An investigation of the optics of an accelerating column for use with a high brightness ion source and a proton microprobe

Abstract

The accelerating column of a 5U Pelletron accelerator is analysed in this paper. This accelerator provides the primary beam for the Melbourne Scanning Proton Microprobe. The finite element method is used to calculate the electrostatic field in the accelerator column, and optical properties are extracted from ray tracing. Gaussian properties are presented which specify object location for the column to produce an exit plane focus for five and three accelerating elements. Column acceptance is discussed and found to match emittance for all practical configurations. Chromatic and spherical aberrations are calculated for the column for a range of image distances and for five and three accelerating elements. The optical combination of the column with an ion source lens and a high brightness ion source is discussed. The contribution of the column is found to be principally dependent on the magnification and accelerating voltage of the lens. Where very low currents are required from the accelerator, beam brightness is limited by chromatic aberration, and for very low divergences by diffraction. At such currents the high brightness phase space “core” of the beam may be degraded by chromatic aberration in the accelerating column if the ion source lens magnification is low, or the lens acceleration is particularly high. Where high currents are required (for example above 100 pA), beam divergence angles are higher, and the brightness is no longer chromatically or diffraction limited. Under these circumstances, accelerating column aberrations will not degrade beam brightness.