Emittance growth in negative ion beam formation: Effects due to secondary positive ions on the extraction sheath

The experimentally measured dependence upon the acceleration voltage of the negative ion current intercepted on a Faraday Cup is compared to theoretical computations. It is shown that the secondary positive ions generated within the electron suppression electrode and accelerated into the plasma source, have a significant effect on the plasma extraction sheath and presheath potentials and explain the experimental data.