A MODEL FOR NEGATIVE-ION EXTRACTION AND COMPARISON OF NEGATIVE-ION OPTICS CALCULATIONS TO EXPERIMENTAL RESULTS

Negative ion extraction is described by a model that includes electron diffusion across transverse magnetic fields in the sheath. This model allows a two-dimensional approximation of the problem. It is used to introduce electron space charge effects in a 2-D particle trajectory code, designed for negative ion optics calculations. Another physical effect, the stripping of negative ions on neutral gas atoms, has also been included in our model; it is found to play an important role in negative ion optics. The comparison with three sets of experimental data from very different negative ion accelerators, show that our model is capable of accurate predictions.