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Motivation

Charge breeding system

Linear accelerator

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Collaboration

REXlogoCharge breeding system

Charge breeding system | REXTRAP | REXEBIS | Mass separator

REXTRAP

Bunching and accumulation of the semi-continuous radioactive beam is required to achieve an efficient ion injection into the EBIS. Moreover, for the low-intensity radioactive beams it is advantageous to compress the ions in bunches to increase the signal-to-background ratio in the measurements. Thus, the ions are injected into a large Penning trap, where they are slowed down by collisions with the atoms of a buffer gas. When aiming for high accumulation efficiency the energy loss during a single oscillation in the trap has to be at least as large as the initial longitudinal energy spread of the ions after electrostatic retardation and injection into the magnetic field of the Penning trap. For a typical 60 keV ISOLDE beam this can be about 50 eV requiring a buffer gas pressure of some 10-3 mbar of Argon or Neon at a trap length of 0.9 m.

To center the ions in radial direction mostly mass selective sideband cooling at the ions cyclotron frequency [1] is applied. Alternatively at high numbers of stored ions (>106) rotating wall compression of the ion cloud [2] can be used.

The trapping
With the REXTRAP on 60 kV, the incoming
ions have just enough energy to climb the first
potential threshold. Inside the trap the ions
collide with a buffer gas and lose energy, and
can therefore not leave the trap. After cooling,
the ions are extracted to the REXEBIS in a
bunch by lowering the ions potential threshold.

REXTRAP1

 

REXTRAP1

The cooling process
In the applied electric and magnetic fields the ions move with three characteristic motions. When the ions cool by buffer gas collisions, the magnetron motion becomes unstable, and its orbital radius increases. The magnetron movement is therefore damped by a combination of an RF-excitation and a buffer gas, i.e. coupling of ω+ (the reduced cyclotron motion) and ω- (the magnetron motion) by shining in ωc.
Since the cyclotron frequency is mass dependent, we obtain a mass selective cooling. Due to the high bufer gas pressure and the short excitation time, the m/Δm is limited to ~300.

 

Transversal emittance of K+ ions ejected from REXTRAP: to the left sideband cooling has been applied, to the right no cooling. The intensity scale in both pictures can not be compared.

REXTRAP2

REXTRAP2

 

REXTRAP2

Time of flight (TOF) spectrum of radioactive 30Mg ion bunches ejected from REXTRAP. The Ne signal originates from buffer gas atoms.

REXTRAP2
The picture to the right shows the superconducting magnet with the trap electrodes inside. The entire set-up is placed on a high voltage platform for the electrostatic retardation of the ions.

Data

Buffer gas = Argon, Neon

Buffer gas pressure < 10-3 mbar

Magnetic field B = 3 T

Trap length = 0.9 m

Cooling time = typically 20 ms

Trap capacity < 108 ions/bunch

Transversal emittance ~ 10 π mm mrad at 30 keV

Efficiency 40-60%

References

  1. G. Bollen, et al., Nucl. Inst. and Meth. A 368 (1996) 675
  2. X.-P. Huang, et al., Phys. Rev. Let. 78 (1997) 875
  3. P. Schmidt, "REXTRAP - Ion Accumulation, Cooling and Bunching for REX-ISOLDE", Doctoral thesis, Johannes Gutenberg Universitaet, Mainz, Germany (2001)
  4. O. Forstner, "Beam-Preparation with REXTRAP for the REX-ISOLDE experiment", Doctoral thesis, T.U. Wien, Austria (2001)
  5. P. Schmidt et al., Nucl. Phys. A701 (2002) 550c
     
  6. F. Ames, et al., Hyperfine Int. 132 (2001) 469
  7. D. Beck, et al., Hyperfine Int. 132 (2001) 473
  8. L. Weissmann, et al., Hyperfine Int. 132 (2001) 535
  9. L. Weissmann, et al., Nucl. Instr. Meth. A492 (2002) 451

  10. F. Ames et al., Nucl. Instrum. Meth. A 538 (2005) 17-32

Last modified 29/01/2009