Electron Donors And Their Role On The Performance Of Microwave Discharge And Electron Cyclotron Resonance Ion Sources

In order to enhance the output of highly charged ions from an electron cyclotron resonance ion source (ECRIS), several techniques like wall coating, biased disk, electron gun have been proposed and are meanwhile employed as standard tools at most of the existing installations. Although the detailed mechanisms are not clear, it has become evident that the additional injection of electrons into the plasma chamber of an ECRIS considerably improves its performances. Depending on the special conditions of the source these additional electrons can either compensate for losses of plasma electrons or even may change global plasma parameters (e.g. plasma potential) and hence positively influence the extraction at high rates of highly charged ions. The first experiments on biased disk were carried out at CEA, Grenoble and KVI, Groningen in 1990-91 and since then, this method is used by almost any ECRIS in the world. Wall coating is not so widely used but it has been proved to be effective not only for ECRIS but also for microwave discharge ion source (MDIS). Other methods to provide electrons to the plasma will be described in the following. In particular, at the Institute of Physics and Nuclear Engineering (IFIN-HH), Bucharest, special metal-dielectric (MD) structures of Al-Al2O3 have been developed. They are characterized by their very high secondary electron emission coefficients and therefore they seem to be ideal to be used in an ECRIS. Preliminary tests at IK in Frankfurt and at KVI have given interesting results. In the following an analysis of the possibilities opened by this method and other methods involving electron donors in ECRIS and MDIS field is carried out.