The impact of the material’s microstructure is related to the amount of intrinsic defects at which He can be trapped and therefore determines the amount of He-retention.2 0 SC tungsten contains fewer defects than powder metallurgically (PM) produced tungsten (grain boundaries) and plasma-sprayed tungsten (grain boundaries and porosity), which is directly related to the thickness of porous/spongy structures (porosity about 90%,256 see Figure 10) that form depending

on energy and temperature.257,258 However, investiga­tions at 1650 K have shown that at such high tempera­tures there exists no difference between SC-W and PM-W, even for ion energies as low as 25 eV. The main trap sites at such high temperatures are thermal vacan­cies while intrinsic defects play a minor role.149,251

With regard to the material’s lifetime under He-exposure, the migration of He-bubbles toward the surface and the formation of pores and porous/ spongy structures seem to prevent the rupture and exfoliation that can accompany blistering. This is important as the exfoliation of blisters creates dust,19 which limits the plasma performance. For an antici­pated flux of 2 x 1018 He+ m~2 s-1 at 850 °C in iner­tial confinement devices, this may lead to a removal of 20mmyear-1 from the wall.109 Therefore, one way to increase the material’s lifetime might be to operate it at higher temperature.253 Another approach would be to develop advanced microengineered materials that have typical feature sizes less than the classical helium migration distance (20 nm).109 However, bubbles, holes, and porous/spongy structures signifi­cantly influence the material’s performance by reducing its thermal conductivity in the near-surface layer. This might play an important role when deter­mining the erosion and melt formation under com­bined He-irradiation and transient thermal loads, which will be shortly addressed in Section 4.17.4.4.3.