The Dα spectral lines emitted from the neutral beam colliding with residual D2 in a beam chamber were observed. The Doppler shifts of these Dα spectral lines indicate that the neutral beam includes deuterium atoms with energies of E, E/2, E/3 and E/9 ˜ E/11 (E : beamaccelerationenergy), which originate from D+, D+2, D+3 and DiO+ (i=1,2,3), respectively. From the intensity ratio of the Dα spectral lines, the fraction of oxygen atoms to deuterium atoms was determined by the analysis on atomic and molecular processes in the neutralizer for the neutral beam. The neutral beam has been found to contain more than 8% of oxygen atoms over deuterium when the neutral beam injection starts. About 50% of the oxygen ions in the core plasma originate from the neutral beam. The oxygen content in the neutral beam decreases gradulally with its injection, and it reaches ˜ 1% around 850 injections. At this time, the fraction of the oxygen ions in the core plasma originating from the neutral beam is less than 20%. In L-mode plasmas, the confinement time of the oxygen ions originating from the neutral beam is estimated to be 0.5 s, and is longer than that of the deuterium ions ( 0.3 s ).
