Fatal Damages due to Breakdown on a Diagnostic Mirror Located outside the Vacuum Vessel in JT-60U

Abstract

Some discharge phenomena seriously damaged the secondary mirror for Thomson scattering diagnostics, which was located outside the vacuum vessel. In this paper, the surface damages recorded on the mirror are observed in detail with an optical microscope. Many fine trails were found on the surface. The trails could be categorized into two different types with respect to the trail width. The mechanisms to lead the damages were discussed based on the observation. This study issues warning on the components to be installed in future fusion devices both inside and outside the vacuum vessel.