The effect of cavity tuning on oxygen beam currents of an A-ECR type 14 GHz electron cyclotron resonance ion source

摘要

The efficiency of the microwave-plasma coupling plays a significant role in the production of highlycharged ion beams with electron cyclotron resonance ion sources (ECRISs). The coupling propertiesare affected by the mechanical design of the ion source plasma chamber and microwave launchingsystem, as well as damping of the microwave electric field by the plasma. Several experimentsattempting to optimize the microwave-plasma coupling characteristics by fine-tuning the frequency ofthe injected microwaves have been conducted with varying degrees of success. The inherent difficultyin interpretation of the frequency tuning results is that the effects of microwave coupling system andthe cavity behavior of the plasma chamber cannot be separated. A preferable approach to study theeffect of the cavity properties of the plasma chamber on extracted beam currents is to adjust thecavity dimensions. The results of such cavity tuning experiments conducted with the JYFL 14 GHzECRIS are reported here. The cavity properties were adjusted by inserting a conducting tuner rodaxially into the plasma chamber. The extracted beam currents of oxygen charge states O3+–O7+were recorded at various tuner positions and frequencies in the range of 14.00–14.15 GHz. It wasobserved that the tuner position affects the beam currents of high charge state ions up to severaltens of percent. In particular, it was found that at some tuner position / frequency combinationsthe plasma exhibited “mode-hopping” between two operating regimes. The results improve theunderstanding of the role of plasma chamber cavity properties on ECRIS performances.