  The caesiated surface negative ion source is the first element of CERN's
LINAC4 a linear injector designed to accelerate negative hydrogen ions to 160
MeV. The IS03 ion source is operated at 35 mA beam intensity and reliably feeds
CERN's accelerator chain, H- ions are generated via plasma volume and caesiated
molybdenum plasma electrode surface mechanisms. Studying the beam extraction
region of this H- ion source is essential for optimizing the H- production. The
3D Particle-in-cell Monte Carlo code ONIX (Orsay Negative Ion eXtraction),
written to study H- beam formation processes in neutral injectors for fusion,
has been adapted to single aperture accelerator H- sources. The code was
modified to match the conditions of the beam formation and extraction regions
of the Linac4 H- source. A set of parameters was chosen to characterize the
plasma and to match the specific volume and surface production modes. Simulated
results of the extraction regions are presented and benchmarked with
experimental results obtained at the Linac4 test stand.
