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场发射电子枪具有亮度高、束斑小和能量分散小等优点。但在三极场发射电子枪的情况下,吸极电压或加速极电压的任何改变都会引起电子光学特性的改变。特别是交叉点位置的变化很大,因而很难使场发射电子枪工作在最佳状态。为了解决这个问题,必须使三极场发射电子枪工作在加速电压对吸极电压的比值大于12的范围;或者采用四极场发射电子枪。场发射电子枪的理论计算是比较复杂的,这里采用法国图卢兹电子光学实验室的方法,把阴极和吸极间的空间当作等电位空间来处理,用“有限元”法计算电场分布,并得到了一组工作在30kV的四极场发射电子枪的特性曲线。
Field emission electron gun has the advantages of high brightness, small beam spot and small energy dispersion. However, in the case of a triple-polar field electron gun, any change in the pick-up voltage or the accelerating voltage causes a change in the electro-optical properties. In particular, the location of the intersections varies greatly, making it very difficult to make the field emission electron gun work in the best condition. In order to solve this problem, it is necessary to make the triode field emission electron gun operate at a ratio of the accelerating voltage to the attracting voltage greater than 12; or use a quadrupole field emission electron gun. The theoretical calculation of the field emission electron gun is quite complicated. Here, the method of electron optical laboratory in Toulouse, France is adopted. The space between the cathode and the absorber is treated as the equipotential space, and the electric field distribution is calculated by the “finite element method” A set of characteristics of a quadrupole field-emission electron gun operating at 30kV.