As mentioned above, hydrogen retention depends on the trapping sites available in the material and their relative energies. Their existence and concentrations are influenced not only by the impinging H-ions, but also by the manufacturing process, thermal pre­treatments, the material’s composition and micro­structure, and the surface quality. Accordingly, the retention increases with the amount of porosity in the material, as it allows a deep penetration of hydrogen and the voids and pores provide the highest

trapping energies264,275 with thermal desorption

occurring at temperatures >700 K.45,262

Another material parameter that increases hydro­gen retention is the number of dislocations,266,286 particularly those introduced during deformation processes used for material densification. However, the recrystallization of the material removes not only dislocations but also vacancies and vacancy clus­ters, which have been introduced by the impinging H-ions286,287 and as grain boundaries. This effectively reduces the trapping sites for hydrogen retention and, consequently, the lowest retention is observed for high-purity SC materials, particularly due to the low diffusion rate compared to polycrystalline tungsten.275,288,289 This low diffusion rate results in a near-surface accumulation of hydrogen, which acts as a diffusion barrier and leads to a saturation of hydrogen retention with increasing fluence.290 Such saturation is not observed for pure polycrystalline tungsten due to the possible hydrogen migration along grain boundaries.291

Finally, the hydrogen retention is influenced by impurities277 and dopants. The addition of La2O3 and TiC particles as well as the formation of pores, for example, by potassium doping, not only introduce traps and increase hydrogen retention,293 but also decrease the diffusion rate.291 In contrast, alloying with up to 10% Re has no measurable effect on the H retention properties of the material,279 as it only creates a slightly deformed crystal lattice structure but no additional hydrogen traps.

In addition to hydrogen retention, material dam­age and particularly blistering is influenced by the material’s microstructure. Blistering occurs preferen­tially when the crystal is oriented with the (111) direction perpendicular to the surface292 and the blisters develop in different shapes from low, large, and spherical to high, small, and dome or cone-

shaped.45,262,267 The blisters in recrystallized materi­als are mainly plateau-shaped, often multilayer structures, which indicate a step-wise build-up, and in few cases also small blisters on top of large ones are formed.263,293 However, it is significant that the blister size is commonly limited by the grain size45,294 indicating that the grain boundaries play an important role in the formation of blisters. Accordingly, SCs and nanostructured materials such as W-TiC provide the strongest resistance against blistering, although the particular reason is different. For SCs, the hydrogen diffusion and accumulation is limited and there is a fast desorption from low — energy traps at elevated temperatures. In contrast, for nanostructured materials the size of individual grains is extremely small and so is the volume for blister formation. Furthermore, the migration of hydrogen is significantly increased by the large num­ber of grain boundaries.292

Further material parameters that reduce blister formation are open porosity and the surface finish, particularly the number of random or artificially introduced scratches that might act similar to grain boundaries.45 In contrast, the introduction of impurities and dopants in commercially available grades of tungsten increases the number of blisters and exfoliation in both their stress relieved and recrystallized states.277,293

4.17.4.4.3 Combined loading conditions

As described above, the damage mechanisms of hydrogen and He-irradiation are rather similar, although they occur in different temperature ranges. Accordingly, their mutual interaction is also strongly influenced by the implantation tempera­ture. Therefore, the testing sequence plays a role in the behavior, as for He preirradiation followed by hydrogen implantation, the implantation tempera­ture of He determines the amount and kind of produced damage and the He-retention, which sub­sequently influences the hydrogen uptake occur­ring as described in Section 4.17.4.4.2. For example, He-implantation at RT either does not change the retention or may increase it due to the formation of additional trapping sites295-297 and the lower diffusion rate of He compared to H. With increasing He implantation temperature up to 800 K, hydrogen retention significantly decreases compared to pure hydrogen irradiation.261,292,296 This may be attributed to the occupation of trap sites by He as a result of its increasing mobility.298 Potential trap sites are the numerous He-induced nanosized bubbles acting as a diffusion barrier.292 A further increase in temperature to 1600 K does create significant material damage by He due to pore and bubble formation or even blistering. This tremendously increases the number of trap sites in the material and leads to He desorption during implantation and accordingly increases the hydro-

299

gen retention.

For simultaneous loading of He and hydrogen, the fraction of He should reach at least 5 at.% to observe significant changes in the material’s response.261,292 Furthermore, for implantation tem­peratures below 900 K, results similar to those described above are observed for sequential ion beam loading.299 However, due to desorption of hydrogen at high temperatures >1000 K, no hydro­gen retention takes place and the damage mechan­isms are dominated by the He-irradiation during such temperature excursions.

Correlated with hydrogen retention, blister for­mation at temperatures <900 K decreases with decreasing hydrogen retention. This is valid until the number of voids and pores, which enhance hydro­gen retention, start to form open porosity and thereby generate small grain structures. These allow a fast hydrogen diffusion through the material and limit the agglomeration of hydrogen necessary to form blis­ters similar to the case of nano-structured materials such as W-TiC described above (Section 4.17.4.4.2).

Investigations of the influence of radiation dam­age (highly energetic hydrogen, neutrons) and impurity irradiation, for example, by carbon atoms, resulted in the depth resolved and particle energy — dependent formation of dislocations, dislocation loops, and even small voids acting as effective trapping sites for hydrogen and influence blister formation.274,276,300-309 Upon annealing, the disloca­tions and dislocation loops were moved and/or

annihilated,310,311 which is positive news as it

would limit the tritium inventory,312 as long as no He is present in the system. In contrast, with the addition of He the dislocations, dislocation loops, and helium bubbles do not vanish at identical annealing conditions, which has a direct impact on the mechanical and thermophysical performance of tungsten. However, He positively influences hydrogen retention in an intermediated tempera­ture range as described above and inhibits the for­mation of a W carbide layer, which is typical for combined hydrogen and carbon loading.311

Finally, the results obtained from the investigation of the mutual influence of ion irradiation and thermal loads are strongly correlated with the choice of the heat source. In the literature, electron beam guns were favored,313,314 which are characterized by heat deposition in a depth range of several micro­meters for W, depending on the acceleration voltage. However, as the thickness of the ion-irradiation — affected surface layer is comparably thin, the majority of the electrons pass through the modified surface layer, which leads to most doubtful conclu­sions. In contrast, lasers are more reliable, as they apply only surface heat loads. The combination of He-irradiation and laser-induced thermal loads (A T = 1400 K, n = 18 000) at high base temperatures (~1700K), resulted in an affected layer thickness (13 pm) about 10 times larger than that without laser irradiation (1-2 pm), which might be attributed to the steep temperature gradient supporting the diffusion of He. This surface modification combined with laser-induced surface roughening, as observed in Section 4.17.4.1.2 for typical thermal shock loads, leads to an enhanced degradation of the thermal diffusivity of W, which further increases the surface roughness and results in local or full melting of the

W surface.199

4.17.5 Conclusion

Along with other favorable properties, tungsten is characterized by the highest melting temperature among all metals, a low energy threshold for sputter­ing, and a low tritium inventory compared to carbon — based materials. These characteristics make tungsten the most promising material for the plasma-facing inner wall of future nuclear fusion devices based on the magnetic confinement principle, and it is also under consideration for inertial fusion applications. Accordingly, it has been selected as the PFM for a large part of the ITER divertor during its start-up phase and will be used for the full divertor as soon as tritium operation starts; in addition, it is the reference material for DEMO.

However, tungsten also offers less favorable prop­erties. Related to these, there are some material issues that have to be resolved before operating tungsten in a fusion environment in an economically reasonable way, which means in DEMO and beyond. These are

• recrystallization, which influences the mechanical properties by reducing the ductility and increasing the DBTT

• embrittlement as a result of neutron-induced damages and transmutation

• resistance to crack formation, depending on the mechanical properties, which is particularly important during transient thermal loads

• He-induced sputtering and modification of a thin surface layer, which is influenced by existing mate­rial damage as well as by temperature and temper­ature gradients, for example, those occurring during transient thermal loads

• melting, which is related to crack formation and the degradation of thermophysical properties as a result of He-irradiation-induced surface modifica­tion; melt splashing and droplet ejection will influ­ence the stable operation of the fusion plasma.

As all grades of tungsten investigated so far have their own individual drawbacks, R&D programs world­wide are aiming for a deeper understanding of the parameters that influence the degradation of tung­sten, and the development of new tungsten grades that are capable of dealing with the above-mentioned requirements. Therefore, the materials are character­ized and qualified with regard to their microstructure before and after recrystallization by

• mechanical tests: evaluation of the material’s strength and DBTT

• thermal shock loading: determination of tempera­ture — and power density-dependent damage, cracking and melting thresholds, which are related to the mechanical and physical properties

• thermal fatigue loading: evaluation of the material’s performance as part of an actively cooled component

• neutron irradiation: characterization of the degrada­tion of the material’s strength and the DBTT as well as the thermal shock and thermal fatigue response

• He — and H-irradiation: determination of the damage mechanisms such as blister, void, and bubble forma­tion as a function of ion energy, fluence, and temper­ature as well as addressing hydrogen retention issues.

However, despite all these efforts, a clear answer on the suitability of tungsten for application in a real fusion environment can only be given by ITER, as it is the mutual interaction of all the different types of loading that determine its lifetime relative to the various material degradation mechanisms.