Final design of electrostatic probes for MITICA beam line components

MITICA is the full-scale prototype of ITER Neutral Beam Injector (NBI), currently under construction in Padova, Italy. In this facility, a 40 A beam of negative deuterium ions will be accelerated to 1 MeV and then neutralized. The emerging beam, after the removal of residual ions, will be dumped onto the calorimeter (up to 17 MW are expected). In each component of the beam-line, monitoring of the presence of plasma and its parameters is crucial: with this aim, various types of electrostatic probes will be installed along the MITICA beam-line. The Neutralizer and the Electric Residual Ion Dump (ERID) panels will be equipped with pairs of Langmuir probes at different vertical and axial positions, with the possibility to be set in single or double probe configurations and to provide local floating potential, ion density and electron temperature measurements. In the neutralizer, the probes will provide information about the beam collisional processes, in view of a neutral fraction optimization. In the ERID, the objective of the probes is to confirm the absence of plasma, which otherwise might invalidate the deflection properties of the device. In the calorimeter, biased electrodes will collect the secondary emission electrons produced by the beam collisions with the cooling tube surfaces. The electrodes will provide vertical and horizontal profiles of the beam at the calorimeter, to study its horizontal steering, divergence and uniformity. Four different probe designs have been developed to fulfill specific scientific and mechanical constrains at the different positions. In this contribution, the realization of the four prototypes, one for each probe, together with a mock-up of the relative housing panel, is described. Problems or difficulties emerged during the manufacturing of specific elements or during the assembly of the probes have been identified and fixed and the prototypes tested. The final design is, thus, presented.