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

Abstract

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 ϵ rms 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 ϵ rms < 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 1þ beam characteristics with a rf beam cooler prior to the injection into an ECR-CB.

The capture of injected 1+ ions in charge breeder electron cyclotron resonance ion source plasma

“…Furthermore, the initial transverse energy dispersion of the 1+ beam in the injection beamline can increase the longitudinal energy spread at the magnetic mirror (injection point) due to the azimuthal deceleration of the ions propagating towards an increasing magnetic field-an inverse process to the emittance increase when extracting into a decreasing magnetic field (see Busch's theorem e.g. in [26])which together with the revised capture model explains the numerically predicted [27] and experimentally observed [16] dependence of the CB efficiency on the transverse emittance of the 1+ beam.…”

Charge breeding time investigations of electron cyclotron resonance charge breeders

“…To achieve that, the first electrode (puller) potential of the ion gun source [20] was modified. The puller electrode controls the ion extraction from the gun and allows a fast change of the beam intensity, changing slightly the beam emittance from 3.1 to 3.3 mm mrad (1σ, rms), which can be considered as a small change for the injection optics and capture process [24]. A fast high-voltage supply TREK 677B, set on the high voltage platform, was used to polarize the puller electrode with respect to the high voltage.…”

Numerical simulations of magnetically confined plasmas

“…The validity of the code was demonstrated by reproducing two experimental results obtained with the PHOENIX Charge Breeder at the Laboratoire de Physique et de Cosmologie, by injecting sodium [10] and rubidium ions [7]. It also revealed to be very useful in understanding the influce of the injected beam parameters on the capture process, in particular the beam emittance and energy spread [11]. The plasma was implemented following the plasmoid/halo scheme [12]: the numerical analysis carried out regarded the effect of different plasma parametrs on the capture process, in particular the plasma density and ion temperature (electrons are involved only in the ionization process).…”

“…The results shown in this paper confirm the validity of the numerical code developed by the INFN ion source group in describing the beam-plasma interaction: figures 1-4, together with those shown in ref. [7,10,11], demonstrate its capability in simulating properly the slowing down and capture of the injected ions for light and heavy species, identifying the beam and plasma parameters playing a key role in the process. The preliminary results of the electrons dynamics show for the first time ever that the ECR plasma automatically assume a plasmoid/halo structure, with a peculiar distribution…”