Effects of nitrogen-seeded deuterium plasma on tungsten surfaces

Abstract

The melting and evaporation of target materials such as tungsten, due to the enormous plasma heat flux, are not compatible with the realization of ITER (International Thermonuclear Experimental Reactor) and nuclear fusion reactor designs. Nitrogen gas seeding has recently been considered as an effective radiator for edge and divertor boundary plasmas to protect the divertor plate from overheating. Although compatibility of nitrogen seeding with tokamak discharge operation was obtained in current tokamaks, the effects of nitrogen seeding on plasma-facing components, especially the divertor target material (e.g. tungsten), have not been fully tested in a fusion-relevant steady-state linear plasma system with a high plasma density and plasma heat flux, in which deuterium and nitrogen mixed-gas discharge plasmas are irradiated onto a tungsten target, closely similar to the boundary region in a magnetic fusion configuration. This study addressed such a detailed irradiation, using a variety of surface analyses. Surface-temperature-sensitive tungsten nitride formation was determined, thus elucidating the interesting surface morphology of whisker, loop, and/or hook-like nanostructures in some surface temperature bands. The surface characteristics were determined using several kinds of surface analysis methods, including x-ray diffraction, energy dispersive x-ray analysis, nanoindentation, Raman spectroscopy, and spectrometry. The possibility of tungsten contamination into facing plasmas is discussed in terms of tungsten nitride melting. The relation to industrial applications of tungsten nitride has also been mentioned.