An optimized and flexible configuration for the magnetic filter in the SPIDER experiment

•The magnetic field topology in the RF drivers of the SPIDER ion source is fundamental for a good plasma confinement and its efficient generation.•The specific bus-bars layout allows a wide modification margin of magnetic configurations inside the RF drivers, keeping it unchanged elsewhere.•An innovative solution to control the current sharing, by thin graphite sheets in some bus-bar joints, has been successfully tested and adopted. Magnetic fields in negative ion driven Neutral Beam Injectors (NBIs) are essential to reduce the electron temperature and density in the extraction region, along with filtering out the otherwise unavoidably co-extracted electrons. In SPIDER, the full scale ion source and extractor of the ITER NBI, a horizontal magnetic field up to 8 mT inside the plasma source is produced by a current of up to 5 kA flowing through the plasma-facing electrode and additional bus-bars. First SPIDER experimental campaigns showed an influence on the plasma generation when the magnetic field is increased beyond a threshold. This effect has been ascribed to the particular topology of the field in the plasma volume inside the 8 inductively-coupled radio-frequency drivers that generate the plasma. The paper describes the experiments aimed at identifying the problem and the design process for a new layout of bus-bars capable of creating an optimized magnetic field topology within the RF driver, while keeping it almost unchanged in the other regions where the required performances were already achieved. Particular attention was given for a design leaving the possibility of easily modifying the new topology, so as to significantly expand the operating margin of the experimental activity.