Caesium oven design and R&D for the SPIDER beam source

In the ITER neutral beam injectors caesium will be used in the negative ion source to enhance the ion yield, by lowering the surface work function, and to reduce the co-extracted electrons. Fresh Cs has to be deposited almost continuously inside the plasma source and its injection is obtained by means of Cs ovens. Three independent Cs ovens have been integrated in the SPIDER (Source for Production of Ion of Deuterium Extracted from Rf plasma) beam source (BS). Since the entire BS is in-vacuum, a completely new design of the Cs ovens was required, assuring their vacuum and high temperature compatibility, and remote operation. They feature an ON-OFF valve and two independent heating systems to control the Cs evaporation and to avoid vapour condensing on the injection duct, by controlling the temperature. A Cs level indicator to measure the quantity of liquid caesium inside the reservoir, based on the electric resistance variation of an electrode, is proposed for the first time and a proof of concept is given. The oven design and the R&D activity on the level measurement system are presented.