Modeling and Design of a BES Diagnostic for the Negative Ion Source NIO1

ITER experiment will operate with two heating Neutral Beam Injectors (NBI), each producing a 16.7 MW H/D beam. In the injectors the H -/D -ions produced by an RF source are first accelerated at 1 MeV and then neutralized. Consorzio RFX in Padova (Italy) will host a facility with the prototypes of the negative ion source (SPIDER) and of the full performance injector (MITICA). A more flexible small negative ion source (NIO1) with much reduced performances (60 keV ions, 9 beamletsof 15 mA each) is going to be built as well, being also an opportunity to test and develop instrumentation for SPIDER and MITICA. As for the main experiments, NIO1 will be monitored with Beam Emission Spectroscopy(BES), a non-invasive diagnostic based on the analysis of the spectrum of the H?emissions produced by the interaction of the energetic ions with the background gas. Aim of BES is to monitor direction, divergence, uniformity of the ion beam and the fraction of ions lost in the accelerator because ofstripping processes. The precision of these measurements depends on a number of factors related to the physics of production and acceleration of the negative ions, to the geometry of the beam and to the collection optics. These elements were considered in a set of codes developed to identify the configuration of the diagnostic which minimizes the measurement errors. The model has been already used to design the BES diagnostic for SPIDER and MITICA. The paper presents the model and describes its application to design the BES diagnostic in NIO1.