Synthesis of an optimized and flexible configuration for the magnetic filter in the SPIDER experiment

Magnetic fields in negative ion driven Neutral Beam Injectors (NBIs) are essential to reduce the electron temperature and density in the extraction region in order to consequently reduce the destruction rate of H- ions, along with filtering out the otherwise unavoidably co-extracted electrons from the plasma source and beam accelerator. In SPIDER, the full scale ion source and extractor of the ITER NBI, a horizontal magnetic field of some mT inside the plasma source, transverse with respect to the beam direction, is produced by a current of some kA flowing through the plasma-facing electrode and additional bus-bars. After an initial characterization phase, the first experimental campaign with relevant plasma generation and particle extraction showed that when the magnetic field is increased beyond a certain threshold, an influence on the plasma parameters is observed. This effect has been ascribed to the particular topology of the magnetic field in the plasma volume inside the 8 inductively-coupled radio-frequency drivers that generate the plasma. Therefore, actions were immediately taken to modify the magnetic field configuration in order to improve the plasma diffusion out of the drivers. The paper describes the experiments aimed at identifying the problem and the design and optimization process for a new layout of the bus-bars creating the magnetic filter field, in order to obtain a different topology within the region of the RF driver, while keeping it almost unchanged in the other regions where the required performances have already been achieved. Another aim of the work concerns the possibility of easily modifying the new topology, so as to significantly expand the operating margin of the experimental activity. After introducing the reasons of the work, the numerical analysis and experimental verification activities necessary for the development and implementation of the new system are presented.