Hydrogen Retention Behavior in Boron Films Affected by Impurities Introduced by Hydrogen Plasma Exposure in the LHD

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The chemical states of impurities such as oxygen and carbon and their effects on hydrogen retention behavior in pure boron films exposed to hydrogen plasma in the Large Helical Device were investigated by X-ray photoelectron spectroscopy and thermal desorption spectroscopy. The atomic concentrations of the boron films after hydrogen plasma exposure changed from 94% B, 4% C, and 2% O to 68% B, 20% C, and 12% O. B-C bonds and free oxygen were the major chemical states of impurities in the boron films after hydrogen plasma exposure. The hydrogen isotope retention behavior of a hydrogen-plasma-exposed boron film and a D+2 -implanted one clearly differed, and the retention of hydrogen as B-H bonds was reduced by hydrogen plasma exposure owing to the chemical sputtering of hydrogen with free oxygen. In addition, the hydrogen desorption stage was observed as B-C-H bonds at approximately 900 K, although the amount of retention as B-C-H bonds was 10% of the total for a pure boron film exposed to hydrogen plasma.