Method and apparatus for controlling alignment and brightness of an

Abstract

An electron beam is aligned with the center of an aperture in a plate in the path of the beam by cyclically scanning the beam across the aperture through equal distances on opposite sides of an initial beam location, measuring the beam current that flows through the aperture when the beam is on opposite sides of the initial location, which opposite sides correspond to adjacent half cycles of the scan, determining the difference in beam currents flowing in adjacent half cycles, and deflecting the beam in a direction toward the center of the aperture until there is reached a condition wherein the difference in currents is zero, which condition indicates that the beam is aligned with the center of the aperture. The currents may be measured by converting them to signals having a frequency proportional to the current, counting the signal cycles during each half cycle, and then subtracting the count for one half cycle from that of an adjacent half cycle. The amplitude of the scan is large enough such that the beam strikes the opposite edges of the aperture at the extreme limits of each scan. The scan frequency is preferably that of the power line frequency to eliminate noise problems. The total current flowing through the aperture during a complete scan cycle is also sensed and used to regulate the brightness of the electron beam.