| 摘要 |
PURPOSE:To enhance the uniformity of focusing, by making the longitudinally sectional form of the opening of the high-potential electrode of a prefocusing lens unit conical to set a larger diameter for the end of the opening in the face of the lower-potential electrode of the unit than for the other end to reduce the aberration of an electron beam. CONSTITUTION:The longitudinally sectional form of the opening of the high- potential grid 4 of a prefocusing lens unit is made conical so that the end of the opening in the face of the lower-potential grid 3 of the unit has a larger diameter than the other end. As a result, the steepness of the envelope for an equipotential line group 8, which is one of equipotential line groups 7, 8 generated between the grid 3, 4 and is convex toward the grid 4, is decreased to diminish the influence of the edge of the grid 4. For that reason, the aberration of an electron beam is reduced to enhance the uniformity of focusing.
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