Abstract
Tungsten (W) is considered a promising plasma-facing material for protecting the divertor of the ITER (International Thermonuclear Experimental Reactor). The effects on W of transient thermal loads of high energy occurring in a tokamak under operative conditions have been simulated through a single laser pulse delivered by an Nd:YAG laser. Bulk and plasma-sprayed (PS) samples have been submitted to tests and successively examined via SEM (scanning electron microscopy) observations. In both types of materials, the laser pulse induces similar effects: (i) a crater forms in the spot central area; (ii) all around the area, the ejection and the movement of molten metal give rise to a ridge; (iii) in a more external area, the surface shows plates with jagged boundaries and cracks induced by thermal stresses; (iv) the pores present in the original material become preferred ablation sites. However, the affected surface area in PS samples is larger and asymmetric if compared to that of bulk material. Such a difference has been explained by considering how microstructural characteristics influence heat propagation from the irradiated spot, and it was found that grain size and shape play a decisive role.
Highlights
W has excellent thermo-mechanical properties, high thermal conductivity, low physical sputtering rate, low tritium inventory, and the highest melting point among all metals, so it is considered a promising plasma-facing material for the armors protecting the divertor of the ITER (InternationalThermonuclear Experimental Reactor) [1,2,3,4,5,6]
The main main effect of the laser shot on bulk W is the formation of a crater with a conical shape, with a effect of the laser shot on bulk W is the formation of a crater with a conical shape, with a diameter diameter of about 75 μm (Figure 5a,b), namely a little smaller than the laser spot, and a depth of of about 75 μm (Figure 5a,b), namely a little smaller than the laser spot, and a depth of about 25 μm
The Gaussian pulse shape implies that the intensity is maximum maximum at the center and decreases towards the periphery with a temperature gradient, which at the center and decreases towards the periphery with a temperature gradient, which leads to liquid leads to liquid movement along the radial direction, resulting in the formation of a ridge around the movement along the radial direction, resulting in the formation of a ridge around the crater
Summary
W has excellent thermo-mechanical properties, high thermal conductivity, low physical sputtering rate, low tritium inventory, and the highest melting point among all metals, so it is considered a promising plasma-facing material for the armors protecting the divertor of the ITER Much work has been devoted to investigate suitable coating technologies and plasma spray (PS) in vacuum [2,9], physical vapor deposition (PVD) [10,11], and chemical vapor deposition (CVD) [12]. Yahiro et al [13] investigated air plasma spray (APS) comparing the characteristics of coatings with those obtained through PS made in vacuum. Metals 2017, 7, 454 to 5 MW·m−2 in conditions relevant for fusion reactor plasma-facing components [2]. The effects of the laser pulse have been examined by scanning electron microscopy (SEM) observations and discussed on the basis of the specific microstructural features of the two types of W considered in the experiments
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