Pressure field and its gradient in electron microscopes

Abstract

In this work the analytical and numerical calculations for the modeling carried out for obtaining the pressure field and its gradient along the structure of an electronic microscope are presented. Electron microscopes in their various types generally have a tubular structure where the electron trajectory occurs. For the proper functioning of the electron microscope, the electron gun is operated at a pressure of the order of 10−7 mbar (10−5 Pa) and on the tubular structure at a pressure of the order of 10−6 mbar (10−4 Pa). The modeling of the vacuum system is based on the assumption that the transport of gases, in the molecular regime, can be considered a diffusion phenomenon. The specific degassing rate and the specific conductance of the tubular structure are defined, as well as the effective pumping speed. Both the steady and transient pressure fields are obtained. In the steady case only the gas source due to the degassing of the tube wall is considered. In the transient case, in addition to the natural degassing of the wall, a pulsed gas source in space and time is considered. This gas source may be originated from the electron beam hitting the wall of the electron microscope tube, due to a malfunction of the magnetic lenses. Through this analysis the time required for the vacuum system to restore the steady pressure field, for the electron microscope to return to normal operation can be determined.