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.

Design optimization of RF lines in vacuum environment for the MITICA experiment.

This contribution regards the Radio Frequency (RF) transmission line of the Megavolt ITER Injector and Concept Advancement (MITICA) experiment. The original design considered copper coaxial lines of 1″ 5/8, but thermal simulations under operating conditions showed maximum temperatures of the lines at regime not compatible with the prescription of the component manufacturer. Hence, an optimization of the design was necessary. Enhancing thermal radiation and increasing the conductor size were considered for design optimization: thermal analyses were carried out to calculate the temperature of MITICA RF lines during operation, as a function of the emissivity value and of other geometrical parameters. Five coating products to increase the conductor surface emissivity were tested, measuring the outgassing behavior of the selected products and the obtained emissivity values.