First characterization of the full scale negative ion source SPIDER in caesium by Beam Emission Spectroscopy

The Neutral Beam Injector (NBI) foreseen for ITER will produce an accelerated particle beam based on H-/D- ions provided by an accelerator capable of delivering a current density of 350 A/m² (H) / 285 A/m² (D), an extracted beam inhomogeneity <10 %, and a co-extracted electron fraction below 1 (H) / 0.5 (D) [1, 2]. At the Neutral Beam Test Facility (NBTF) of Padova, Italy, the full-scale negative ion source SPIDER represents an intermediate key step towards the realization of MITICA, the full-scale prototype of the entire NBI for ITER, currently under construction. SPIDER consists of a plasma, coupled to a set of grids, biased at increasing electrical potential, to extract and accelerate H-/D- ions up to 110 kV. To enhance the production of negative ions by surface processes, caesium is evaporated on the plasma grid, lowering the work function. One of the techniques currently employed to characterize the beam is Beam Emission Spectroscopy (BES), which is based on the collection of emission spectra due to the de-excitation of H/D atoms generated by the collision of the accelerated H-/D- ions with the residual background gas [4]. This technique is routinely employed to measure the deflection, divergence and brightness of the observed part of the beam. In this contribution the recent results of the first Cs campaign are described, showing the effect of this new operational regime on beam properties as measured by BES. Compared to Cs-free campaign, beam brightness has increased by a factor of 2-3, and divergence has dropped to less than 15 mrad, from previous typical minimum values of 20-30 mrad.A full characterization of beam properties as a function of source and accelerator parameters is here presented. The isotopic effect H/D was also investigated showing behaviors similar to other prototypes of negative ion sources for fusion.