Design, Installation, Commissioning and Operation of a Beamlet Monitor in the negative ion beam test stand at NIFS

In the framework of the accompanying activity for the development of the two neutral beam injectors for the ITER fusion experiment, an instrumented b eam calorimeter is being designed at Consorzio RFX, to be used in the SPIDER test facility (particle energy 100keV; beam current 50A), with the aim of testing beam characteristics and to verify the source proper operation. The main components of the instru mented calorimeter are one - directional carbon fibre carbon composite tiles. Some prototype tiles have been used as a small - scale version of the entire calorimeter in the test stand of the neutral beam injectors of the LHD experiment, with the aim of characterising the beam features in various operating conditions. The extraction system of the NIFS test stand source was modified, by appliyng a mask to the first gridded electrode, in order to isolate only a subset of the beamlets, arranged in two 3x5 ma trices, resembling the beamlet groups of the ITER beam sources. The present contibution gives a description of the design of the diagnostic system, including the numerical simulations of the expected thermal pattern. Moreover the dedicated thermocouple mea surement system is presented. The beamlet monitor was succesfully used during a full experimental campaign, during which the main parameters of the source, mainly the arc power and the grid voltages, were varied. This contribution describes the methods of fitting and data analysis applyed to the infrared images of the camera to recover the beamlet optics characteristics, in order to quantify the response of the system to different operational conditions. The results concerning the beamlet features are prese nted as a function of the source parameters.