| 摘要 |
An electron source for, for example, an electron microscope cannot exhibit a high brightness and a large beam current at the same time, because the virtual emitter dimension is enlarged by Coulomb repulsion in the electron beam in the case of a large beam current, thus reducing the brightness. In a conventional electron source switching-over could take place from a high brightness to a large beam current by varying the dimension of a beam-limiting diaphragm; however, this is objectionable because the location of such a diaphragm is not readily accessible. In accordance with the invention said switching-over can take place by arranging two lenses (26, 28) in the source, which lenses parallelize. In the described circumstances the beam is either directly behind the emitter (4) (large current) or directly in front of the diaphragm aperture (32) (high brightness).
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