Hydrogen atoms effusing out from an extraction hole of a 6 cm diam and 6 cm long ion source was measured using a quadrupole mass spectrometer coupled to a neutral beam chopping system. The system background for atomic signal detection was negligible. As the discharge current was increased, the signal of atomic hydrogen increased almost linearly with the discharge current. Meanwhile the signal of molecular hydrogen slightly decreased with increasing discharge current. The signal to noise ratio of the present system is as low as 1/80 for typical operation of the ion source. The status of the system modifications to reduce residual hydrogen gas is reported.

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