Effects of carbon impurity on deuterium retention in VPS-tungsten coatings exposed to JT-60U divertor plasmas

Carbon eroded from carbon armor tiles during plasma discharge was implanted into and accumulated in tungsten coating exposed to JT-60U divertor plasmas. The D/C ratio of 0.06 +/- 0.02 evaluated in the tungsten coating was half to one-quarter that in carbon codeposits formed at similar temperature of the tungsten coating. These results suggest that simultaneous use of carbon and tungsten coating would enhance tritium retention in the tungsten coating in future deuterium-tritium fusion devices. To investigate the carbon diffusion mechanism in the tungsten coating exposed to JT-60U divertor plasmas, the carbon diffusion coefficient in tungsten coating was measured by tracer methods. Using the apparent carbon diffusion coefficient obtained in this study (similar to 8 x 10(-19) m(2)/s), the carbon diffusion length in the tungsten coating exposed to JT-60U divertor plasmas was evaluated to similar to 100 nm. This diffusion length was quite shorter than that observed in the tungsten coating exposed to JT-60U divertor plasmas. Therefore, it remains possible that diffusion of implanted carbon in tungsten coating would be enhanced by other diffusion mechanisms which did not arise in the diffusion experiments or heat loads to the tungsten coating during transient events and plasma discharges with a strike point positioned on the tungsten-coated tiles. (C) 2011 Elsevier B.V. All rights reserved.