W erosion due to low energy O+ and D+ impact

The erosion of polycrystalline tungsten by 0.15–10 keV D3+, O2+ and Ar+ has been studied at temperatures of 300–1900 K, using line-of-sight quadrupole mass spectroscopy (LOS-QMS) and mass-loss measurements. The overall rates of erosion at 300 K are consistent with standard Monte Carlo calculations (TRIM 2000), indicating that physical sputtering is the process dominating erosion. During simultaneous bombardment by 500 eV/D+ and 75 eV/O+ the mass-loss due to bombardment by D+ appeared to be unaltered by the addition of O+. During O+ bombardment the LOS-QMS signals show clear peaks at masses 184 (W) and 200 (WO). These peaks are significantly larger during 5 keV/O+ bombardment than during 10 keV Ar+ bombardment, indicating the presence of a chemical component of erosion by O+ ions. This chemical component is estimated to be <10% of the physical sputtering yield for 5 keV/O+. The `chemical erosion' process is fairly complex: it is sensitive to the composition of the surface; the yield (mass 184 net peak height divided by the O+ flux) decreases with decreasing O+ energy; it has an apparent threshold energy of 350 eV/O+, which suggests that bond-breaking or displacement creation is involved; the yield decreases with increasing O+ flux and temperature; and the yield generally decreases when D+ is incident on the W simultaneously with the O+.