Design of an optimised movable electrostatic diagnostic for the investigation of plasma properties in a large negative ion source

This contribution describes the design of a movable Langmuir probe system to be permanently installed in the SPIDER negative ion source. This diagnostic aims at measuring the axial profiles of plasma density, electron temperature and floating potential in both the driver and the expansion region, with a view to further investigate the positive ion dynamics. Furthermore, a deeper understanding of the plasma formation mechanism and of the RF coupling can be provided, e,g. via the estimation of the Electron Energy Distribution Function (EEDF) inside the driver. Thermal analyses were performed to define both the allowable duration of the probe exposure to the plasma and the extent of the voltage sweep. The maximum exposure time for the probe shaft is 10 s, thus a fast manipulation system is included in the design. The integration in the SPIDER source is described, considering all the limitations due to the space availability, the presence of the RF generators and the vacuum-compatibility requirements. To comply with all the constraints, several solutions are proposed for the probe head design.