Effect of sputtering on self-damaged recrystallized W mirror specimens

Abstract

The effect of heavy sputtering and of neutron irradiation simulated by displacement damaging with of 20 MeV W 6+ ions on the optical properties of tungsten mirrors was studied. Ar + ions with 600 eV of energy were used as imitation of charge exchange atoms ejected from fusion plasma. The ion fluence dependence of the surface topography and the optical properties of polycrystalline, recrystallized tungsten (grain size 20 -100 µm) were studied by optical microscopy, interferometry, reflectrometry and ellipsometry. Furthermore, after sputtering in total a layer of 3.9 μm in thickness, the orientation and the thickness of the eroded layer of many individual grains was determined by electron backscattering diffraction and confocal laser scanning microscopy. Concluding from the obtained data the neutron irradiation, at least at the damage level would be achieved in ITER, has not to make an additional contribution in the processes developing under impact of charge exchange atoms only.

Effect of sputtering on self-damaged ITER-grade tungsten

“…The independence of normal incidence reflectance degradation under long-term sputtering on whether W mirror specimens were pre-irradiated with 20 MeV W ions or not [12], is in agreement with previously published results on sputtering of copper and stainless steel mirror specimens pre-irradiated to varied doses with Cu and Cr ions of 3 MeV energy, respectively [11], as well as with recently published data for recrystallized tungsten [7].…”

“…In agreement with the results of [7] the areas composed dominantly of grains with orientation close to (110) have highest sputtering rate, forming "valleys", and the areas composed dominantly of grains with orientation close to (100) have much lower sputtering rate forming the main parts of "ridges" with "peaks" composed of grains close to (111) orientation having lowest sputtering rate. Such surface relief is very much different from the relief of polycrystalline mirror specimens being sputtered under similar conditions [7,10,11,14], and this is a consequence of a quite complicated technology procedures used for preparation of this particular tungsten kind. Note that surface peculiarities due to sputtering are qualitatively similar for undamaged and damaged specimens, independently on the dose (0.3 or 3.0 dpa).…”

“…5(b), the height difference between lowest and highest levels reaches ~1.8 μm when the layer of ~3.9 μm was sputtered in average (weight loss measurements). In accordance with the EBSD results on the surface of recrystallized tungsten specimens [7], in "valleys" the grains with (110) orientation are predominant, and in "ridges" -the (100) grains with "peaks" corresponding to (111) orientations.…”

“…The size of these areas exceeds (by more than one order of magnitude) the size of individual grains. The sputtering yield depends on the orientation [7]. Therefore, the obtained relief consisting of "ridges" and "valleys" corresponds to these areas of closely related orientation.…”

“…The irradiated area (face side) was a circle with 8 mm in diameter. The calculated dose depth profile (shown in [7], Fig. 1) has a maximum (0.3 or 3.0 dpa) near 1.4 μm and the maximal ion range was ~2.2 μm; the details of calculations can be found in [8].…”