Surface morphology and deuterium retention in tungsten oxide layers exposed to low-energy, high flux D plasma

Surface morphology and deuterium retention in tungsten oxide layers (WO3-z, z ≤ 0.25) grown on polycrystalline and recrystallized W substrates have been examined after exposure to a low-energy (38 eV/D), high flux (1022 D/m2 s) D plasma to an ion fluence of 1026 D/m2 at various temperatures (up to ∼700 K). Characterization methods used were scanning electron microscopy, X-ray diffraction, Rutherford backscattering spectroscopy, and the D(3He,p)4He nuclear reaction analysis. During exposure to the D plasma at temperatures of 340-615 K, a partial reduction of the tungsten oxide takes place in the near-surface layer up to 0.3 μm in depth. Even at around room temperature, deuterium atoms diffuse several micrometers into the tungsten oxide. The high D concentration of about 0.1 D/W observed in the first micrometers below the surface at temperatures below 500 K can be related mainly to D atoms chemically bonded to O atoms. As the exposure temperature increases, the D concentration decreases, reaching about 2 x 10-4 D/W at 615 K. At plasma exposure temperatures of about 700 K, the oxide layer shrinks and loses a large fraction of oxygen.