Structural response of transient heat loading on a molybdenum surface exposed to low-energy helium ion irradiation

Abstract

The advancement of fusion reactor engineering is currently inhibited by the lack of knowledge surrounding the stability of plasma facing components (PFCs) in a tokamak environment. During normal operation, events of high heat loading occur periodically where large amounts of energy are imparted onto the PFC surface. Concurrently, irradiation by low-energy helium ions present in the fusion plasma can result in the synthesis of a fibre form nanostructure on the PFC surface, called ‘fuzz’. In order to understand how this heterogeneous structure evolves and deforms in response to transient heat loading, a pulsed Nd:YAG millisecond laser is used to simulate these events on a fuzz form molybdenum (Mo) surface. Performance was analysed by three metrics: nanostructure evolution, particle emission, and improvement in optical properties. Experiments performed at the upper end of the expected range for type-I edge-localized modes (ELMs) found that the helium-induced nanostructure completely disappears after 200 pulses of the laser at 1.5 MJ m−2. In situ mass loss measurements found that the amount of particles leaving the surface increases as energy density increases and the rate of emission increases with pulse count. Finally, optical properties assisted in providing a qualitative indication of fuzz density on the Mo surface; after 400 pulses at 1.5 MJ m−2, the optical reflectivity of the damaged surface is ~90% of that of a mirror polished Mo sample. These findings provide different results than previous studies done with tungsten (W), and further help illustrate the complicated nature of how transient events of high heat loading in a tokamak environment might impact the performance and lifetime of PFCs in ITER and future DEMO devices (Ueda et al 2014 Fusion Eng. Des. 89 901–6).

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

“…A QCM presents an in situ alternative to measure the mass loss per pulse. Results obtained in 29 demonstrated the viability of the QCM as a possible tool to measure material loss, yielding consistent results with SEM imaging and optical reflectivity measurements. Work done in 51 further discuss the use of a QCM as a mass sensor.…”

“…Energy density dependent testing was done by exposing mirror-finished Mo samples to 100 pulses of 1064 nm Nd:YAG millisecond (ms) laser irradiation at increasing intensities, from 0.6 MJ m −2 to 1.8 MJ m −2 . The ms laser utilized a pulse duration of 1 ms and a repetition rate of 1 Hz, to match expected heat loading conditions during an unmitigated type-I ELM event 29 . A schematic of the experimental setup can be found in Fig.…”

Surface Morphological Features of Molybdenum Irradiated by a Single Laser Pulse

“…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].…”

Tuning surface porosity on vanadium surface by low energy He+ ion irradiation

“…This study is also significant in the understanding of microstructure evolution and fuzz formation on high-Z refractory metals for fusion applications. As with W [8,9,[36][37][38][39], Mo [40][41][42], Ta [43], and Nb [44], the He + ion-irradiated V surface could also have a weakness against plasma and/or ion heat flux. Therefore, such V nano-and submicron-structures may not be favorable for protecting the V surface from possible melting and erosion.…”