JPS57151160A - Electron lens - Google Patents

Abstract

PURPOSE:To improve a focusing characteristic by dividing an excitation coil, varying excitation currents of the divided coils to obtain predetermined magnetomotive forces, and setting the individual excitation currents so that the total heating value of the coils is kept constant. CONSTITUTION:An excitation coil 5 is divided into two coils 5a, 5b, and excitation currents are fed from a power source 7 to each of them through control elements 8, 18. Excitation of the coils 5a, 5b is changed over under control of the control elements 8, 18 by adjusting the contacts of excitation correlation value setting circuits 12, 22. The inner coil 5a is connected to change-over contacts 13a-13f which are controlled by a relay 13, while the relay 13 is opened or closed by moving contacts engaged with the moving contacts of the setting circuits 12, 22. Excitation currents of opposite polarities are fed when the individual moving contacts are at 11a-11c, 21a-21c, 15a-15b, and excitation currents of the same set polarity are fed when the moving contacts are set otherwise. Accordingly, the square sum of the excitation currents remains constant even by the change-over, and the total heating value can be kept constant.