  Consorzio RFX and INFN-LNL are building a flexible small ion source (NIO1)
capable of producing about 130 mA of H- ions accelerated at 60 KeV. Aim of the
experiment is to test and develop the instrumentation for SPIDER and MITICA,
the prototypes respectively of the negative ion sources and of the whole
neutral beam injectors which will operate in the ITER experiment. As SPIDER and
MITICA, NIO1 will be monitored with Beam Emission Spectroscopy (BES), a
non-invasive diagnostic based on the analysis of the spectrum of the $H_\alpha$
emission produced by the interaction of the energetic ions with the background
gas. Aim of BES is to monitor direction, divergence and uniformity of the ion
beam. 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 was 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.
