Measurement of the Bidirectional Reflectance Distribution Function of Tungsten Surface Sputtered in Argon Plasma

Abstract

For ITER spectroscopic measurements, it is important to understand the optical reflection characteristics of the divertor surface for an accurate measurement because the stray light in the divertor may be large. We set up a goniophotometer that measures the optical reflection characteristics and investigated the bidirectional reflectance distribution function of tungsten samples sputtered in an argon plasma. The specimens sputtered at temperatures lower than the recrystallization temperature of tungsten exhibited smooth surfaces and strong specular reflections in their optical reflectance characteristics. Recrystallized crystals likely grew for the specimens sputtered at temperatures approximately equal to the recrystallization temperature, resulting in a weak specular reflection.