Tungsten sputtering by simultaneous deuterium and nitrogen irradiation

In magnetic fusion devices, extrinsic impurities like nitrogen (N) are introduced in hydrogen plasmas to reduce the power load onto tungsten (W) plasma facing components. Evaluating the sputter characteristics of W under simultaneous irradiation of hydrogen isotopes and nitrogen species is required to quantify the effects of W-contamination on plasma confinement. Due to the rich chemistry involving hydrogen- and nitrogen- species, the W surface is expected to evolve in a complex manner with significant influence on the W sputtering. In this study, we report on systematic W sputtering experiments as a function of incident ion energy (300-1000 eV) and sample temperature (400-700 K) for fixed D/N ratio (3.5% N). Experimental W-sputtering yields are compared to the results of a dynamic BCA code (TRIDYN). In the energy range between 700-1000 eV, experiments and simulation results agree within 20% as a function of incident ion energy, with very weak dependence on temperature. However, in the energy range between 300-500 eV, sputtering yields show stronger temperature dependence. Therefore, we conclude that physical sputtering is the dominant mechanism at incident energy greater than 700 eV, while deposited nitrogen is desorbed by chemical sputtering at incident energy less than 500 eV.