The effect of cavity tuning on oxygen beam currents of an A-ECR type 14 GHz electron cyclotron resonance ion source Tarvainen, Olli; Orpana, J.; Kronholm, Risto; Kalvas, Taneli; Laulainen, Janne; Koivisto, Hannu; Izotov, I.; Skalyga, V.; Toivanen, V.Tarvainen, O., Orpana, J., Kronholm, R., Kalvas, T., Laulainen, J., Koivisto, H., . . . Toivanen, V. (2016) The direct injection of intense ion beams from a high field electron cyclotron resonance ion source into a radio frequency quadrupolea) Rev. Sci. Instrum. 85, 02A740 (2014) The efficiency of the microwave-plasma coupling plays a significant role in the production of highly charged ion beams with electron cyclotron resonance ion sources (ECRISs). The coupling properties are affected by the mechanical design of the ion source plasma chamber and microwave launching system, as well as damping of the microwave electric field by the plasma. Several experiments attempting to optimize the microwave-plasma coupling characteristics by fine-tuning the frequency of the injected microwaves have been conducted with varying degrees of success. The inherent difficulty in interpretation of the frequency tuning results is that the effects of microwave coupling system and the cavity behavior of the plasma chamber cannot be separated. A preferable approach to study the effect of the cavity properties of the plasma chamber on extracted beam currents is to adjust the cavity dimensions. were recorded at various tuner positions and frequencies in the range of 14.00-14.15 GHz. It was observed that the tuner position affects the beam currents of high charge state ions up to several tens of percent. In particular, it was found that at some tuner position / frequency combinations the plasma exhibited "mode-hopping" between two operating regimes. The results improve the understanding of the role of plasma chamber cavity properties on ECRIS performances. Published by AIP Publishing.[http://dx