Abstract
Numerical calculations were conducted on the electron optical characteristics of the accelerating tube (AT) for the high voltage electron microscope. The emitted electrons are firstly accelerated to V<SUB>0</SUB> by the TE gun or the FE gun and finally to V<SUB>a</SUB> by the AT which consists of 34 electrodes with the inner diameter of 33 mm and has the overall length of 1423 mm. The AT is treated as a thick electrostatic accelerating lens. Several electron optical problems arising from a combination of the AT with a thermionic emission (TE) gun or a field emission (FE) gun are studied. For the TE gun the aberration effect of the AT lens is found to be safely neglected for any combination of Va and V<SUB>0</SUB>. In the case of the FE gun, on the other hand, the aberration effect of the AT lens can not be neglected and deteriorates the brightness of the beam. This situation can be overcome by placing an electron lens between the FE gun and the AT.
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