  Beam-focusing characteristics of negative ion beams have been experimentally
investigated with a superimposition of a controlled perturbation of RF field in
a filament-arc discharge negative ion source. Oscillations of a negative-ion
beamlet width and axis responding to the RF perturbation were observed, which
may be a cause of the larger beam divergence angle of the RF negative ion
source for ITER. It is pointed out that the oscillation of the beamlet width
depends on the perveance and on the RF frequency such that the oscillation is
suppressed at perveance-matched conditions and at low RF frequency.
