Design and comparison of the Cs ovens for the test facilities ELISE and SPIDER.

Negative ion sources for fusion rely on the formation of negative hydrogen (or deuterium) ions by conversion of atomic hydrogen and positive hydrogen ions at a low work function caesiated surface. Cs is thus evaporated into the source to decrease the surface work function, which may change due to the removal and redistribution of Cs during plasma phases. To maintain a temporarily stable low work function during 1 h plasma, continuous evaporation of caesium is required, and this is performed by temperature controlled Cs ovens. The Cs ovens for ELISE (IPP Garching) and SPIDER (Consorzio RFX) are based on the evaporation of liquid Cs from a reservoir located at one end of the oven, which is controlled by the reservoir temperature. The ampoule Cs oven of ELISE is in operation since 2015, allowing for controllable and stable evaporation. The SPIDER oven is based on the ELISE oven although it required significant changes due to the vacuum environment and the oven location (at the back-plate instead of the sidewalls), leading to a different design of the oven and the nozzle. First investigations on the SPIDER oven in a dedicated test stand show that Cs evaporation is controllable, stable, and reproducible.