Morphology and optical property changes of nanostructured tungsten in LHD

Nanostructured tungsten formed by the exposure to helium plasma in a linear plasma device was installed in the large helical device (LHD). After the exposure in a series of experiments in the 2012 fiscal year campaign in LHD, the samples were analyzed by scanning electron microscope (SEM), transmission electron microscope (TEM), and energy dispersion x-ray spectroscopy (EDX). It was found that part of the nanostructures was totally covered with carbon based material probably from divertor, while some other parts were eroded by sputtering. On the erosion dominant region, it was revealed that the head part of nanostructures was sputtered and the surface became rounded, but the nanostructures still remained near the surface. Optical reflectance of the material was measured, and it was found that the morphology changes increased the optical reflectivity up to ∼10% from typically less than 1%. The possibility and limitation of the nanostructured tungsten as a light absorber (viewing dump) are discussed. (author)