Influence of harmonic disturbances on electron beam spot characteristics of the large aspect ratio emitter field emission gun

Abstract Quasi-one-dimensional carbon materials such as carbon fiber (CF) have good mechanical, electrical and field emission properties so that they are used as emitters in field emission guns (FEG). Due to the large aspect ratio (LAR) of such materials, it is easy to be affected by external force disturbances, which leads to an off-axis deformation of the emitter and further causes drift of the electron beam spot (EBS). This greatly reduces the working accuracy of electron gun. In this work, the finite element method (FEM) is used to analyze the harmonic response and EBS characteristics of the FEG with an LAR emitter (LARE-FEG). The results show that when the harmonic disturbance is applied, the emitter tip's off-axis displacement increases with the aspect ratio (AR) of CF, which leads to an increase in the EBS drifting range. Particularly, three EBS drifting types are obtained. When the emitter's tip off-axis displacement is between 22.8 μm and 391.1 μm (the range of CF's AR is between 200 and 600), drift increases the diameter of converging EBS by 17% to 136% compared with drift-free. The research results can provide a technical route for harmonic disturbance analysis of the LARE-FEG and optimization of the whole machine.
Keywords: Quasi-one-dimensional material; Large aspect ratio emitter; Field emission gun; Vibration deformation; Electron beam spot drift