Efficiency of arc discharge and beam extraction of the DIII-D neutral beam ion source

DIII-D neutral beam ion sources are operated in the emission limited arc discharge regime. Filament temperature has been observed to play an important role in the ion source operation. Effects of the filament temperature on the arc discharge efficiency (Langmuir probe signal of the arc discharge/arc power) and arc efficiency (extracted beam current/arc power) have been experimentally measured. The results show that both efficiencies reach optimum values with respect to filament temperature. The optimum arc efficiency is independent of the beam energy, but the filament temperature at which optimum arc efficiency is obtained shifts to higher values with higher beam energy. Gas puff into the neutralizer is necessary to achieve optimum neutralization efficiency, but it also increases both particle collisions in the accelerator column and gas flow into the arc chamber. Tests show that arc efficiency decreases only slightly with increasing particle collisions, since loss in the extracted beam current is compensated for by the decreasing we power required to sustain the arc discharge with additional gas flow. The most significant effect of particle collisions is the sharp increase of the gradient grid current, which can cause damage to the grid.