Machicoane G, Doleans M, Stetson J, Wu X, Závodszky P A
National Superconducting Cyclotron Laboratory, MSU, East Lansing, MI 48824, USA.
Rev Sci Instrum. 2008 Feb;79(2 Pt 2):02B714. doi: 10.1063/1.2816662.
Solenoids are widely used to provide initial focusing of beams extracted from an ion source. However, in the case of an electron cyclotron resonance (ECR) ion source, the extracted beam will usually include different ion species and for each of them a wide distribution of charge states. When such a multicomponent beam is focused by a solenoid, the ions with a Q/A larger than the beam of interest are overfocused and usually go through a waist before reaching the analyzing magnet. If the beam currents obtained for these ions are sufficient, the resulting space charge forces can significantly degrade the emittance of the beam components with a lower Q/A and result for those in a hollow beam. Using a beam viewer and an emittance-measuring device, this paper reports on experimental findings that confirm the existence of such an effect for low charge states of argon. Moreover, by changing the experimental conditions of the ECR plasma in order to modify the charge state distribution of the extracted ion beam, it is shown that the threshold where this space charge effect starts to be significant can be changed.
螺线管被广泛用于对从离子源引出的束流进行初始聚焦。然而，在电子回旋共振（ECR）离子源的情况下，引出的束流通常会包含不同的离子种类，并且每种离子都有很宽的电荷态分布。当这样的多组分束流由螺线管聚焦时，比感兴趣的束流具有更大Q/A值的离子会过度聚焦，并且通常在到达分析磁铁之前经过一个束腰。如果这些离子获得的束流足够大，由此产生的空间电荷力会显著降低具有较低Q/A值的束流组分的发射度，并导致这些组分形成空心束。本文使用束流观测仪和发射度测量装置，报告了实验结果，证实了氩的低电荷态存在这种效应。此外，通过改变ECR等离子体的实验条件以改变引出离子束的电荷态分布，结果表明这种空间电荷效应开始变得显著的阈值可以改变。
