4.2 This work’s contribution
4.2.3 Surface structure
The Be surface morphology, as the main wall material, can experience significant changes such as surface melting and erosion. The erosion may not be homogeneous leading to surface roughening. Moreover, deposition of D on the Be surface can cause surface amorphization at higher temperatures. In the present work, after only few D impacts, the surface was damaged and the Be atoms often had D atoms bound to them.
Figure 4.9 shows the influence of D irradiation and substrate sputtering on the Be surface. The morphology of the Be cell after 2000 impacts at different temperatures, for a particle flux of 2.02·1028 m−2s−1 (one impact/10 ps) are shown. On the surface
of the cells, Be-D chains can be seen. These loosely bounded atoms could sputtered easily through the next impacts. Also the D atoms at the surface can weaken the surface binding energy of Be atoms and make them easier targets for sputtering.
Figure 4.9: Surface structure of Be cell after 2000 impacts at different temperatures, for a particle flux of 2.02·1028m−2s−1(an impact/10 ps). The D atoms are represented by small light pink spheres and the Be atoms are the larger blue spheres.
A comparison of the structures highlights the increase in the Be erosion with the temperature.
On one hand, regarding the depth profile for Be atoms at 200 K, a D pile-up can be seen in the center of the cell, with a separate atomic layer containing mostly D2
molecules. The D concentration at these layers increases with increasing the D influ- ence.
On the other hand, with increasing temperature, the atomic motion at the surface increases, results in weaker atomic bonds, and thus larger erosion. The 1440 K tem- perature is close to the melting temperature of Be, causing the Be bond to break easily
and sputter or desorb. At this temperature, around half of the surface is eroded along the z direction as illustrated in Fig 4.9.
5
Summary
ITER is expected to be the first fusion reactor reaching ignition. Its performance is based on a deuterium (D)-tritium plasma. ITER’s success highly depends on un- derstanding the interaction between plasma particles and the wall materials of the reactor. Because of the variable conditions in the reactor, such as temperature and particle fluxes, the requirements for the wall materials vary with their location in the reactor. Plasma particles cause the wall materials to erode due to their high particle flux and energy. This erosion of the wall materials is not desirable not only because the walls get thinner, but also because the eroded particles cause radiation losses when entering the plasma.
One of the best candidates for the main wall of the ITER is Beryllium (Be) due to its low atomic number. Therefore, it is essential to understand the behavior of Be under reactor-relevant conditions and to obtain insight on how Be behave in fusion reactor first walls, such as identifying the sputtered species under D bombardment and quantifying the Be erosion both from wall life-time point of view and to explain the impurity transport, re-deposition and re-erosion patterns.
A wide range of experiments have been performed on Be exposed to D plasma mainly in linear plasma devices, where the exposure conditions can be controlled. A broad database for the sputtering yields of Be is provided by these experiments. Un- fortunately, experiments are not always able to describe the underlying mechanisms completely. Hence, modeling is necessary for a complete description of the system. Molecular dynamics (MD) simulations can be a powerful tool to study many-body effects, such as chemical erosion and molecule formation. This work presents a MD study on the Be erosion by D irradiation, under fusion relevant conditions, scanning over the plasma flux (1027− 1028m−2s−1) and surface temperature (200 - 1440 K) at
low irradiation energy (50 eV).
swift chemical sputtering mechanism. The sputtering yields show little dependence on the D flux range studied here, but strongly vary with the surface temperature. The Be sputtering yield increased rapidly at temperatures above 600 K, as does the Be molecule sputtering yield. A large fraction of sputtered Be is for different Be molecules, mainly BeD or BeD2, but some larger ones are also observed.
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