Effect of Filter Field and Biased Double-Bias Plate on Volume Process in Negative Ion Sources

The RF negative ion source NIO1, built at Consorzio RFX in Padua (Italy), is aimed to investigate general issues on ion source physics, as well as innovative solutions for electrical efficiency, in view of their use as sources of neutral beam injectors (NBIs) in future fusion experiments, like MITICA, the ITER NBI prototype. NIO1 has been designed to produce 9 H- beamlets (in a 3x3 pattern) of each (when using cesium) and 60 keV, using a three-electrode system (plasma grid, extraction grid and post accelerated grid) downstream the plasma source. Since no cesium has been used yet in NIO1, the negative ion production relies on the volume process only. In order to maximize its efficiency, some solutions have been proposed. Inside the source chamber, in front of the plasma grid, an electrically insulated bias plate has been located, which can be independently biased with respect to the plasma grid or, in turn, with respect to the source walls. The original design solution for the filter field (generated by current flowing through the plasma grid and external return coil), was modified in order to increase its absolute value inside the source chamber, by a different configuration of the cusp field magnets. The aim of this contribution is to show the beneficial effect of these two design solutions in improving negative ion current in NIO1.