Fig. 2-8 Schematic of the boron coating system Decaborane vapor produced by heating the container was mixed with deuterium and helium gases and injected through many nozzles. The 100 g of decaborane was injected in about 50 hours. The layer of 200 nm in average thickness was formed on 200 m2 of the inside area of the JT-60 vessel.

Fig. 2-9 Oxygen concentrations before and after boron coating Boron easily binds with oxygen and fixes it inside the film. Simultaneously, the boron film reduces the rebounding of particles. As a result, convection loss is reduced. This means the enhancement of confinement characteristics; that is, the plasma temperature is remarkably increased at the same heating power.

Plasma impurities such as carbon and oxygen should be less than 1% of the hydrogen plasma density in realistic fusion facilities. The vacuum vessel is exposed to plasma particles caused by the convection loss from the core plasma. As a result the particles hitting the vessel wall cause the emission of impurities contained inside the wall. The wall material always includes oxygen and water because the wall material is usually manufactured under atmospheric conditions. We have attempted to give in-situ boron coatings to the wall surface to reduce plasma impurities in the JT-60 Tokamak plasma. The chemical activity of diborane (B2H6) is too high for ordinary handling. Decaborane (B10H14), which is in the solid state at room temperature, was used. In this case, decaborane was vaporized by heating and introduced to the vacuum vessel. Decaborane was dissociated to boron and hydrogen by glow discharge (like a fluorescent light). The boron was stuck on the wall surface. This process is usually called Chemical Vapor Deposition (CVD). The schematic of the system arrangement is shown in Fig. 2-8. The oxygen concentrations of the plasma before and after the boron CVD are shown in Fig. 2-9. It is known that the boron film fixes the oxygen and increases plasma purity. A plan was made to get a homogeneous coating on the surface of the large vacuum vessel. Improvement of surface characteristics compatible with the structural material could be one of the promising ways to develop new materials.

Reference M. Saido, Study of Plasma Facing Materials in JT-60, Purazuma Kakuyugo Gakkai-Shi, 71 (5), 372 (1995).

Select a topic in left column