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The capture of many electrons by Ar !7+ ions, at low velocity, near a metallic surface, has been studied. Multiexcited bound states with many electrons in the outermost shells (hollow atoms) have been observed. The surrounding of an ionic excited core by many outermost electrons greatly decreases the lifetimes of the states. This characteristic decrease explains the main striking features of the relaxation of the ions.

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Due to the production methods of exotic nuclei, an efficient acceleration of radioactive ion beams needs charge breeding of weakly charged ions. The upgrade of existing isotope separator on-line facilities ͑TRIUMF-isotope separation and acceleration, CERN-isotope separation on-line detector, etc.͒ or the development of projects for the acceleration of radioactive ion beams ͑GANIL-SPIRAL2, MAFF, EURISOL, etc.͒ requires charge breeders with high efficiency, fast charge breeding time, low background levels, and high intensity acceptance either in continuous or in pulsed mode. The optimization of these parameters is a challenge for the electron cyclotron resonance ͑ECR͒ community and is useful to get a better understanding of plasma physics in ECR ion sources ͑ECRISs͒. The ECR charge breeding technique has been developed for more than ten years at LPSC ͑former ISN͒ Grenoble, typical 1 + → n+ efficiencies are in the 3%-10% range depending on the nature of the incoming beam ͑metallic, alkaline, and gaseous͒ and remain constant for high intensity injection. Different laboratories have developed programs to study ECR charge breeding, and the progress of these studies will be presented. ECRISs have main advantages to be efficient charge breeders and a few limitations which will be discussed.