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 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. As result of the physics-optical analyses carried out on the accelerator, the number and energy spectrum of the back accelerated positive ions are known, so it was possible to estimate the sputtering rate and the power density profile on the impinged surfaces of the plasma source. As a 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 minimum required thickness is approximately 0.5mm. Among the few technologies suitable for the manufacturing of such thick armour layer, according to the strict technical requirements and with a quite complex geometry, plasma spraying and explosion bonding techniques have been investigated. The explosion bonding process has been found indeed to be the most promising. In the paper the activities carried out for optimizing the explosion bonding process of molybdenum on copper and the forming of the samples according to the final components specifications are presented. The resulting prototypes made by explosion bonding and the sample obtained by plasma spraying have then been tested: microscopic, outgassing, delamination, thermal shock, and thermal fatigue analyses have been carried out. 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.