RFQ beam cooler

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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.[ citation needed ]

Contents

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 1011 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

NameFacilityInput beamInput emittanceCooler lengthR0RF voltage, freq, DCMass rangeAxial voltagePressureOutput beam qualitiesImages
Colette [1] [2] CERN 60 keV ISOLDE beam decelerated to ≤ 10 eV~ 30 π-mm-mrad504 mm (15 segments, electrically isolated)7 mmFreq : 450 – 700 kHz0.25 V/cm0.01 mbar HeReaccelerated to 59.99 keV; transverse emittance 8 π-mm-mrad at 20 keV COLETTE1

COLETTE2

LPC Cooler [3] GANIL SPIRAL type beamsUp to ~ 100 π-mm-mrad468 mm (26 segments, electrically isolated)15 mmRF : up to 250 Vp, Freq : 500 kHz – 2.2 MHzup to 0.1 mbar LPC1

LPC2

SHIPTRAP Cooler [4] [5] [6] GSI SHIP type beams 20–500 keV/A1140 mm (29 segments, electrically isolated)3.9 mmRF: 30–200 Vpp, Freq: 800 kHz – 1.2 MHzup to 260 u Variable: 0.25 – 1 V/cm~ 5×10-3 mbar He SHIPTRAP1

SHIPTRAP2

JYFL Cooler [7] [8] University of Jyvaskyla IGISOL type beam at 40 keVUp to 17 π-mm-mrad400 mm (16 segments)10 mmRF: 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] FRM II 30 keV beam decelerated to ~100 eV450 mm30 mmRF: 100 –150 Vpp, Freq: 5 MHz~0.5 V/cm~0.1 mbar Heenergy spread = 5 eV, Emittance @ 30keV: from = 36 π-mm-mrad to eT = 6 π-mm-mrad
ORNL Cooler [10] ORNL 20–60 keV negative RIBs decelerated to <100 eV~50 π-mm-mrad (@ 20 keV)400 mm3.5 mmRF: ~400 Vp, Freq: up to 2.7 MHz--up to ±5 kV on tapered rods~0.01 mbarEnergy spread ~2 eV ORNL1

ORNL2

ORNL3

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

LEBIT2

LEBIT3

ISCOOL [12] [13] CERN 60 keV ISOLDE beamup to 20 π-mm-mrad800 mm (using segmented DC wedge electrodes)20 mmRF: up to 380 V, Freq: 300 kHz – 3 MHz10–300 u~0.1V/cm0,01 – 0,1 mbar He ISCOOL1

ISCOOL2

ISCOOL3

ISCOOL4

ISOLTRAP Cooler [14] CERN 60 keV ISOLDE beam860 mm (segmented)6 mmRF: ~125 Vp, Freq: ~1 MHz.~2×10-2 mbar Heelong ≈ 10 eV us, etrans ≈ 10p mm mrad. ISOLTRAP1

ISOLTRAP2

TITAN RFCT [15] TRIUMF continuous 30–60 keV ISAC beamRF: 1000 Vpp, Freq: 300 kHz – 3 MHz6 π-mm-mrad at 5 keV extraction energy TITAN1

TITAN2

TITAN3

TRIMP Cooler [16] University of Groningen TRIMP beams660 mm (segmented)5 mmRF= 100 Vp, Freq.: up to 1.5 MHz6 < A < 250--up to 0.1 mbar-- TRIMP1

TRIMP2

TRIMP3

SPIG Leuven cooler [17] KU Leuven IGISOL Beams124 mm (sextupole rod structure)1.5 mmRF= 0–150 Vpp, Freq.: 4.7 MHz~50 kPa HeMass Resolving Power (MRP)= 1450 SPIG1

SPIG2

SPIG3

Argonne CPT cooler Argonne National Laboratory CPT Cooler1

CPT Cooler2

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

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Bibliography