Influence of diffusion on W sputtering by carbon

This paper concerns with temperature dependence of tungsten (W) sputtering by carbon (C). Weight loss measurements of W samples during sputtering by 2.4 keV C ions at different temperatures in the range between 300 and 1000 K showed a strong temperature dependence. At a certain fluence, which depends on the temperature, the probe's weight starts to increase again. This is due to C enrichment in the surface during ion bombardment, which eventually leads to the formation of a pure C surface. This C film then shields the underlying W from further sputtering and further growth of this film increases the probe's weight. The growth of this film depends critically on the diffusion of C into the W bulk material since this removes C from the surface and thereby increases the W concentration and W erosion. This in turn results in an increased reflection of the incoming C ions. These two coupled mechanisms hinder the growth of the C film with the diffusion accounting for the temperature dependence of the weight loss. To verify this model, C depth profiles in W were determined by means of Rutherford backscattering (RBS) with He ions. To increase sensitivity, the increased cross-section at 5715 keV was used. Analysis of depth profiles measured in the temperature range between 300 and 1000 K show that relevant diffusion processes appear at approximately 800 K. By performing Boltzmann Matano analysis on the measured depth profiles, a concentration dependant diffusion coefficient for C diffusion in W was deduced. The weight loss measurements were simulated using a combination of the Monte Carlo program TRIDYN and the diffusion code PIDAT.