不对称单场透镜的电子光学性质

The Electron Optical Properties of Asymmetrical Condenser-objective Lenses

对于不对称单场透镜的电子光学性质进行了系统的分析和计算。单场透镜的性质不仅决定于极靴的几何参数,而且决定于它的磁饱和程度。下极靴适当地强饱和时,不对称单场透镜可以产生低的球差系数。研究球差系数和色差系数的被积函数沿轴的分布,可以帮助设计者分析象差系数和极靴磁饱和的关系。D_1/D_2=4的不对称透镜是值得推荐的。不对称单场透镜在机械结构方面和电子光学性质方面都有其特点,值得加以注意和利用。

The electron optical properties of a condenser-objective lens are determinednot only by its structure parameters,but also by the degree of saturation of its pole-pieces.A systematic theoretical investigation has been carried out for a series of asy-mmetrical condenser-objective lenses.Comparison between symmetrical and asymmetricallenses has been made.Results show that for unsaturated or mildly saturated pole-pieces the performance of a symmetrical lens is better than its asymmetrical counter-part;but when the lower pole-piece is properly highly saturated,an asymmetricallens possesses lower spherical aberration coefficient than the corresponding symmetricalone.To investigate the correlation between the spherical and chromatic aberration coe-fficient C_s,C_c and saturation condition in the lens pole-piece,and therefore,toprovide useful diagnostic information in the early design stages,it is useful to displaythe distribution of integrated function of spherical and chromatic aberration coefficientalong the axis.An asymmetrical condenser-objective lens has its own characteristics,for example,the large bore of its upper pole-piece can provide a practical advantagein the mechanical design of the lens,and therefore is noticeable.The asymmetricalcondenser-objective lens of D_1/D_2=4 is recommendable.