Origin of metal impurities in DIVA

The metal impurity origin is studied in DIVA which is a tokamak with an axisymmetric divertor. Ion sputtering is demonstrated to be the dominant process of the release of gold from the shell coated with gold. A cone-like formation due to the sputtering is found on the gold protection plate exposed to about 10000 discharge by using the scanning electron micrograph technique. The SEM pictures shows that the spatial distribution of the cone-like formations on the shell surface due to the ion sputtering is well correlated with the spatial distribution of the intensity of the Au-I line radiation. It is also demonstrated that a honeycomb structure can decrease the release of the metal impurity. (author)