EDDY-CURRENT COMPENSATION FOR MAGNETIC ELECTRON LENSES

The decay of eddy currents in magnetic electron lenses poses a fundame ntal limit to their speed of operation. The effect of eddy currents on the focusing properties of an electron lens is considered and it is s hown that response of the lens field to excitation can be well approxi mated as a linear system. Using Fourier transform techniques, the tran sfer function of the lens is derived from a measurement of the peak ax ial 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 fi lters to improve the transient response of magnetic lenses is presente d. The procedure involves approximating the step response of the lens field by exponential functions to derive the transfer function of the filter.