Study of plasma uniformity on JT-60U negative ion source

The negative ion source developed at JAERI for the N-NBI was intended to accelerate a 500 keV, 22 A D(-) beam for 10 s. Two of these ion sources are mounted in the beamline. It has been found that the spatial uniformity of the source plasma is not as good as expected in the design phase of the negative ion sources for JT-60U. The source plasma uniformity has been estimated through measuring spatial distributions of the arc current flowing into each filament group and also the ion saturation current with Langmuir probes placed at ten positions around the extraction area of the plasma grid. We then altered the relative values of the arc current-limiting resistors to the eight filament groups in each source to balance the arc. As a result of the optimization of the arc resistors, the nonuniformity of the source plasma has been reduced. (C) 2000 American Institute of Physics. [S0034-6748(00)57402-4].