Helium effects on tungsten under fusion-relevant plasma loading conditions

Abstract

Experiments were performed in the Pilot-PSI linear plasma device to study the formation of helium-induced nanostructures under high heat fluxes (>10MW.m-2), the erosion of a tungsten surface at the elevated temperatures where those structures typically form and most importantly the behaviour of a helium-induced fibreform structure during ELM-like pulses. A strong correlation between the characteristic size of the helium-induced morphology and the size of the voids observed in the near-surface region has been found, and both increase in size with increasing temperatures. Temperature-dependent erosion of tungsten surfaces was observed despite the ion energy being below the sputtering threshold, with an exponential increase of the erosion for temperatures higher than 2400 o C. The same effect was observed during ELM-like plasma pulses. Finally, a complete disappearance of the tungsten fibreform structure was observed after only one plasma pulse for energy densities higher than 0.5MJ.m-2 .

From Field Emission to Vacuum Arc Ignition: A New Tool for Simulating Copper Vacuum Arcs

“…A plausible mechanism that could cause such an increased sputtering is a combination of electric field [55] and thermally-enhanced [56] sputtering, since the arc induces both a high field and a high temperature at the surface. Studies are ongoing to determine the exact yield and threshold for the ion energies reported in this paper.…”

Nanostructured fuzz growth on tungsten under low-energy and high-flux He irradiation

“…In a fully three-dimensional W lattice, the {101} planes have higher areal density and larger spacing than crystal planes in other directions. Our DFT calculation predicts that He atoms prefer to form mono-layer He clusters between {101} planes in single crystal W. The formation of He clusters or defects can be strongly dependent on He + fluence and W surface temperature3456. Due to favorable energetics, a number of He clusters can be formed between {101} planes with increasing He accumulation at elevated temperature (1400 K), resulting in He-enriched strips, as shown in Fig 5(a).…”

“…The bombardments of W by the high-flux and low-energy He ions often lead to serious irradiation damages, such as the formation of voids, wave-like microstructures, or nanostructured fuzzes (nano-fuzzes) at the surface3456. The growth of nano-fuzzes in W is a major concern because it may cause serious etching of W surface and shorten the lifetime of the W components in ITER.…”

Competing Kinetics and He Bubble Morphology in W

“…This growth rate can be associated to an impinging He flux by considering the fraction of incoming ions that reach the bubble [20]. The slowest growth rate considered in this work corresponds to a flux of approximately 1.2 × 10 24 He m −2 s −1 , which is on the order of magnitude expected at ITER [34]. An initial bubble is created by placing eight He atoms inside a preexisting W vacancy.…”