Beam current regulation of DIII-D neutral beam long pulse ion sources

The slow increase of arc and beam currents during the beam pulse is an intrinsic characteristics of the neutral beam long pulse ion source installed on the DIII-D tokamak. This ramping is attributed to the heating of the filaments by energetic electrons backstreaming from the accelerator into the arc chamber. The corresponding change in beam perveance causes the beam optics to vary during a beam pulse, often resulting in an overdense condition. Stepping down the voltage applied to the filaments at beam turn-on was helpful, but its disadvantages called for a better scheme. A technique using a Langmuir probe signal for feedback regulation in the arc power supply is presented. Plasma density within the arc chamber is maintained at a constant value, as is beam current. The arc regulation method also features arc notching at beam turn-on to provide perveance matching during initial beam formation; this is crucial to obtaining smooth initial beam extraction, a high perveance beam, and thus higher-power beam operation. The beam power achieved with this arc notching and regulation technique is about 10% higher than that obtained with the filament voltage step-down method