周期场透镜的成象特性

由三个或五个透镜元素组成的周期场,可以作为离子透镜使离子束成象。通常它具有比单个透镜元素强得多的聚焦能力(在相同的电压比下)。这种周期场透镜曾用于静电加速器离子源初聚系统中,但它的成象特性尚未有详细的分析。本文提出一种研究这类透镜特性的方法。将周期场看作由若干个透镜元素合成的组合透镜,然后以Ferraris特征行列式求它的基点表示式。作为普遍情况,以浸没透镜作为透镜元素,导得了三个和五个透镜元素的周期场透镜基点表示式。式中只包含透镜元素的基点参量和电极系统的几何参量。式子十分简洁,便于计算,并有足够的精确度。通过解析表示式中各个因子的分析,可以方便地掌握透镜的特性。由短磁透镜组合的周期磁场是上述普遍情况的一个特例。文中以等径小间隙双圆筒透镜为浸没透镜元素,对周期场透镜的特性用导得的基点表示式进行了计算和分析。

THE IMAGE-FORMING CHARACTERISTICS OF PERIODIC FIELD LENSES

A periodic field formed with three or five lens elements can be used as an image-forming lens in ion-optics. Generally, it possesses much stronger focusing ability than a single lens element under the same voltage ratio. Periodic field lenses have been used as initial focusing systems of ion sources in some electrostatic accelerators, yet its image-forming characteristics have not been fully studied.This paper proposes a method to investigate the image-forming characteristics of periodic field lenses. Treating periodic field lens as a lens-element-combination, the expressions of its cardinal points can be obtained by using Ferrarias characteristic determinant. For a general case, with immersion lens as lens element, the expressions of cardinal points of periodic field lenses of three and five lens elements have been given. In these expressions only cardinal parameters of lens element and geometrical parameters of electrode system are involved. These derived expressions are quite simple in form, easy for calculation with sufficient accuracy. Thus there is no difficulty in studying the general characteristics of these lenses.Using these expressions, the image-forming characteristics of some periodic field lenses with symmetrical two-tube lens of small gap-spacing as lens element have been calculated and analysed.