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.