| 摘要 |
PROBLEM TO BE SOLVED: To properly maintain the degree of vacuum in a device and to provide a stable beam emission characteristic by installing an electric field leakage preventing shield plate in a space between an emitter for emitting charged particles and an extraction electrode for accelerating the charged particles. SOLUTION: A shield plate 21 is formed into a disc-like shape by the use of a conductive material such as stainless steel or titanium, integrally has a cylindrical part 21a surrounding the lower end part of a Wehnelt electrode 3 at its circumferential part, and is fitted to a small diameter part 4d of the central part 4b of an extraction electrode 4. The shield plate 21 can prevent an electric field between the extraction electrode 4 and a positive electrode 5 from leaking to a space between the Wehnelt electrode 3 and the extraction electrode 4, through an evacuating hole 8 of the extraction electrode 4. Thereby, reflected electrons present in the space between the Wehnelt electrode 3 and the extraction electrode 4 are never attracted to the positive electrode 5 by the leakage electric field, new reflected electrons are never generated from the positive electrode 5 nor electrifies an insulator 6, so that discharge due to electrification can be prevented.
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