Modeling of a negative ion source. III. Two-dimensional structure of the extraction region

The self-consistent production and transport of H- in the extraction region of a hybrid negative ion source is modeled by means of a two-dimensional particle-in-cell/Monte Carlo simulation. The normal coordinate and one parallel coordinate with respect to the plasma grid are considered to analyze the transport of negative ions. Results show that, in order to establish space charge compensation, the extraction of surface-produced negative ions is limited by the flux of positive ions directed toward the plasma grid surface. An electrostatic barrier appears just in front of the wall, reflecting the majority of surface-produced H- and reducing by this their extraction probability to only 8.5%. Results reproduce the experimentally observed influence of the plasma grid bias voltage on the extraction identifying as a key element the presence of a saddle point in the electric potential distribution.