Laser Pulse Effects on Plasma-Sprayed and Bulk Tungsten

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.

Surface Morphological Features of Molybdenum Irradiated by a Single Laser Pulse

“…The effects of the laser pulse on the sample surface have been then investigated through 3D surface analysis and scanning electron microscopy (SEM) observations. The same experimental procedure was adopted in previous works of present authors to study the behavior of W (bulk and plasma sprayed) [21,22] and W-1%La 2 O 3 alloy [23]. The results are discussed in comparison to those previously obtained by us on W, the principal candidate material for building the armours of ITER, and in general to literature data.…”

“…From this calculation, the ablated volume in the case of Mo is about ten times that of W. This strictly depends on the latent heat of fusion EM and latent heat of vaporization EV of W, which are remarkably higher than those of Mo (see Table 1). After melting, the liquid metal is flushed to the periphery, and its viscosity and surface tension determine the morphological features of the crater and surrounding zones, which are different from those of W because in that case [21,22] solidified drops decorating the crater walls have been observed but the long filaments displayed by Mo have not (see Figure 5).…”

“…After melting, the liquid metal is flushed to the periphery, and its viscosity and surface tension determine the morphological features of the crater and surrounding zones, which are different from those of W because in that case [21,22] solidified drops decorating the crater walls have been observed but the long filaments displayed by Mo have not (see Figure 5).…”

“…The experiments were carried out in a vacuum chamber (P ≈ 10 −5 bar), and the laser beam hits the samples with an incidence angle of 45 • to minimize debris projection near the target. More details about the experimental set-up are reported in previous works [21][22][23] on W (bulk and plasma sprayed) and W-1%La 2 O 3 alloy, which were irradiated in the same conditions.…”

Surface Plasmon Nanolaser: Principle, Structure, Characteristics and Applications

“…At the end of the 20th century, the rapid development of nano-materials and nano-technology had a profound influence on the fields of information and materials, providing conditions for the miniaturization of lasers. However, conventional semiconductor lasers are subject to the diffraction limit, and so their size cannot be less than half a wavelength, hindering the miniaturization of lasers [7][8][9][10][11][12][13].…”