R&D activities for the production of 1,0 mm thick molybdenum armour layer on copper substrates

In the framework of the activities for the development of the Neutral Beam Injector (NBI) for ITER, the design of the Radio-Frequency plasma source has been carried out. The most critical components of the plasma source are the rear vertical plates facing the plasma, since they are hit by the back-streaming positive ions that are generated, mostly for stripping losses, inside the 1 MV electrostatic accelerator. Such high energetic particles, impinging the rear vertical surfaces of the plasma source, cause heat deposition and physical sputtering. As consequence, the need of an armour layer having low sputtering yield was established to be necessary. Molybdenum and tungsten are the most suitable materials; nevertheless the required thickness is around 0.5 mm. Different technologies for the manufacturing of such a thick armour layer, that is not common, have been investigated and samples have been manufactured by explosion bonding and atmospheric plasma spraying. Samples have then been tested: microscopic, outgassing, delamination, thermal shock, and thermal fatigue analyses have been carried out. The results of tests performed on the explosion bonded samples are presented in the paper, giving an overview on the critical technological aspects and open issues.