Abstract
According to earlier measurements with the two-aperture method, the brightness of electron guns has sometimes a maximum value near the cut-off potential. This result has been confirmed by means of the lens method. Experiments with well-oriented monocrystalline spherical cathodes lead to the result that a brightness maximum only occurs when the cathode apex has a lower emission density than its environment. In this case, the axial brightness is increased by electrons emitted from the off-axial cathode area which feed the space-charge cloud in front of the cathode. Under space-charge conditions, the cathode adjustment is not very critical, because the axial brightness differs less than the emission densities of different crystal surfaces
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