Charge breeding techniques

Abstract

The numerous newly built and 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 increasing the ion charge-tomass ratio directly after the radioactive ion production stage, a short and compact linear accelerator can be employed. Not only the efficiency, but also the rapidity of such a charge-to-mass increasing process, called charge breeding, is a crucial factor for the often short-lived radioisotopes.The traditional foil or gas stripping technique was challenged some five to ten years ago by novel schemes for charge breeding. The transformation from 1 + to n + charged ions takes place inside an Electron Beam Ion Source/Trap (EBIS/T) or Electron Cyclotron Resonance Ion Source/Trap (ECRIS/T) by electron-ion collisions. These charge breeders are located in the low-energy part of the machine before the accelerating structures. Because of the capability of these devices to produce highly charged ions, mass-to-charge ratios between 4 and 9 are easily obtained.In this article the performance and the features of the above listed charge breeding concepts will be compared and discussed. An outlook on charge breeders for the next generation of radioactive ion beam facilities is also given.

Charge state breeders: On-line results

“…In addition, a large number of other stable elements/isotopes, covering 6 Li to 238 U, have been charge bred for setting up and calibration purposes. Remarkable results in terms of versatility and efficiency were noted during the 2006 measurement campaign.…”

“…The total efficiency comprises REXTRAP transmission, the beam transport from the trap to the EBIS, the injection-breeding-extraction cycle, and the transmission through the mass separator. The total efficiency can be at least 4% over the 6 Li to 238 U mass range, demonstrating the element independency. Nevertheless, with poor injection conditions, the efficiency drops dramatically, and the breeding time rises.…”

“…In addition, foil stripping may be used in combination with the charge breeding technique, to further accelerate the beam, or to improve beam purity. See also [6,7] for further treatment of the subject.…”

Evaluation of charge breeding options for EURISOL

“…For example, at GSI the High Current Injector of UNI-LAC, a 30 m long, 36 MHz, 2 MW accelerator consisting of an IH-type RFQ and two IH-DTL cavities, is required to accelerate the ions to 1.4 MeV/u for the first stripping stage. The pre-stripper induces a significant additional cost to the facility [3]. This method, although the most rapid and robust one, might not be the best choice for EURISOL due to the drawbacks given above (although stripping can be used for additional purification of the beam from isobaric contamination).…”

Selective containment in EBIS for charge breeding applications

“…Until now, in fact, two high-charge-state sources are considered as charge breeder for RIB production with the ISOL technique: ECR and EBIS sources. Both those sources have some encouraging features but also some drawbacks [14]. The main drawback of the EBIS is given by the impossibility of injecting the 1+ ions continuously in the trap.…”