Melt layer erosion during ELM-like heat loading on molybdenum as an alternative plasma-facing material

Abstract

Transient events that occur during plasma instabilities in fusion reactors impart large heat fluxes onto the surrounding plasma-facing components (PFCs). Erosion and splashing of PFCs can contaminate the plasma and shorten material lifetime. Although tungsten is currently considered the most promising candidate material for future PFCs, concerns over the thermal shock performance during type-I ELMs (transient events expected in fusion devices) necessitate the study of other comparable materials. ELM-like heat loading was applied via a pulsed Nd:YAG millisecond laser on a pristine molybdenum (Mo) surface to measure surface melting and mass loss. One potential advantage of Mo is its higher specific heat of vaporization, which could lead to reduced particle emission. Imaging of the surface after loading revealed that complete surface melting began at 1.0 MJ m−2 (heat load parameter of 31.62 MJ m−2 s−1/2). Photon excitation also increased significantly above 1.0 MJ m−2, indicating possible phase change. At 1.4 MJ m−2 (44.27 MJ m−2 s−1/2), in situ mass loss measurements found an exponential increase in particle emission, indicating the presence of droplet formation and boiling. Direct comparisons of erosion during pulsed heat loading between PFC candidate materials will ensure that future fusion devices design components with optimal thermal strength.

Surface Morphological Features of Molybdenum Irradiated by a Single Laser Pulse

“…Figure 5b displays a drop of the ejected metal fallen and re-solidified in a zone near the crater. Sinclair et al [16] demonstrated that the surface melting of Mo begins when the energy density reaches a value of ~1.0 MJ•m −2 and that the droplet formation and boiling starts from ~1.4 MJ•m −2 , namely values much lower than those involved in the present experiments.…”

“…Recently, molybdenum (Mo) has attracted increasing interest as a possible PFM alternative to W [12][13][14][15][16]. As shown in Table 1, Mo is a high-Z refractory metal with very good physical properties (density, melting point, boiling point, and thermal conductivity), even if it is a little inferior to W. On the contrary, the ductile to brittle transition temperature (DBTT) of Mo is much lower, and it exhibits a better resistance to thermal shocks [15][16][17]. Another relevant advantage of Mo to W is its behavior under prolonged irradiation by 14 MeV neutrons of a fusion power plant.…”

Morphological and structural damage investigation of nanostructured molybdenum fuzzy surface after pulsed plasma bombardment

“…Rapid cooling will accumulate a lot of stress in the metal. Once the surface faces even higher power-density transient plasma, when the surface stress reaches the critical limit, the growing crystallite is no longer resistant to plastic deformation, [45] which is evidenced by observed widening cracks left in FE-SEM after pulsed load treatment.…”

OpenFOAM modeling of beryllium melt motion and splashing from first wall in ITER

“…The computational models were greatly refined and enhanced to account for many relevant physics effects and realistic conditions. The HEIGHTS simulation package [30] that consists of several integrated models has continued to be developed over the years and applied to many melting problems [39][40][41][42]. The MEMOS-1.5D code that implements the shallow water approximation [43,44] was upgraded to MEMOS-3D [45] and more recently to MEMOS-U [46].…”