DESIGN OF ION MIRROR AS AN ABERRATION CORRECTOR USING BOUNDARY VALUES METHOD

      The idea of using  an electron mirror to correct the chromatic and spherical aberration of around lens dates back more than half of century.Extensive studies of electrons mirrors have been performed by the Kelman, Rempfer, Shao, and Rose and co-workers [Barnett and Nixon 1966]. In Rempfer’s work and the work of Preikszas and Rose, it was found that an electron mirror could correct the spherical and chromatic aberrations of the accelerating field of an immersion objective lens [Schmid et al 2005].An electron mirror is create when an electrode with sufficiently high negative potential is placed in the path of an electron beam. The negative electrode forms a potential hill that decelerates the incident electrons. The electrons lose their kinetic energy before reaching the electrode and are re-accelerated in the reverse direction away from the electrode [Kuehler 1960].An electron mirrors are capable of introducing chromatic and spherical aberrations of arbitrary sign. Hence, we can utilize mirrors to compensate for the corresponding aberrations of round lenses  Preikszas and Rose 1997]. Unlike a light optical mirror, where the reflection occurs at the physical surface, the electron mirror represents a “soft” mirror, which allows the electrons to penetrate into the inhomogeneous reflecting medium formed by the electrostatic potential [Rose and Wan 2005].