OPTIMIZATION OF THE OPERATING CONDITION OF THE ACCELERATING TUBE FOR THE HIGH-VOLTAGE ELECTRON-MICROSCOPE EQUIPPED WITH THE FIELD-EMISSION GUN

Numerical calculations have been conducted on the electron optical cha racteristics of the accelerating tube (AT) for the high voltage electr on microscope (HVEM) equipped with the field emission gun; (FEG). The emitted electrons are firstly accelerated to V-0 by the FEG and finall y to V-a by the AT which consists of 34 stages of accelerating electro des with an inner diameter of 33 mm and has an overall length of 1423 mm. The AT is treated as a thick electrostatic accelerating lens. Seve ral electron optical problems arising from a combination of the AT wit h the FEG are studied. In order to reduce an unfavorable aberration ef fect of the AT-lens, the beam crossover must be brought to a position near the entrance plane of the AT. This can be done by a transfer lens which is placed between the FEG and the AT. The introduction of a par tial retarding field in the AT is also very effective for a flexible o peration of the FEG system without serious aberration effects of the A T-lens.