Co-extracted electrons and beam inhomogeneity in the large negative ion source SPIDER

The SPIDER ion source has been designed to generate a current of H- ions up to 60 A and accelerate them up to 100 keV. In addition, the ratio between the co-extracted electrons and negative ions has to be below 1, and a beam inhomogeneity within 10 % has to be attained. A magnetic filter field is produced by a current flowing through the plasma electrode in the vertical direction with the aim of reducing electron temperature and density in the extraction region, so as to enhance the survival probability of H- ions while reducing the co-extracted electrons. Another strategy to reduce the co-extracted electrons is to apply a small bias voltage to the plasma grid with respect to the ion source body. In addition to this, a so-called bias plate is installed in the vicinity of the plasma grid, which can also be biased with respect to source body. However, the application of a bias voltage affects the beam inhomogeneity by modifying the ExB plasma drift within the ion source. Recently, the filter field configuration of SPIDER was modified so as to improve the plasma diffusion towards the extraction region. In addition, the voltage ratings of the bias power supplies were increased. Both these modifications permit to efficiently lower the electron co-extracted current. The present contribution describes such improvements while focusing on the effects they have on the beam inhomogeneity.