Abstract

The accelerating tube (AT) which accelerates electrons up to a desired energy also functions as an electrostatic lens, In the present paper, numerical calculations were conducted on the electron optical characteristics including the spherical aberration of the AT for the high voltage electron microscope. Several electron optical problems arising from a combination of the AT with a thermionic electron gun (TEG) or a field emission gun (FEG) were studied.<Estimation of aberration for AT lens> The AT consists of 34 electrodes with the inner diameter of 3.3 cm and has the overall length of 142.3 cm (Fig.l). The voltage Va is applied between the cathode and the final electrode of the AT. The initial energy of the electron incident to the AT is eVo, where Vo is the anode voltage of the electron gun mounted on the AT. The electric field inside the AT was calculated using the surface charge method. The ray tracing was carried out on the basis of the relativistic paraxial ray equation, and the cardinal elements of the AT lens were obtained.

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