purity. When heated above the recrystallization temperature, the structure of tungsten is altered due to grain growth causing an increase in DBTT and reducing other mechanical properties, that is, strength and hardness.67

There are several possibilities for increasing the recrystallization temperature. Particle reinforcement and controlled formation of porosity are the best and most investigated options.68 For example, the higher recrystallization temperature of dispersion strength­ened alloys results from the interaction between dispersoids and dislocations during hot-working; the higher the amount of hot-work, the finer are the dispersoid particles and the higher is the recrystalli­zation temperature. During recrystallization, these particles prevent secondary grain growth and conse­quently, the recrystallization temperature of disper­sion strengthened alloys may increase compared to pure W.67 Another example is highly creep-resistant doped/nonsag materials with aligned porosity acting as obstacles for dislocation movement as they are used in the lighting industry.69

Experience shows that incomplete recrystalliza­tion often helps to achieve the desired balance in material properties. If the operating temperature is well known, controlled recrystallization during application might be feasible as well.67 However, for operational conditions in nuclear fusion devices, it is expected that the very high thermal strain rates experienced in the thin layer heated by plasma dis­ruption or any other transient thermal event will significantly affect the material’s microstructure and properties.