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

Abstract Surface morphology and deuterium retention in tungsten oxide layers (WO 3− 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 (10 22  D/m 2  s) D plasma to an ion fluence of 10 26  D/m 2 at various temperatures (up to ∼700 K). Characterization methods used were scanning electron microscopy, X-ray diffraction, Rutherford backscattering spectroscopy, and the D( 3 He,p) 4 He 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 × 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.