The influence of impurities on the erosion of tungsten by low energy high flux deuterium plasma irradiation

Abstract

The erosion behavior of tungsten (W) exposed to low-energy, high flux deuterium (D) plasma has been investigated. The samples were characterized by scanning electron microscopy (SEM), atomic force microscopy (AFM) and ion beam analysis (IBA). The eroded thicknesses increase with increasing irradiation fluence from 4.8 × 10 25 to 1.64 × 10 26 m −2 for 100 eV D plasma bombardment at a temperature of approximately 600 K, however, the corresponding sputtering yields gradually decrease as a function of fluence. A number of possible erosion mechanisms for enhanced sputtering of W by D near the energy threshold which include impurities in the plasma and sample, retained D, surface roughness and morphology, were analyzed. As the sputtering threshold energy predicted by theory and simulation for deuterium on pure W is ~216 eV and for W with carbon contamination is ~85 eV, a sputtering yield of 2.32 × 10 −6 which was measured for W with only C impurity exposed to a 60 eV deuterium fluence of 1.08 × 10 26 m −2 indicates that there were a little amount of impurities such as carbon or oxygen in the D plasma but they show an important contribution to the sputtering yield of the W under 100 eV D plasma erosion. The C impurity in the sample can significantly enhance the sputtering yield of W through increasing energy transfer efficiency in a triple collision cascade system, and the change of C concentration from 14 at.% at the surface to 5.6 at.% in the deeper region of the sample dominates the change of erosion rate under 100 eV plasma bombardment. It is recommended to reducing the impurity concentration to very low levels, whether from background D plasma or contaminants in the near-surface of W wall, will be very beneficial for decreasing W erosion in Tokamak operation.