Impurity Effects on Hydrogen Isotope Retention in Boronized Wall of LHD

Full text: Boronization is one of useful wall conditioning techniques. It is thought that boron will be deposited on the first wall with the impurities and energetic hydrogen isotopes escaped from plasma will be implanted into boron film. Therefore, for the evaluation of hydrogen isotopes recycling in fusion devices, impurity effects on retention behaviors of hydrogen isotopes in boron films were investigated using CVD boron films with controlled impurity concentrations and ones produced by boronization in the Large Helical Device (LHD). It was found that two different existing states of impurities were observed for both boron films. One was the atomic state and the other was that bound to boron chemically. The hydrogen isotopes retention decreased as the total impurity concentration increased, indicating the hydrogen isotopes were desorbed with the formation of heavy water and hydrocarbons. However, some of hydrogen isotopes were retained with much stable chemical forms of O-D and/or C-D bonds with impurities. It was found that these hydrogen isotope trapping and detrapping processes would be influenced by the impurities existing as the atomic state and bounding state with boron, respectively. Furthermore, for the boronized film in LHD, the impurity concentration increased after the exposure to H-H main discharge in LHD, indicating that the impurities in plasma of fusion devices would be dynamically contained into boron film and the removal of hydrogen isotope will largely correlated with that of impurity. It was concluded that these facts will be important to the evaluation for the recycling of hydrogen isotopes in fusion devices. (author)