Molecular dynamics simulation of beryllium oxide irradiated by deuterium ions: sputtering and reflection

Abstract

The sputtering and reflection properties of wurtzite beryllium oxide (BeO) subjected to deuterium (D) ions bombardment at 300 K with ion energy between 10 eV and 200 eV is studied by classical molecular dynamics. Cumulative irradiations of wurtzite BeO show a D concentration threshold above which an 'unphysical dramatic' sputtering is observed. From the cumulative irradiations, simulation cells with different D concentrations are used to run non-cumulative irradiations at different concentrations. Using a D concentration close to the experimentally determined saturation concentration (0.12 atomic fraction), the simulations are able to reproduce accurately the experimental sputtering yield of BeO materials. The processes driving the sputtering of beryllium (Be) and oxygen (O) atoms as molecules are subsequently determined. At low irradiation energy, between 10 eV and 80 eV, swift chemical sputtering is dominant and produces mostly ODz molecules. At high energy, the sputtered molecules are mostly BexOy molecules (mainly BeO dimer). Four different processes are associated to the formation of such molecules: the physical sputtering of BeO dimer, the delayed swift chemical sputtering not involving D ions and the detachmentinduced sputtering. The physical sputtering of BeO dimer can be delayed if the sputtering event implies two interactions with the incoming ion (first interaction in its way in the material, the other in its way out if it is backscattered). The detachment-induced sputtering is a characteristic feature of the 'dramatic' sputtering and is mainly observed when the concentration of D is close to the threshold leading to this sputtering regime.

Sputtering of beryllium oxide by deuterium at various temperatures simulated with molecular dynamics

“…For these interactions, D atoms are considered as H atoms with a mass of 2.014 atomic mass units. The Be-Be and Be-H potential are both taken from [12] (version II), the H-H potential is taken from [13], the O-O is taken from [14], the Be-O is taken from [15] and the O-H part is taken from [8]. The full Be-O-H potential was previously shown to give good agreement between simulated and experimental sputtering yields [8], and is therefore well-suited for plasma-wall interaction simulations.…”

“…In [8], it is shown that the sputtering yield increases with c D in the material. The best comparison between MD and experimental sputtering yields is obtained at 300 K for c D ≈ 0.12 atomic fraction (at.fr.)…”

“…same as in [8]: a wurtzite BeO cell with a (0001) surface is irradiated with ion energies of 10 eV, 30 eV, 50 eV, 80 eV, 140 eV and 200 eV. The irradiated temperatures are 400 K, 500 K, 600 K, 700 K and 800 K. The sputtering data at 300 K are taken from [8] and have been obtained with the exact same procedure. The cell is 25×24Å 2 in the (x,y) plane and is elongated in the z direction (Z=68Å for E inc < 80 eV and Z=104 A for ≥80 eV).…”

“…Each irradiation lasts for 7000 fs and prior to any irradiation, the box is randomly shifted over the periodic boundaries in the x and y direction to simulate a uniform bombardment. More details on the simulation set-up (relaxation of the box, temperature control, incident angle) can be found in [8].…”

“…In this work, we use molecular dynamics (MD) to estimate the sputtering yields of BeO by D ions. In a previous study, MD has been used to determine these sputtering yields at 300 K [8]. During tokamak operations, the temperature of the first wall can rise above 300 K. Here, the MD simulations have been extended to provide estimations of sputtering yields of BeO up to 800 K.…”