| 摘要 |
PURPOSE:To obtain a function equivalent to multi-stage configuration of focusing lenses with small bore diameters by fixing a plurality of annular magnetic material pieces, which have taper parts on their inner surfaces, inside a yoke so as to form lens gaps and configuring a vacuum shield pipe, which has a conical outer surface along the taper parts, inside the magnetic material pieces. CONSTITUTION:A plurality of annular magnetic material pieces 3 to 7 which have taper parts on their respective inner surfaces are surrounded with a lens yoke 1 and configured coaxially. These annular magnetic material pieces 3 to 7 are fixed with non-magnetic material spacers 8 so that magnetic lens gaps A to F are formed between adjacent annular magnetic material pieces themselves or between the yoke terminal parts 1a and 1b. A vacuum shield pipe 9 having a conical outer surface along the taper parts on the inner surfaces of the respective annular magnetic material pieces 3 to 7 is configured inside the magnetic material pieces 3 to 7, and a single excitation coil 2 is assembled in the lens yoke 1. Thus, a focusing lens which functions equivalently to multi- stage configuration of a focusing lens having a small magnetic gap and a small bore diameter can be obtained easily in manufacture and profitably in cost.
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