Optimized caesium oven nozzles for reduced loss and improved uniformity of Cs deposition in SPIDER

SPIDER is the full-scale prototype for the ITER Heating Neutral Beam (HNB) ion source. In order to reach the current density requirements the production of H-/D- ions is greatly enhanced by covering the source's converter surfaces with caesium, which lowers their work function. SPIDER is equipped with 3 caesium ovens located at the rear part of the source. During the first experimental campaign with Cs in SPIDER, experimental evidence called for two improvements concerning the Cs evaporation: the optimization of the uniformity of Cs deposition in the beam extraction region and the minimization of the Cs lost in the rear part of the source. This work describes the modifications made to the Cs oven nozzles aiming to tackle both improvements. We discuss the simulations supporting the choice of the new design (suggesting a 40% reduction of the direct flux of Cs at the backplate and a possible reduction of the vertical non-uniformity at the Plasma Grid from 23% to 13%) as well as the experiments performed in the CAesium Test Stand (CATS) to characterize the new nozzles.