Achromatic deflector from six poles-electrodes

Abstract

A combined electrostatic–magnetic (electromagnetic) deflector for beam rastering has been studied theoretically. It consists of a cylinder made of magnetic material cut along generatrices into six equal parts that represent magnetic poles as well as electrodes. Analytical equations for calculating the coordinates and deflection angles for such a deflector with a finite length have been derived with a precision of up to the third order. Using the rectangular field model we have obtained analytical expressions for these parameters as well as for chromatic aberration coefficients. The relation between electrostatic and magnetic components as a function of combined deflector strength has been found under the condition that the chromatic aberration is corrected in one direction. In the perpendicular direction, the chromatic aberration is reduced considerably. The conditions for achromatic deflection for various object positions have been calculated; we have found the object position for which this condition does not depend on the deflection direction. The residual chromatic aberration with such correction is less than one in the classical achromatic deflector.