Effect of filter field and double-bias plate on volume process in negative ion sources

The negative ion source NIO1, at Consorzio RFX in Padova (Italy), is aimed at investigating 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. 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 installed, which can be independently biased with respect to the plasma grid or, in turn, with respect to the source walls. The original magnetic filter field configuration (generated by current flowing through the plasma grid and external return coil), has been modified in order to increase its absolute value inside the source chamber, using 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 and reducing the extracted electron current in NIO1.