MD and BCA simulations of He and H bombardment of fuzz in bcc elements

Abstract

a b s t r a c tWe present results of MD simulations of low energy He ion bombardment of low density fuzz in bcc elements. He ions can penetrate several micrometers into sparse fuzz, which allows for a sufficient He flux through it to grow the fuzz further. He kinetic energy falls off exponentially with penetration depth. A BCA code was used to carry out the same ion bombardment on the same fuzz structures as in MD simulations, but with simpler, 10 million times faster calculations. Despite the poor theoretical basis of the BCA at low ion energies, and the use of somewhat different potentials in MD and BCA calculations, the ion penetration depths predicted by BCA are only~12% less than those predicted by MD. The MD-BCA differences are highly systematic and trends in the results of the two methods are very similar. We have carried out more than 200 BCA calculation runs of ion bombardment of fuzz, in which parameters in the ion bombardment process were varied. For most parameters, the results show that the ion bombardment process is quite generic. The ion species (He or H), ion mass, fuzz element (W, Ta, Mo, Fe) and fuzz element lattice parameter turned out to have a modest influence on ion penetration depths at most. An off-normal angle of incidence strongly reduces the ion penetration depth. Increasing the ion energy increases the ion penetration, but the rate by which ion energy drops off at high ion energies follows the same exponential pattern as at lower energies.

A review of late-stage tungsten fuzz growth

“…Since fuzz layers have been found to have around 90% porosity, Klaver et al [47,49] proposed that He + ions may be able to penetrate much further than the several nanometres expected. They observed that tendrils favour growing perpendicular to the W surface, leading to open channels that He could scatter down.…”

“…The MD calculations were repeated with the computationally simpler binary collision approximation (BCA) approach [49], where ion movement is treated as a sequence of collisions. BCA produced similar results for all but the lowest ion energies, where the approximation is inaccurate.…”

“…Figure 5 shows the cylinders used in Ref. [50] to build the structure, whilst [47,49] used ellipsoids instead. Determination of the complete structure would allow much more accurate modelling to be done, so long as the representations of the structure can be constructed in a way that do not increase computing times.…”

Pore sealing mechanism in OSG low‐k films under ion bombardment

“…Currently, computer simulation can be considered as an effective tool to give an appropriate interpretation of obtained experimental results and to reveal the mechanisms of the processes under investigation. The Monte Carlo (MC) and molecular dynamics (MD)‐based modeling were applied to study effects of the surface roughness and the presence of nanosized structural objects on the sputtering yield, and some important features of these phenomena were explained . To investigate structural changes in nanoporous materials due to ion bombardment, an atomistic approach is needed as it is necessary to analyze trajectories of all atoms, to take into account the energy deposited by incident ions and the excess energy of atoms located on pore walls as well as to consider the possibility of phase transitions caused by the fast energy release.…”

Triple Hybrid Simulation Method for Tungsten Fuzzy Nanostructure Formation

“…Molecular dynamics (MD) [10][11][12][13][14][15][16][17][18][19][20][21][22][23], the binary collision approximation (BCA) [21,24,25], and several models [26,27] have been employed in simulations aimed at understanding the formation of fuzzy nanostructures. In particular, multi-scale simulations [28] are necessary to treat such complex processes as helium bubble formation and the growth of fuzzy nanostructures.…”