Molybdenum armour layer on copper plates: Manufacturing technologies and tests of prototypes

In the framework of the activities for the development of the Neutral Beam Injector (NBI) for ITER, the design of the MITICA device, including a plasma Source, a negative-Ion Accelerator and a Neutralizer has been recently finalized at Consorzio RFX. Among the most critical components of the plasma source are the rear vertical plates which enclose the plasma in the vicinity of the RF drivers and are hit by the back-streaming positive ions that are generated, mostly for stripping losses, inside the 1 MV electrostatic accelerator. On the basis of the physics-optical simulations of the negative-ion accelerator, the geometrical power density pattern and the energy spectrum of the back accelerated positive ions have been calculated, together with their sputtering rate on the impinged surfaces of the plasma source. The use of copper plates with an armour layer having low sputtering yield was established to be necessary for minimizing the pollution of the plasma and for preserving the component functionality. Molybdenum and Tungsten are the most suitable materials for the aim; nevertheless the minimum required thickness is approximately 0.5 mm. In order to verify the possibility of manufacturing such a thick armour layer, having a quite complex geometry and strict tolerance requirements, plasma spraying and explosion bonding techniques have been selected and investigated. The explosion bonding process has been found indeed to be the most suitable. In the paper the set up of the explosion bonding process of molybdenum on copper and the forming of the samples according to the component specifications are presented. The results of the test on explosion bonding and on plasma spraying prototypes are also reported, together with microscopic, outgassing, delamination, thermal shock, and thermal fatigue analyses. The results of the tests are presented in this paper, giving an overview on the critical technological aspects and on the still open issues in view of the manufacturing of the final components.