Ionic densities measurement in an electron cyclotron resonance ion source plasma by high resolution X-ray spectroscopy

Electron cyclotron resonance (ECR) ion sources currently represent the best compromise for the supply of intense beams of highly charged positive ions. Their performances have, over the last few years, constantly evolved. Nevertheless, their improvement requires a better understanding of the mechanisms governing the production and confinement of ions in these machines. The aim of this work was to measure the densities of multi-charged ions in an RCE plasma by X-ray emission spectroscopy. The density distribution obtained could then be compared to the distribution of the currents extracted from the source. For this purpose a high resolution X-ray spectrometer was developed and built. At the same time, a detailed study of the Bremsstrahlung emitted by the plasma electrons allowed the determination of the electronic distribution function. A strong evolution of the high resolution X-ray spectra with the parameters of the ion source was observed. It appeared, moreover, a strong correlation with the extracted currents. This phenomenon could be confirmed by an absolute estimation of the different ion densities in the plasma. They proved to be a faithful image of the extracted currents. The ionic confinement times were estimated and a quasi-linear evolution with the charge appeared whereas a strongly exponential evolution was predicted by the theoretical models. These results constitute the first direct evidence that the commonly accepted theoretical model of ion confinement in an ECR source plasma is not adapted. (O.M.)