Reflection and implantation of low energy helium with tungsten surfaces

Abstract

Reflection and implantation of low energy helium (He) ions by tungsten (W) substrate are studied using molecular dynamics (MD) simulations. Motivated by the ITER divertor design, our study considers a range of W substrate temperatures (300 K, 1000 K, 1500 K), a range of He atom incidence energies ($\le$100 eV) and a range of angles of incidence ($0^{\circ}$-$75^{\circ}$) with respect to substrate normal. The MD simulations quantify the reflection and implantation function, the integrated moments such as the particle/energy reflection coefficients and average implantation depths. Distributions of implantation depths, reflected energy, polar and azimuthal angles of reflection are obtained, as functions of simulation parameters, such as W substrate temperature, polar angle of incidence, the energy of incident He, and the type of W substrate surface. Comparison between the MD simulation results, the results obtained using SRIM simulation package, and the existing experimental and theoretical results is provided.Comment: 51 pages, 6 tables, 27 figures. Submitted to Journal of Nuclear Material

Triple Hybrid Simulation Method for Tungsten Fuzzy Nanostructure Formation

“…The reflection ratio is 0.73 to 0.78 at the incident energy of 50 eV, and variations in the reflection ratio are smaller than 5 percent. The present reflection ratio, which is derived from the BCA part of the BDoG code, agrees with a BCA simulation using the MARLOWE code [41], with experiments [41], and with a MD simulation [42]. Sputtering, in which a tungsten atom is expelled from the surface of the target material, did not occur in the present simulation.…”

Nanoscale modification of silicon and germanium surfaces exposed to low-energy helium plasma

“…These differences indicate that the nanostructures form as a result of a dynamic equilibrium between plasma-driven nanostructure growth and temperature-driven surface relaxation. The exact reason for this difference has not yet been determined, but it could be related to a reduction in the proportion of helium that is reflected from the sample surface at higher implantation energies 15 , which in turn would increase the equilibrium concentration of helium within the material during irradiation.…”

“…At higher temperatures adatoms are able to overcome the Ehrlich-Schwoebel energy barriers, leading to first the formation of larger surface features, and later surface recovery towards favored crystallographic orientations via back-diffusion of adatoms once the diffusion barriers can be overcome 23 . Nanostructure formation would be enhanced by increasing the helium flux, or by increasing the incident helium energy to reduce the fraction of helium ions that are reflected from the surface at these low energies 15,25 .…”

An Overview of Recent Standard and Accelerated Molecular Dynamics Simulations of Helium Behavior in Tungsten

“…Borovikov et al [7] studied this topic via MD simulations, significantly extending the scope of previous computational efforts [8] by considering the effect of temperature in the substrate (300 K, 1000 K, and 1500 K), incidence energy Ei (≤100 eV), deposition angles θi (0–75∘), and substrate surface orientation ((100), (110), and (310)). The interaction between W atoms was determined by an Ackland–Thetford potential [9], modified at short distances by Juslin and Wirth [10].…”

“…Given a substrate temperature and orientation and a deposition angle θi≤75∘, Borovikov et al [7] showed that an increase of impact energy (Ei>10 eV), in general, implies a decrease of R and RE and an increase of L . For θi=75∘, all the atoms were reflected.…”