Temperature dependent surface modification of molybdenum due to low energy He+ ion irradiation

Tailoring molybdenum nanostructure evolution by low-energy He+ ion irradiation

“…To see the effect of fluence and flux, beyond the temperature window [30], fluence-and flux-dependent studies have also been performed at 1223 K. 2.6 × 10 24 ions m −2 fluence (7.2 × 10 20 ions m −2 s −1 flux) irradiation induces uniformly distributed well-defined circular nanodiscs however, the same fluence but a flux of 1.2 × 10 21 ions m −2 s −1 deteriorates the Mo surface in netlike nanomatrix network structure (having ∼105 nm nanowall width), in patches (Fig. 6).…”

“…Base pressure of the ultra-high vacuum (UHV) chamber was 6.0 × 10 −8 torr, although the pressure during ion irradiation was 2.0 × 10 −4 torr. The Mo sample temperatures 923 and 1223 K were chosen as favorable and nonfavorable target temperatures, respectively, for Mo fuzz formation, as observed in our previous report [30]. For precise temperature measurement, we used thermocouples rather than radiation thermometers.…”

“…The selected range of ion fluence was 2.1 × 10 23 to 1.1 × 10 25 ions m −2 , which was based on the estimated fluence ranges of 1.0 × 10 23 -1.0 × 10 24 ions m −2 for ITER-like conditions [28,29]. Selection of a constant target temperature for these experiments, 923 K, was based on our recent study in which we reported a clear temperature window of 823-1073 K for Mo fuzz formation [30]. Similar temperature window has also been observed by Takamura [31] where he has shown that at the upper boundary temperature, Mo fuzz growth competes with shrinkage due to plasma annealing and at the lower boundary temperature; the speed of growth is very low.…”

“…On the other hand, although the majority of the surface structure as observed for 1.2 × 10 21 ions m −2 s −1 ion-flux did not change significantly after fluence was increased by a factor of 3.3 (for constant flux of 1.2 × 10 21 ions m −2 s −1 ), average nanowall width decreases significantly to as low as ∼45 nm. Although there have been several reports on similar topics [27,[33][34][35][36][37][38], the majority of these are on W refractory metal; only a few reports, by us [30] and others [31] discuss low-energy He + ion-irradiation-induced surface modification on Mo refractory metal as a function of temperature, but none is on fluence-and flux-dependent Mo surface deterioration. As with W [39], the He + ion-irradiated Mo surface could have a weakness against plasma and/or ion heat flux.…”

Temperature-dependent surface modification of Ta due to high-flux, low-energy He+ ion irradiation

“…On the other hand, a simultaneous sequential enhancement in the large pore density, i.e., 6.1 (923K) to 8.2 (1023K) to 61.2 (1123K) to 73.5 pores µm -2 (1223K) was also observed (Fig 1). A summary of both small and large pore density variation as a function of target temperature is shown in Figure 1 high-Z metals such as W [14] and Mo [15][16][17] where clear temperature windows and lower ion fluence thresholds for nanoscopic filaments and/or fibers, commonly known as fuzz growth, were observed at similar fluxes and fluences of low-energy He + ion radiation. Therefore, it seems that under high-flux, low-energy He + irradiation conditions studied in this work, Ta is free from the formation of fine nano-tendril structures that are observed in Mo irradiated under similar conditions [15][16].…”