AN ANALYSIS OF THE OPTICS OF A FIELD-IONIZATION ION-SOURCE FOR APPLICATION WITH A SCANNING PROTON MICROPROBE

This article analyzes a field ionization source for use within a pelletron accelerator which provides the primary beam for a scanning proton microprobe. The charge simulation method is used to calculate the electrostatic field, and ray tracing is used to determine optical properties. Current characteristics are taken from experimental results. Gaussian properties indicate an effective source radius of below 10(-3) mum at low angles. Chromatic aberration is calculated by perturbing initial particle energies, then tracing back from field-free trajectories. Calculations indicate that at typical source voltages, the beam is never chromatically limited. Spherical aberration is also calculated and the source is found to be spherically limited above a divergence of approximately 0.1 rad. Finally, calculations indicate that a brightness of 10(6) A m-2 rad-2 V-1 is achieved by the source producing 150 pA of current at a tip electric field of 25 V/nm.