RFQ beam cooler

A radio-frequency quadrupole (RFQ) beam cooler is a device for particle beam cooling, especially suited for ion beams. It lowers the temperature of a particle beam by reducing its energy dispersion and emittance, effectively increasing its brightness (brilliance). The prevalent mechanism for cooling in this case is buffer-gas cooling, whereby the beam loses energy from collisions with a light, neutral and inert gas (typically helium). The cooling must take place within a confining field in order to counteract the thermal diffusion that results from the ion-atom collisions.

The quadrupole mass analyzer (a radio frequency quadrupole used as a mass filter) was invented by Wolfgang Paul in the late 1950s to early 60s at the University of Bonn, Germany. Paul shared the 1989 Nobel Prize in Physics for his work. Samples for mass analysis are ionized, for example by laser (matrix-assisted laser desorption/ionization) or discharge (electrospray or inductively coupled plasma) and the resulting beam is sent through the RFQ and "filtered" by scanning the operating parameters (chiefly the RF amplitude). This gives a mass spectrum, or fingerprint, of the sample. Residual gas analyzers use this principle as well.

Applications of ion cooling to nuclear physics

Despite its long history, high-sensitivity high-accuracy mass measurements of atomic nuclei continue to be very important areas of research for many branches of physics. Not only do these measurements provide a better understanding of nuclear structures and nuclear forces but they also offer insight into how matter behaves in some of Nature’s harshest environments. At facilities such as ISOLDE at CERN and TRIUMF in Vancouver, for instance, measurement techniques are now being extended to short-lived radionuclei that only occur naturally in the interior of exploding stars. Their short half-lives and very low production rates at even the most powerful facilities require the very highest in sensitivity of such measurements.

Penning traps, the central element in modern high-accuracy high-sensitivity mass measurement installations, enable measurements of accuracies approaching 1 part in 10^11 on single ions. However, to achieve this Penning traps must have the ion to be measured delivered to it very precisely and with certainty that it is indeed the desired ion. This imposes severe requirements on the apparatus that must take the atomic nucleus out of the target in which it has been created, sort it from the myriad of other ions that are emitted from the target and then direct it so that it can be captured in the measurement trap.

Cooling these ion beams, particularly radioactive ion beams, has been shown to drastically improve the accuracy and sensitivity of mass measurements by reducing the phase space of the ion collections in question. Using a light neutral background gas, typically helium, charged particles originating from on-line mass separators undergo a number of soft collisions with the background gas molecules resulting in fractional losses of the ions’ kinetic energy and a reduction of the ion ensemble’s overall energy. In order for this to be effective however, the ions need to be contained using transverse radiofrequency quadrupole (RFQ) electric fields during the collisional cooling process (also known as buffer gas cooling). These RFQ coolers operate on the same principles as quadrupole ion traps and have been shown to be particularly well suited for buffer gas cooling given their capacity for total confinement of ions having a large dispersion of velocities, corresponding to kinetic energies up to tens of electron volts. A number of the RFQ coolers have already been installed at research facilities around the world and a list of their characteristics can be found below.

List of facilities containing RFQ Coolers
Name Input beam Input emittance Cooler length R0 RF voltage, freq, DC Mass range Axial voltage Pressure Output beam qualities Images
Colette[1]

[2]

 60 keV ISOLDE beam decelerated to ≤ 10 eV ~ 30 π-mm-mrad 504 mm (15 segments, electrically isolated) 7 mm Freq : 450 – 700 kHz – 0.25 V/cm 0.01 mbar He Reaccelerated to 59.99 keV; transverse emittance 8 π-mm-mrad at 20 keV COLETTE1

COLETTE2

LPC Cooler[3] SPIRAL type beams Up to ~ 100 π-mm-mrad 468 mm (26 segments, electrically isolated) 15 mm RF : up to 250 Vp, Freq : 500 kHz – 2.2 MHz – – up to 0.1 mbar – LPC1

LPC2

SHIPTRAP Cooler[4]

[5] [6]

 SHIP type beams 20–500 keV/A – 1140 mm (29 segments, electrically isolated) 3.9 mm RF: 30–200 Vpp, Freq: 800 kHz – 1.2 MHz up to 260 u Variable: 0.25 – 1 V/cm ~ 5×10-3 mbar He – SHIPTRAP1

SHIPTRAP2

JYFL Cooler[7]

[8]

 IGISOL type beam at 40 keV Up to 17 π-mm-mrad 400 mm (16 segmentes) 10 mm RF: 200 Vp, Freq: 300 kHz – 800 kHz – ~1 V/cm ~0.1 mbar He ~3 π-mm-mrad, Energy spread < 4 eV JYFL1

JYFL2

JYFL3

MAFF Cooler[9] 30 keV beam decelerated to ~100 eV – 450 mm 30 mm RF: 100 –150 Vpp, Freq: 5 MHz – ~0.5 V/cm ~0.1 mbar He energy spread = 5 eV, Emittance @ 30keV: from = 36 π-mm-mrad to eT = 6 π-mm-mrad –
ORNL Cooler[10] 20–60 keV negative RIBs decelerated to <100 eV ~50 π-mm-mrad (@ 20 keV) 400 mm 3.5 mm RF: ~400 Vp, Freq: up to 2.7 MHz -- up to ±5 kV on tapered rods ~0.01 mbar Energy spread ~2 eV ORNL1

ORNL2

ORNL3

LEBIT Cooler[11] 5 keV DC beams – – – – – – ~1×x10−1 mbar He (high-pressure section) – LEBIT1

LEBIT2

LEBIT3

ISCOOL[12]

[13]

 60 keV ISOLDE beam up to 20 π-mm-mrad 800 mm (using segmented DC wedge electrodes) 20 mm RF: up to 380 V, Freq: 300 kHz – 3 MHz 10–300 u ~0.1V/cm 0,01 – 0,1 mbar He – ISCOOL1

ISCOOL2

ISCOOL3

ISCOOL4

ISOLTRAP Cooler[14] 60 keV ISOLDE beam – 860 mm (segmented) 6 mm RF: ~125 Vp, Freq: ~1 MHz. – – ~2×10-2 mbar He elong ≈ 10 eV us, etrans ≈ 10p mm mrad. ISOLTRAP1

ISOLTRAP2

TITAN RFCT[15] continuous 30–60 keV ISAC beam – – – RF: 1000 Vpp, Freq: 300 kHz – 3 MHz – – – 6 π-mm-mrad at 5 keV extraction energy TITAN1

TITAN2

TITAN3

TRIMP Cooler[16] TRIMP beams – 660 mm (segmented) 5 mm RF= 100 Vp, Freq.: up to 1.5 MHz 6 < A < 250 -- up to 0.1 mbar -- TRIMP1

TRIMP2

TRIMP3

SPIG Leuven cooler[17] IGISOL Beams – 124 mm (sextupole rod structure) 1.5 mm RF= 0–150 Vpp, Freq.: 4.7 MHz – – ~50 kPa He Mass Resolving Power (MRP)= 1450 SPIG1

SPIG2

SPIG3

Argonne CPT cooler – – – – – – – – – CPT Cooler1

CPT Cooler2

SLOWRI cooler – – 600 mm (segmented sextuple rod structure) 8 mm RF= 400 Vpp, Freq.: 3.6 MHz – – ~10 mbar He – –
– – – – – – – – – – –

See also

Quadrupole mass analyzer

References
  1. M. Sewtz; C. Bachelet; N. Chauvin; C. Guénaut; E. Leccia; D. Le Du & D. Lunney (2005). "Deceleration and cooling of heavy ion beams: The COLETTE project". Nuclear Instruments and Methods in Physics Research Section B. 240 (1–2): 55–60. Bibcode:2005NIMPB.240...55S. doi:10.1016/j.nimb.2005.06.088.
  2. David Lunney; Cyril Bachelet; Céline Guénaut; Sylvain Henry & Michael Sewtz (2009). "COLETTE: A linear Paul-trap beam cooler for the on-line mass spectrometer MISTRAL". Nuclear Instruments and Methods in Physics Research Section A. 598 (2): 379–387. Bibcode:2009NIMPA.598..379L. doi:10.1016/j.nima.2008.09.050.
  3. Guillaume Darius (2004). "Etude et Mise en oeuvre d'un Dispositif pour la Mesure de Paramètre de Correlation Angulaire dans la Désintégration du Noyau Hélium 6". PhD Thesis. Université de Caen / Basse-Normandie, France. Cite journal requires |journal= (help)
  4. S. Rahaman; M. Block; D. Ackermann; D. Beck; A. Chaudhuri; S. Eliseev; H. Geissel; D. Habs; F. Herfurth; F.P. Heßberger; et al. (2006). "On-line commissioning of SHIPTRAP". International Journal of Mass Spectrometry. 251 (2–3): 146–151. Bibcode:2006IJMSp.251..146R. doi:10.1016/j.ijms.2006.01.049.
  5. Jens Dilling (2001). "Direct Mass Measurements on Exotic Nuclei with SHIPTRAP and ISOLTRAP". PhD Thesis. University of Heidelberg, Germany. Cite journal requires |journal= (help)
  6. Daniel Rodriguez Rubiales (2001). "An RFQ Buncher for Accumulation and Cooling of Heavy Radionuclides at SHIPTRAP and High Precision Mass Measurements on Unstable Kr Isotopes at ISOLTRAP". PhD Thesis. University of Valencia, Spain. Cite journal requires |journal= (help)
  7. A. Jokinen; J. Huikari; A. Nieminen & J. Äystö (2002). "The first cooled beams from JYFL ion cooler and trap project". Nuclear Physics A. 701 (1–4): 557–560. Bibcode:2002NuPhA.701..557J. doi:10.1016/S0375-9474(01)01643-8.
  8. Arto Nieminen (2002). "Manipulation of Low-Energy Radioactive Ion Beams With an RFQ Cooler; Applications to Collinear Laser Spectroscopy". PhD Thesis. University of Jyväskylä, Jyväskylä, Finland. Cite journal requires |journal= (help)
  9. J. Szerypo; D. Habs; S. Heinz; J. Neumayr; P. Thirolf; A. Wilfart & F. Voit (2003). "MAFFTRAP: ion trap system for MAFF". Nuclear Instruments and Methods in Physics Research Section B. 204: 512–516. Bibcode:2003NIMPB.204..512S. doi:10.1016/S0168-583X(02)02123-7.
  10. Y. Liu; J.F. Liang G.D. Alton; J.R. Beene; Z. Zhou; H. Wollnik (2002). "Collisional Cooling of Negative-Ion Beams". Nuclear Instruments and Methods in Physics Research Section B. 187 (1): 117–131. Bibcode:2002NIMPB.187..117L. doi:10.1016/S0168-583X(01)00844-8.
  11. G. Bollen; S. Schwarz; D. Davies; P. Lofy; D. Morrissey; R. Ringle; P. Schury; T. Sun; L. Weissman (2004). "Beam cooling at the low-energy-beam and ion-trap facility at NSCL/MSU". Nuclear Instruments and Methods in Physics Research A. 532 (1–2): 203–209. Bibcode:2004NIMPA.532..203B. doi:10.1016/j.nima.2004.06.046.
  12. I. Podadera Aliseda; T. Fritioff; T. Giles; A. Jokinen; M. Lindroos & F. Wenander (2004). "Design of a second generation RFQ Ion Cooler and Buncher (RFQCB) for ISOLDE". Nuclear Physics A. 746: 647–650. Bibcode:2004NuPhA.746..647P. doi:10.1016/j.nuclphysa.2004.09.043.
  13. Ivan Podadera Aliseda (2006). "New Developments on Preparation of Cooled and Bunched Radioactive Ion Beams at ISOL-Facilities: The ISCOOL Project and Rotating-Wall Cooling". PhD Thesis. CERN, Geneva, Switzerland. Cite journal requires |journal= (help)
  14. T. J. Giles; R. Catherall; V. Fedosseev; U. Georg; E. Kugler; J. Lettry & M. Lindroos (2003). "The high resolution spectrometer at ISOLDE". Nuclear Instruments and Methods in Physics Research Section B. 204: 497–501. Bibcode:2003NIMPB.204..497G. doi:10.1016/S0168-583X(02)02119-5.
  15. J. Dilling; P. Bricault; M. Smith; H. -J. Kluge; et al. (TITAN collaboration) (2003). "The proposed TITAN facility at ISAC for very precise mass measurements on highly charged short-lived isotopes". Nuclear Instruments and Methods in Physics Research Section B. 204 (492–496): 492–496. Bibcode:2003NIMPB.204..492D. doi:10.1016/S0168-583X(02)02118-3.
  16. Emil Traykov (2006). "Production of Radioactive Beams for Atomic Trapping". PhD Thesis. University of Groningen, The Netherlands. Cite journal requires |journal= (help)
  17. P. Van den Bergh; S. Franchoo; J. Gentens; M. Huyse; Yu.A. Kudryavtsev; A. Piechaczek; R. Raabe; I. Reusen; P. Van Duppen; L. Vermeeren; A. Wiihr (1997). "The SPIG, improvement of the efficiency and beam quality of an ion-guide based on-line isotope separator". Nuclear Instruments and Methods in Physics Research Section B. 126 (Pages 194– 197).

Bibliography
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