Besides He, hydrogen isotopes, particularly the fuel elements deuterium and tritium, are the main inci­dent ion species contacting PFMs and PFCs. The energy of these particles corresponds with the plasma temperature at the edge, which is in the range of some eV, but also includes highly energetic particles (<10keV) escaping from the inner core of the

(g)

Figure 10 Cross-sectional scanning electron microscopic images for nine different grades of W relevant to fusion engineering practice. All target specimens were exposed to consistent pure He plasmas at 1120 K for 1 h. The He+ impact energy was ~40eV; (a) PLANSEE stress-relieved W, (b) single crystal (100) W, (c) ITER ASTM B760 compliant W,

(d) PLANSEE W-5% Re, (e) PLANSEE W-1% La2O3, (f) ultrafine-grained W-1.5% TiC, (g) ULTRAMET CVD W-10% Re,

(h) VPS W (EAST), and (i) recrystallized W. Reproduced from Baldwin, M. J.; Doerner, R. P. J. Nucl. Mater. 2010,404,165-173, with permission from Elsevier.

plasma. The impact of the energetic hydrogen ions is influenced by the incident ion energy, the ion flu — ence, the temperature, and the material’s composition and microstructure. The resulting damage, that is, vacancy formation, vacancy clustering, bubble formation, and blistering, determine not only the amount of material degradation and erosion but also the hydrogen (tritium) retention in the material. For active control of the hydrogen retention, short-term thermal treatments of the surface are being investi­gated. However, the short thermal load required to effectively remove the deuterium and tritium may also destroy the thin material layer (nm to low-pm range) that is responsible for the majority of the

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retention.