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A theoretical study has been made of the focusing of high-current electron beams by means of the aperture effect. The aperture effect is obtained by a triode electron optical system. An equation for describing the beam profile in the system has been described. Both the condition for obtaining the minimum radius of the beam and the condition for optimum focus have also been worked out. Two numerical examples are treated to illustrate the triode focusing of electron beams with two different total current magnitudes of 50 and 250 µa. It is found that by using this triode system the final energy of the electrons and the total current of the electron beam may be adjusted separately.