OPTICAL-PROPERTIES OF PLANAR IMMERSION OBJECTIVES IN ELECTRON-IMPACT ION SOURCES

The ion optical properties of an immersion ion source have been calcul ated in the presence of electron space charge. The ion source was an e lectron impact ionizer with planar geometry and a crossed beam configu ration, assuming a well defined atomic beam. We present the focal prop erties and the third-order aberration coefficients, as a function of b oth the electron beam current and the ion extraction geometry. The ima ge position is relatively unchanged by the electron beam or extraction geometry; however, there are significant changes to the magnification . The aberrations indicate that it is possible to preserve a spatial r esolution in a typical imaging ion source of 50 mu m over a 6 mm field of vision.