Material probe analysis of boronized wall in LHD

Abstract

In the Large Helical Device (LHD), boronization using glow discharge with a mixture of helium and diborane gas was conducted in the 6th experimental campaign. In this campaign, material probes made of silicon and 316L stainless steel were placed on the plasma-facing wall in the toroidal direction. The thickness of the deposited boron and retained amounts of discharge gases (hydrogen and helium) were investigated. The toroidal distribution of the boron thickness significantly depended on the positions of the anodes and the diborane-inlet nozzles. The thickness of boron film and the atomic concentration of boron at the wall close to the anode and the nozzle were 400 nm and 80 at.%, respectively. At the wall far from the anodes and the nozzles, the boron film was very thin, only several nm. The trapped amount of oxygen had a tendency to increase with the thickness of boron film. The retained amount of helium at the wall close to the anodes was one order of magnitude larger than that far from the anodes. The retained amount of hydrogen at the wall with a thick boron film was observed to be smaller than that at the wall with a thin boron film.