| Quasi-one-dimensional carbon material with large aspect ratio has good electrical,mechanical and field emission properties.As a cold electron gun filament,it has a good application prospect in cold field emission electron source.However,due to the large aspect ratio of this kind of cathode filament material,when it is disturbed by the external during operation,the emitter tip of filament will generate off-axis effect,which will lead to the change of the position and shape of the electron beam converging spot,and then affect the working accuracy of the equipment.In order to reveal the influence of external force disturbance on the beam spot position and shape of cathode filaments with different aspect ratios,a cold electron gun filament device with quasi-macroscopic carbon fiber(QMCF)placed in a coaxial sleeve base was prepared in this paper.The influence of external mechanical vibration on electron beam convergence characteristics under this structure was studied.The results are mainly as follows:(1)Based on the finite element method,the modal analysis and harmonic response analysis of the electron gun filament device were carried out by ANSYS software,and the off-axis range of the filament device with different carbon fiber aspect ratios was obtained.The results show that the off-axis displacement at the tip of carbon fiber increases with the increase of its aspect ratio,and the maximum off-axis displacement occurs at the tip of carbon fiber.Among them,when the length of the copper tubing bracket is 3 mm,the diameter is 0.3 mm,and the aspect ratio of the QMCF is less than200,the structure of the filament device is more stable.When the aspect ratio of the filament is within 200～600 and the excitation load is 3000 mm/s~2,the off-axis displacement of the filament device tip is between 22～391μm.(2)Based on the finite element method,the COMSOL software is used to establish the two-pole bipolar structure model of the cold field emission filament device based on QMCF and simulate the electron emission.Finally,the experimental verification is carried out.The results show that the beam spot diameter is between 16 mm and 17.5mm,and the beam spot diameter increases with the increase of carbon fiber filament length and tip emission area.Under external disturbance,when the filament aspect ratio is 428 and the off-axis displacement is 298.2μm,the beam spot diameter is 18.2 mm.The field emission test experiment of the filament device with the aspect ratio is carried out.The measured current density is 1000 A/cm~2,and the beam spot diameter is about17.5 mm,which is consistent with the simulation results and proves the reliability of the simulation results.(3)On the basis of the above,COMSOL software is used to establish a three-pole structure model for the cold field emission filament device based on QMCF and simulate the electron emission.Finally,the electron beam spot drift problem is analyzed.When the cathode and gate spacing is 4 mm,the grid and anode spacing is 4 mm,and the grid hole diameter is 2 mm,the diameter of the convergence beam spot is between0.91 and 1.02 mm.When the tip off-axis displacement caused by external disturbances such as vibration is small,the convergence beam spot only drifts in the same direction and the shape distortion is small.In this case,the maximum diameter of convergence beam spot is about 1.12 mm.When the off-axis displacement of the tip is large,the shape distortion is more serious,and the drift amount is 2 times that of the original beam spot.Through the above work,the common research methods and models of the influence of external disturbance on the electron beam spot drift of the filament cold electron gun system with large aspect ratio are established,and the rule of electron beam spot drift under the influence of external vibration is found.It has certain reference value for the research and development of cold electron gun devices and electronic optical systems based on QMCF and other quasi-one-dimensional materials. |
