ABSTRACT
This chapter begins the actual process of linearization with the linearization of the fields, which corresponds to quadratic potentials. It provides a discussion with the case in which the potentials on the reference orbit vanish, which describes the situation of electric and magnetic multipoles as well as in deflectors. The case of electric and magnetic lenses do require the presence of potentials on axis. The chapter considers the zeroth order of the motion. It describes two important regularly employed classes of lenses, the magnetic round lens and the electric round lens. Different from the cases of the electric and magnetic quadrupoles, their focusing properties arise from different mechanisms, and the magnetic and electric round lenses require a quite different treatment.
