Temperature Programmed Desorption diagnostic for SPIDER Cs operations

A dedicated Neutral Beam Test Facility (NBTF) has been set up to verify ITER requirements for the neutral beam injectors. The facility, located at Consorzio RFX in Padua (Italy), hosts two experiments: SPIDER and MITICA. SPIDER (Source for the Production of Ions of Deuterium Extracted from Rf plasma) is the full-scale prototype of the ITER source injectors. Its accelerator can be set up to 100 kV and its aim is to investigate and optimize the properties of the ion source. MITICA (Megavolt ITER Injector and Concept Advancement) is the full-scale prototype of the entire injector, devoted to the issues related to the accelerator, including voltage holding at low gas pressure. SPIDER has started operations in 2019 and the plasma without caesium has been characterized. Caesium evaporation, aimed at enhancing the production of negative ions, has recently started during SPIDER experimental campaigns. Understanding the behaviour of caesium during plasma operation is necessary to optimize the generation of negative Hydrogen ions. There are several kinds of contribution to the caesium evaporation in a plasma source. To determine the relative contribution of these fluxes, it is necessary to know the desorption energy of the Cs compounds deposited over the source walls. The Temperature Programmed Desorption (TPD) diagnostic allows to to estimate these energies thanks to a pair of tungsten filaments. The design and development of the device are presented. The diagnostic has been duplicated so it was possible to test its reliability while installing the twin one in SPIDER. First SPIDER experimental results, both using the diagnostic as a TPD and as a Langmuir-Taylor Detector, are shown. Finally, first estimations of the desorption energy are determined in order to finalize plasma-Cs model for SPIDER.