Reduction of co-extracted electrons and beam inhomogeneity in the large negative ion source SPIDER

The SPIDER ion source has 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, while a beam inhomogeneity within 10% has to be attained. The source plasma is generated inside 8 inductively coupled radio-frequency drivers; it then diffuses through a so-called expansion region and finally negative ions are extracted and accelerated by means of a three-grid system comprising a plasma grid (facing the source plasma), an extraction grid and a grounded grid. A magnetic filter field is produced by flowing a current through the plasma grid 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. The main drawback is that the coupling of the magnetic filter field with the electric field and the pressure gradients within the ion source induces a vertical plasma drift which possibly turns into a vertical beam inhomogeneity. Another strategy to reduce the co-extracted electrons is to bias the plasma grid with respect to the ion source body and to install a so-called bias plate in the vicinity of the plasma grid, which can also be biased. Also, such biases affect the beam inhomogeneity by modifying the ExB drift. After the first beam campaign, the filter field configuration of SPIDER was modified so as to improve the plasma diffusion outside of the drivers. In addition, the voltage ratings of the bias power supplies were increased. Both these modifications permit to lower the electron co-extracted current while increasing the negative ion current. The present contribution describes such improvements while focusing on the effects they have on the beam inhomogeneity.