EBIS as charge breeder for radioactive ion beam accelerators

Abstract

The numerous forthcoming post-accelerators for radioactive ions produced with the isotope separator on-line (ISOL) technique all have a need for an efficient method to accelerate the precious primary ions. By including an Electron Beam Ion Source (EBIS) as a charge breeder after the radioactive ion production stage, a short and compact linear accelerator can be employed as a result of the increased ion charge-to-mass ratio. In view of the constraints on a post-accelerator for radioactive beams, an EBIS appears to be an attractive solution. It is independent of the element injected, and an overall efficiency as high as 30% for the peak charge state can be attained. The mean charge state of the extracted ion beam is easily varied by changing the breeding time and the beam has excellent properties (time structure and emittance) for LINAC injection. Furthermore, the extraction of a bunched beam improves the signal-to-noise ratio for low-intensity beams and the UHV inside the source permits the mass separation of very weak radioactive beams from the residual gas contamination. With a high electron-beam density, the breeding time can be kept short, so decay losses are minimized. To obtain a high efficiency for a charge breeding EBIS, it is advantageous if the beam from the ISOL system is bunched and has its emittance reduced before injection. To achieve this a Penning trap can be introduced in front of the EBIS. The space-charge limit of the Penning trap restricts the number of ions per bunch to a couple of orders of magnitude lower than the constraint set by the EBIS. A positive side effect though, is that a mass selection can take place inside the trap. In this article the above listed features will be discussed. As an example, the charge breeding system for the REX-ISOLDE post-accelerator will be introduced. In addition, future possible developments of an EBIS leading to shorter breeding times, the acceptance of a continuous beam, and an increased number of charge-bred ions, will be discussed.