A key development for the future of high-energy radioactive ion-beam facilities is the efficient and fast “charge-state breeding” from singly charged ions at low energies (10-60 keV). Recently, a major breakthrough has been made with the first charge breeding with an electron cyclotron resonance (ECR) source at the ISOLDE facility at CERN.
Two processes for the charge breeding of 1+ states to multi-ionized ones are currently available. At ISOLDE the 1+ beam is cooled into a Penning trap before injection into an electron-beam ion source (EBIS), which performs the multi-ionization. At the Laboratory for Subatomic Physics and Cosmology (LPSC), Grenoble (CNRS/IN2P3-UJF-ENSPG), the 1+ beam is captured directly into the dense plasma of a dedicated ECR ion source, the PHOENIX Booster, which ensures the cooling and multi-ionization.
At the end of an EU contract on charge breeding (HPRI-CT-1999-50003), which set up a collaboration of European laboratories to study this important topic, the IS397 experiment at ISOLDE was agreed upon to compare the characteristics of the two techniques. The concept of the PHOENIX Booster has already been chosen for two future facilities, ISACII at TRIUMF in Vancouver, Canada, and SPIRALII at GANIL in Caen, France.
Charge breeding of stable ions with an ECR ion source (ECRIS) has been conceived, studied and improved over the past 10 years at LPSC. The first ECRIS used was MINIMAFIOS, by Richard Geller, and since 2000 a dedicated one, the PHOENIX Booster, has been developed together with its associated injection optics by Pascal Sortais and Thierry Lamy. For the primary beams more than 20 elements have been produced by various ion sources (thermo-ionization, glow discharge, ECRs). The parameters measured are the efficiency yields, the charge-breeding times, and eventually, for pulsed modes, the time the multicharged ions are trapped in the device.
The IS397 set-up at ISOLDE duplicates the experiment at LPSC to assure a fully comparable operation. The CLRC Daresbury Laboratory made its PHOENIX source available to the collaboration, together with all its power supplies; the analysing magnet for the multi-ionized (n+) beams came from the test bench at LPSC; and the detection device was provided by ISOLDE. Several other institutes have also contributed with equipment, including Ludwig-Maximilians-Universität with a double Einzel lens and GSI with power supplies.
In the experiment a 50 nA 238U+ beam from the REX-ISOLDE target was injected in the PHOENIX source. The extracted spectrum (see figure) shows a 2% efficiency for 238U26+. The 96Sr15+ and 94Rb15+ states have also been produced, as well as stable and radioactive ions of the noble gases argon, xenon and krypton.
The main advantages of the technique are the simple technology, the fast tuning and the reliability of the system. For the future development of high-intensity accelerators, it is important to note that there is no limitation on the 1+ primary beam current accepted by the process. This is due to the high density of the ECR plasma, which varies as the square of the radiofrequency used to produce the ionizing plasma. Currently 14 GHz is used, giving a density of a few 1012 ions per cm3. The limitation currently is the ECR breeding of the light elements like lithium, sodium and neon, which is less efficient than the Penning trap-EBIS system. However, one of the goals of IS397 is to conclude in which context each technique is more convenient.
• The charge-breeding collaboration is Ludwig-Maximilians-Universität, Munich, Germany; LPSC Grenoble, France; CLRC Daresbury Lab, UK; ISOLDE, CERN, Switzerland; GANIL, Caen, France; INFN, Legnaro, Italy; Kungliga Tekniska Högskolan, Stockholm, Sweden; and the University of Jyväskylä, Finland.
