  A test facility for the development of the Neutral Beam Injection system for
ITER is under construction at Consorzio RFX. It will host two experiments:
SPIDER, a 100 keV H-/D- ion RF source, and MITICA, a prototype of the full
performance ITER injector (1 MV, 17 MW beam). A set of diagnostics will monitor
the operation and allow to optimize the performance of the two prototypes. In
particular, Beam Emission Spectroscopy will measure the uniformity and the
divergence of the fast particles beam exiting the ion source and travelling
through the beam line components. This type of measurement is based on the
collection of the H{\alpha}/D{\alpha} emission resulting from the interaction
of the energetic particles with the background gas. A numerical model has been
developed to simulate the spectrum of the collected emissions in order to
design this diagnostic and to study its performance. The paper describes the
model at the base of the simulations and presents the modeled $H_\alpha$
spectra in the case of MITICA experiment.
