G. Veilleux

Preliminary observations of blistering of niobium by 1–15 keV helium ions

Abstract We have bombarded at room temperature polycrystalline targets of cold-rolled Marz grade niobium with beams of helium ions of 1, 5, 10 and 15 keV at doses varying from 0.1 to 10.0 C/cm 2 . We have chosen the doses so as to obtain surfaces where the thickness removed by sputtering is respectively less than, comparable to, and greater than the implantation depths. The targets were not further treated beyond mechanical polishing. The vacuum during bombardment was 2 × 10 −8 torr. Blisters of 0.1 to 3.0 μm size have been observed. The blister covered area increases with dose up to a point, but at the higher doses the blisters, if any, are masked by a micro-relief of wide valleys, presumably sputtering-induced, and tiny pock-marks which could be small ruptured helium blisters.

Inexpensive, quantitative hydrogen depth-profiling for surface probes

Abstract We present a novel nuclear microanalysis technique to profile hydrogen and deuterium in materials; it should be particularly useful in fusion technology. It uses a small 350 keV light-ion accelerator and an E × B filter (crossed electric and magnetic fields) whose optimization is discussed. The charge fraction measurements required are also briefly reported. The depth probed is 1000 A (H only) or 600 A (H and D simultaneously), the sensitivity 1 at%, improvable to 0.1%, and the resolution 50–200 A depending on depth (in Si). Examples of measured H profiles are discussed.

Evidence that helium irradiation blisters contain high‐pressure gas

Trapping of ∼20‐keV helium in Be, Cu, and Nb at fluences near the blistering threshold has been investigated by proton backscattering and by elastic recoil detection. Blister cavities at a depth close to the mean range contain up to 2.3×1017 He/cm2 (or more), corresponding to pressures in kbars, in agreement with gas pressure models. Depending on the metal, and its condition (annealing, stresses), the blisters may or may not rupture and release a large burst of helium. The re‐emission data of Bauer et al. agrees qualitatively with these trapping data.

Measurement of the depth distribution of light impurities in first-wall materials: He in Nb

Abstract We have measured the concentrations and depth profiles of implanted helium in niobium by a method demonstrated previously with hydrogen and lithium in copper. The three targets, bombarded at room temperature with 10 keV He+ at doses of 0.01, 0.16 and 0.98 C/cm+, were respectively: unblistered; covered with circular blisters; and marked with “microrelief”, without blisters. The corresponding doses retained in the metal were 0.0076, 0.039 and 0.052 C/cm2 (i.e.≈3 × 1022 He atoms/cm3) with a 10% normalization uncertainty. The profile shapes did not change much: in particular we did not observe, as the dose increased, an accumulation near the surface, which is receding by erosion (sputtering, blistering). These results show that a mechanism of helium loss starts operating at a dose ⩽0.16 C/cm2, i.e. before the bursting of blisters (if they burst at all), and it is most effective near the surface.

Dose rate and temperature effects on blistering phenomena in helium bombarded niobium

Abstract Blistering of niobium under bombardment with 5 to 15 keV He ions has been investigated. At these energies there is only one generation of blisters, which are sputtered and give way to a microrelief. A high dose rate of 3–5 mA/cm 2 has been used to find the energy and temperature dependance of the blistering cut-off effect. At 20° C this cut-off dose increases from about 1 to 3 C/cm 2 when the energy is increased from 5 to 15 keV. The temperature effect has been investigated with 10 keV ions. The cut-off dose decreased to about 0.2 C/cm 2 at 700° C. Since in a fusion device, the dose rate may be much smaller than the one used above, the dose rate effect has been investigated. With a dose rate reduced to 0.05 mA/cm 2 the critical dose for blistering and the cut-off dose have not been found to vary by any large amount.

Surface swelling produced in Nb and Ti-6A1-4V by He irradiation

Abstract Surfaces exposed to the plasma will become greatly swollen. It has already been established that this swelling has to be considered in the study of the phenomenon of blistering. Moreover it could affect other properties of these surfaces. Measurements of surface lifts in Nb at 300 K have already been reported. This paper extends these measurements at 773 K for different fluences and energies. For a given fluence the lift increases with temperature. The linear relationship between surface lift and fluence observed at 300 K up to the blistering threshold exists also at 773 K. This paper also reports measurements of surface lifts in Ti-6A1-4V at 300 and 773 K. The effect of the homologous temperature and mechanical properties are considered and comparison are made with literature data for Al, Cu, Ni and 304 stainless steel.

Tem Investigation of Hydrogen Implanted and Laser Annealed Silicon

In order to explain the relatively easy laser-induced desorption of hydrogen implanted in silicon, and particularly the lower temperature needed for desorption at higher implantation energy, the microstructural modifications produced by laser pulses were studied by means of transmission electron microscopy. The structural damage, such as defect clusters and hydrogen gas bubbles was observed. In the case of low dose implantation (H/SI {le} 15%), most of the bubbles were produced during laser annealing rather than during implantation. This bubble formation in the course of desorption explains the higher temperature needed. When blisters are already present on the as-implanted surface, desorption starts at a lower temperature.

LOW-ENERGY HELIUM BOMBARDMENT OF COPPER AND NIOBIUM: GAS DEPTH PROFILES MEASUREMENTS

The operation of Fusion reactors will result in the implantation of many isotopes of H, He, Li (Be?) in various components. To study their effects on materials, in particular, possible cooperative effects, the method of Elastic Recoil Detection for non-destructive simultaneous depth-profiling of all light atoms was developed. Firstly we have measured the depth profiles of He implanted in Cu at 1, 5, 10, 15, 20 and 25 keV, room temperature, 2 and low dose (≤ 0.01 C/cm 2 ), undisturbed by sputtering or blistering. The average depths are larger by a fairly constant 150–200 a than the theoretical values of Oen and Robinson (O.-R.). The doses retained are consistent with a picture where the reflection coefficient is in agreement with O.-R. and re-emission is nil at 10 keV and above, small at 5 keV (≈5%) and important (43%) at 1 keV, at the dose studied (4 × 10 −3 C/cm 2 ). Secondly we have measured the re-emission and profile shapes of He in Nb at 10 keV, room temperature, for doses of 0.01 C/cm 2 (no surface change), 0.16 C/cm 2 (abundant blistering), and 0.98 C/cm 2 (“micro-relief”). Again, profiles appear to be deeper than theory. Strong re-emission takes place at the middle and higher doses with near saturation of the He concentration.