Late in 2016 the international experimental fusion reactor ITER is planned to become operational. One of the technical issues is the removal of the fusion ashes in the divertor. The divertor's surface is expected to be subjected to hydrogen atoms of energies in the order of 1 to 10 eV and particle fluxes of 1024/m2s. The surface temperature is expected to be around 1000 K. A possible material choice is carbon, since it can withstand the high heat flux. However, previous experiments and theoretical studies show that carbon
can retain large amounts of hydrogen. This is a problem for the operation of ITER, where no more than 350 g of tritium must be present in the entire device.
The fundamental processes involved in the interactions between hydrogen and carbon under these conditions are not well understood yet. To gain more insight, the retention of hydrogen in a diamond substrate is investigated using molecular dynamics (MD) simulations.
We use an updated version of Cameron Abrams' MD code (Abrams and Graves 1999) for our simulations. The inter-atomic forces are calculated by using the empirical Brenner-Beardmore potential energy function with parameter set II of the Brenner potential (Brenner 1990, 1992; Beardmore 1996). The Berendsen heat bath (Berendsen, 1984) serves as thermostat.
The conditions are chosen to be comparable to those expected in the ITER divertor: the hydrogen energy ranges from 0.1 to 50 eV and the surface temperature is in the order of 1000 K. For the hydrogen flux two values are chosen, one where the surface can return to thermal
equilibrium before the impact of the next H-atom, and one where the surface is still out of thermal equilibrium when the next H-atom hits.
The results show that the number of retained hydrogen atoms has a minimum at the H-atom energy where the transition from surface interaction to surface penetration takes place. For high surface temperatures and for high flux values less hydrogen is retained than in cool surfaces and under low flux. |