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Motivation

Charge breeding system

Linear accelerator

Target and Miniball

Collaboration

REXlogoCharge breeding system

Charge breeding system | REXTRAP | REXEBIS | Mass separator

REXEBIS

In contrast to a plasma ion source, an Electron Beam Ion Source (EBIS) uses mono-energetic electrons from an electron gun focused by a strong magnetic field to produce highly charged ions [1,2]. In an EBIS the ions are confined radially by the potential depression of the negative space charge of the electrons, while the longitudinal confinement is arranged by potential barriers established by cylindrical electrodes surrounding the electron beam. Trapped low-charged ions will undergo stepwise ionisation via electron impact until they are extracted when the outer barrier is lowered.

https://isolde.web.cern.ch/ISOLDE/REX-ISOLDE/rexebi2.jpg

Figure 1.

Level 1: The EBIS structure (not to scale) showing the crucial elements.

 

Level 2: The electrical potential along the axis for closed trap (green) and ramped extraction (purple).

 

Level 3: Potential created by the electron and ions.

The REXEBIS has attained a current density of ~150 A/cm2 and beam currents of <0.4 A. A 1.5 m solenoid provides a trap length of 0.8 m with a magnetic field strength of 2 T. The electron beam energy is adjustable between 3 and 6 kV. For these parameters Mg and Cs are charge bred to 7+ and 32+ in 8 ms and 150 ms, respectively. To obtain a high breeding efficiency, the phase space overlap of injected ions and the electron beam has to be large. Hence, a rather low extraction emittance of <10 π mm mrad (95% at 60kV) from the Penning trap is required for a successful injection into the EBIS. Since only one specific charge state from the total charge state distribution coming out of the EBIS is selected in the successive mass separator, the maximum breeding efficiency is about 30%. The space charge from the electron beam determines an upper limit for the maximum number of positive ions that can be stored in each pulse, for the above parameters this gives 5.1010 charges. Assuming 50% neutralization, 3.109 number of Na+ can be charge bred to Na8+ per pulse. This is almost one order of magnitude larger than the ion number which can be accumulated in the Penning trap.

https://isolde.web.cern.ch/ISOLDE/REX-ISOLDE/rexebi1.jpg

Figure 2. By varying the breeding time the charge state distribution is changed and different q/A-values can be optimised.

 

Figure 3. Time structure of extracted EBIS pulse measured at the Miniball. Outer barrier lowered and no ramping voltage applied on the trapping tubes. 90% of the beam is extracted within 80 us.

https://isolde.web.cern.ch/ISOLDE/REX-ISOLDE/rexebi3.jpg

While the voltage of the trap platform is fixed to 60 kV to decelerate the ions from ISOLDE, the platform of the EBIS is pulsed between injection and extraction from 60  to about 20 kV. Pulsing of the EBIS high voltage platform allows for a fixed ion extraction velocity independent of the q/A-value. The low energy (5 keV/u) leads to an efficient  adiabatic bunching and small longitudinal output emittances of the RFQ. In contrast to the Penning trap with 10-3 mbar buffer gas pressure, the EBIS requires a vacuum of better than 10-10 mbar. Therefore,  several differential pumping stages are inserted along the transfer line between trap and EBIS. In spite of the excellent vacuum inside an EBIS, the residual gas peaks are often comparable to or larger than the radioactive ions peaks, as illustrated in a figure in the Mass Separator section.

https://isolde.web.cern.ch/ISOLDE/REX-ISOLDE/rexebi4.jpg

Figure 4. The REXEBIS superconducting magnet standing on the HV platform. The vacuum cross containing the electron gun is closest to the camera and the injection/extraction is performed from the opposite side.

The EBIS was built by the Manne Siegbahn Laboratory in Stockholm in collaboration with Chalmers University of Technology in Gothenburg.

Data

Solenoidal magnetic field = 2 T

Eelectron = 3-6 keV

τconfinement = 3 to >200 ms

Beam current Ie = 0.4 A

Current density je = 100-150 A/cm2

Trap length Ltrap < 0.8 m

Warm bore

A/q < 4.5

References

  1. E.D. Donets and V.I. Ilyushchenko, JINR R7-4124, 1968
  2. Handbook of ion sources, Ed. B.Wolf (Springer, 1995)
  3. F. Wenander, Proc. of the 5th Radioactive Nuclear Beam conference, Divonne, France (2000). ed. H. L. Ravn et al., Nucl. Phys. A701 (2002) 528-536c.
  4. F. Wenander et al., Proc. of the 6th European Particle Accelerator Conference, Stockholm, Sweden, IOP Bristol (1999) p. 1412-1414.
  5. F. Wenander et al., Proc. of the 8th Int. Symposium on EBIS/T, Upton, NY, USA, Nov. 2000, AIP-Conference-Proceedings, no.572, (2001) p59-73.
  6. F. Wenander et al., ‘REXEBIS - the electron beam ion source for the REX-ISOLDE project’, CERN-OPEN-2000-320.
  7. B. Wolf et al., F. Wenander et al., Proc. of the 9th International Conference on Ion Sources, ICIS01, Oakland California, USA, Rev. Sci. Inst. 73 no.2 (2002) p.682-684.
     
  8. B. Wolf et al., Nucl. Instr.Meth. B204 (2003) 428-432. 

  9. F. Wenander, ‘Charge state breeders: on-line results’, Nucl. Instrum. Meth. B266 (2008) p.4346-4353.

 

Last modified 29/01/2009