Retention behaviors of hydrogen isotope in boron film exposed to H–H discharge in LHD

Retention behaviors of hydrogen isotopes in boron film exposed to a H–H discharge in the LHD was evaluated by TDS and compared with that in the D2+ implanted one. D2 TDS spectrum for D2+ implanted LHD-boron film showed that desorption stages were consisted of B–X–B, B–X, B–O–X and B–C–X bonds (X: hydrogen isotope). The ratios for B–O–X and B–C–X bonds to hydrogen isotope retentions were reached up to 60% for the LHD-boron film by H–H discharge, although those for the D2+ implanted were about 35%. The atomic concentrations of impurities were increased after H–H discharge. Therefore, it is suggested that the impurities in plasma would be introduced to boron film dynamically during H–H discharge. The impurity concentration during the H–H discharge or ion implantation would have a large impact on the retention. The hydrogen isotope trapping by impurities should be refrained to enhance the recycling of hydrogen isotope and the impurity control will be one of critical issues in LHD.