Vacuum diagnostic method of deuterium and helium partial pressures using ion backscattering technique

We have developed a new diagnostic method to measure a partial pressure of helium (He) in the presence of deuterium (D2). The principle of this diagnostic method is based on the difference between monatomic and molecular ions in a backscattering process on a solid surface. The mass separation of both gases which have the same mass of 4 is easily done by energy analysis and intensity measurement of the scattered ions. The pressure of 0.1 % He in the D2 gas can be measured with 8.8 % statistic error. It is concluded that this diagnostics can be applied to He ash removal experiments of ALT-II project. (author)