Independent of the ion energy, blistering (see Figures 11 and 12) due to H occurs only during irradiation at temperatures below 900-950 K and as a function of the ion fluence199,260 at 500 K.261-263 This temperature dependence of blistering is attrib­uted to the formation, movement, and agglomeration
of vacancies containing trapped hydrogen,264 which is dominant at temperatures <500 K, while the detrap­ping of hydrogen from the defects is prevalent at temperatures >500 K.263

The fluence threshold for blister formation increases with decreasing ion energy and signifi­cantly increases to values >1025 D+ m~2 at ion ener­gies <20 eV. This is assumed to be the result of thin (a few monolayers) which act as oxide diffusion bar­riers at the material’s surface.265 Furthermore, with increasing fluence, the size and number of blisters can increase up to a few 100 pm260 until saturation is reached, which is assumed to be related to the rupture of blisters.265-267 The rupture and related dust formation is the result ofhydrogen accumulation and pressure build up, which is effectively released by the failure of the blister cap. However, whether the blister has vented or not, the thermal contact with the substrate has been significantly reduced (see Figure 11). This eventually results in melting or vaporization of the thin blister cap, particularly during transient thermal loading conditions as described in Section 4.17.4.1, which contributes to further materi­al’s erosion and probably plasma contamination.268

(a)

Large blisters

Small blisters

(b)

(b)

Large blisters

r>’

(c)

Crack/void along grain boundary

(c)

Figure 12 Scanning electron micrographs of small blisters appearing at tungsten exposed to a hydrogen fluence of 1026 D m~2 at 480 K (45° tilt). (a) Initial stage; (b) growing; (c) bursting. Reproduced from Shu, W. M.; Kawasuso, A.; Yamanishi, T. J. Nucl. Mater. 2009, 386-388, 356-359, with permission from Elsevier.

Figure 11 Scanning electron micrographs of tungsten exposed to a hydrogen fluence of 1026 D m~2 at 480 K (45° tilt). (a) Overall image; (b) cross-sectional image of a large blister; (c) internal image of small blisters. Reproduced from Shu, W. M.; Kawasuso, A.; Yamanishi, T. J. Nucl. Mater. 2009, 386-388, 356-359, with permission from Elsevier.

In contrast, from the point of view of H retention, blistering is favorable because tritium accumulates in blisters, which act as a diffusion barrier for hydrogen, and which can otherwise penetrate deep into the

material even at rt269-274 until it finally ends up in the heat sink structure and the coolant. Accordingly, as tritium retention is, in general, strongly correlated with the generation of blisters,275 it shows a maxi­mum at an irradiation temperature of about 500 K. , , , , However, the retention of

tritium and deuterium is also dependent on the trapping sites existing in the material. These are, in ascending order of their trapping potential, resid­ual impurities, from which slow desorption occurs even at RT,277 grain boundaries and dislocations,
radiation-induced vacancies and vacancy clusters, and pores. Depending on the occurrence and domi­nance of particular sites, the temperature at which the maximum hydrogen retention is observed varies between 450 and 850 K.138,262,264,268,277-281

With increasing temperature (>1000K), such as that occurring at the strike point of the divertor, and with the lack of blisters, continuously decreasing hydrogen retention is observed.2,255,278,282,283 The remaining amount of retained hydrogen might be attributed to the presence of hydrogen as a solute, which depends only slightly on the incident ion energy, but scales with the implantation fluence and which is assumed to be of the same order of mag­nitude as the trapped concentration. It decreases only slightly with increasing temperature and at 1600 K still amounts to about 10% of the initial hydrogen content retained at 300 K.274

In addition to blisters, the high amount of hydrogen out-gassing at temperatures of 873 K and above results

in the formation of bubbles and pores.253,277,284,285

This effect depends on the ion energy and fluence, which determines the amount and penetration depth of trapped hydrogen. Even though a beneficial smoothing effect on the surface quality is observed in comparison to pure annealing without hydrogen impact, at high temperatures up to 2500 K the surface smoothening might be accompanied by detrimental

crack formation.284