AN INVESTIGATION OF THE OPTICS OF A 5-ELEMENT ELECTROSTATIC LENS FOR USE WITH A HIGH-BRIGHTNESS ION-SOURCE

The optics of a configuration consisting of a biased ion source exit c anal, followed by a four-electrode electrostatic lens is investigated. This effectively operates as a five-electrode electrostatic lens (alt hough two electrodes are in fact wired at the same potential). This le ns displays three degrees of freedom in achieving a required beam focu s. In particular, this lens is investigated to determine its optimal c onfiguration for the present, low voltage ion source and its suitabili ty for use with a high voltage field ionization ion source. The finite element method is used to calculate the electrostatic field in the le ns, and optical properties are extracted from ray tracing. A full rang e of ''accelerating'' and ''decelerating'' focusing modes are analysed with a range of final to initial voltage ratios of between 1 and 16, and with and without a beam crossover inside the lens. It is found tha t aberrations are lowest for large initial acceleration, and with no b eam crossover, with the optimal aberrations being relatively insensiti ve to the final electrode voltage. Calculations suggest, however, that the introduction of a high voltage field ionization source would almo st certainly preclude the use of the optimal lens configuration in pra ctice.