Experiments With Fundamental Aspects Performed in a Small ECR Ion Source for a New Carbon Therapy Facility

A small permanent magnet electron cyclotron resonance ion source (ECRIS) has been developed for the production of C⁴⁺ ions for the third carbon-therapy facility in Japan. The emphasis of the design is on stability, easy operation and prevention of maintenance, where high charge state production is not a requirement. Tests with different feed-gases have shown that carbon-hydrogen compounds are favorable, in particular with carbon to hydrogen ratio of 1:1. A parallel study of (highly charged) C⁵⁺ production revealed that the various compounds of feed-gas are behaving as if feeding the two components separately. This includes the behavior of replacing hydrogen by deuterium. From these a plausible reason was found for the observed failure of hydrogen as a mixing gas. Other experiments with the small source were performed to study the effect of inserting an aluminum (-oxide) cylinder inside the electron cyclotron resonance plasma-chamber. Earlier research found an increase in production of high charge states, but a good explanation was still missing. By adapting an insulated cylinder that can be biased positively, it was shown that ion diffusion to the side walls can be reduced, in particular when the cylinder is made of strips covering the pole pieces of the hexapole magnet. The new tool can be used to study the mechanism in more detail and also has possibilities of further improving a source for higher charge state production.