Eddy current compensation for magnetic electron lenses

Abstract

The decay of eddy currents in magnetic electron lenses poses a fundamental limit to their speed of operation. The effect of eddy currents on the focusing properties of an electron lens is considered and it is shown that response of the lens field to excitation can be well approximated as a linear system. Using Fourier transform techniques, the transfer function of the lens is derived from a mesurement of the peak axial field in response to step-like excitation from which the necessary excitation to achieve a specific transient response can be calculated. For practical implementation, the design of analogue pre-emphasis filters to improve the transient response of magnetic lenses is presented. The procedure involves approximating the step response of the lens field by exponential functions to derive the transfer function of the filter.