Influence of the injected beam parameters on the capture efficiency of an electron cyclotron resonance based charge breeder

Electron cyclotron resonance ion sources based charge breeders (ECR-CB) are fundamental devices for Isotope Separation On Line (ISOL) facilities aiming at postaccelerating radioactive ion beams (RIBs). Presently, low intensity RIBs do not allow a conventional tuning of the ECR-CB: as a consequence, it has to be set with a stable 1+ pilot beam first, switching then to the radioactive one without changing any parameter; this procedure is usually called “blind tuning.” Besides having different masses, pilot and radioactive beams can also differ in terms of the rms transverse emittance erms and/or longitudinal energy spread ΔE, so the choice of a given pilot beam can determine the overall performances of the final breeding stage. This paper shows a numerical study of how the capture efficiency of the PHOENIX charge breeder is affected by the aforementioned beam paramaters: the analysis reveals the two-step nature of the process, highlighting the role of the injection optics and the plasma capture capability in the overall performances of this device. The simulations predict highest efficiency for erms<5π  mm mrad and ΔE<5  eV in a optimum energy range between 2 and 6 eV, thus giving important information on the possibility of blindly tuning an ECR-CB. No isotopical effects were observed, while it clearly came out the necessity to improve the 1+ beam characteristics with a rf beam cooler prior to the injection into an ECR-CB.