- Email: [email protected]
Segmented linear RFQ traps for nuclear physics
Nuclear Instruments and Methods in Physics Research B 204 (2003) 587–591 www.elsevier.com/locate/nimb
Segmented linear RFQ traps for nuclear physics F. Herfurth
*
CERN EP/ISOLDE, 1211 Geneva 23, Switzerland
Abstract Segmented linear radio-frequency quadrupole (RFQ) ion traps are nowadays widely used in experimental nuclear physics. Within recent years this type of device proved to be very valuable for an improved matching of the requirements of nuclear physics experiments with the boundary conditions dictated by the production technique of radioactive beams. Due to the success of the systems already in operation at on-line facilities, a number of new systems are designed or under construction. Ó 2003 Elsevier Science B.V. All rights reserved. PACS: 29; 41.85.)p Keywords: Beam manipulation; Linear Paul trap; Ion trap; Nuclear physics
1. Introduction and principle of operation Radio frequency quadrupole (RFQ) or Paul traps, devices for three-dimensional confinement of charged particles, are widely applied in physics and chemistry. The two-dimensional (linear) version of the RFQ trap, also called Paul mass filter, has turned out to be the most promising device for beam manipulation. The reason is that in this device the ion motion in direction of beam propagation is independent of amplitude, frequency and phase of the applied radio frequency (RF). The radial confinement is achieved by applying RF voltages to four rods in quadrupolar arrangement as shown in Fig. 1. The ion motion in a quadrupolar RF field can be described by the Mathieu equations [1] and is subject to the known
*
Tel.: +41-22-767-2780; fax: +41-22-767-8990. E-mail address: [email protected] (F. Herfurth).
stability conditions. It can be split into two characteristic motions, the micromotion that is the direct response to the applied RF and the macromotion, which is the oscillation in the harmonic pseudo-potential created by the RF. The macromotion can be damped by collisions with buffer gas atoms if the ion is heavier than the buffer gas atom while the micromotion is repeatedly fed with new energy by the RF [2]. In other words, the macromotion is random in nature and can thus be assigned a temperature while the micromotion is not random since it is strongly correlated with the driving RF field and can therefore not be expressed in temperature terms. This is one of the limits for the application of three-dimensional RFQ traps as ion motion cooling devices since the lowest temperature reachable by cooling is then not only determined by the buffer gas temperature. Longitudinal confinement in linear RFQ traps is generally realized by electrodes at appropriate DC potentials placed at both ends of the linear
0168-583X/03/$ - see front matter Ó 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0168-583X(02)02135-3
588
F. Herfurth / Nucl. Instr. and Meth. in Phys. Res. B 204 (2003) 587–591 buffer gas
ions
ions
U DC
trapping
z
r0
ejection/DC mode
U = UDC( z) + URF sin(wRF t ) U = UDC( z) - URF sin(wRF t )
Fig. 1. Principle of a segmented linear RFQ trap. The upper part shows a side view of such a system as well as a possible DC potential along the axis of beam propagation, the z-axis. The lower part shows how the RF voltage is applied to the four rods to create the oscillating quadrupolar field.
structure. Another possibility is to segment the rods and apply varying DC voltages to different segments. This is the most flexible solution that allows for two different modes of operation: the continuous beam cooling mode and the beam cooling and bunching mode. In both cases the device is filled with a buffer gas to achieve cooling of ion motion via ion-buffer-gas-atom collisions. For continuous-mode beam cooling the ion beam is continuously extracted. During the passage through the linear RFQ the radial motion will be damped and the longitudinal energy spread will be reduced. To avoid the ions drifting back as well as to shorten the time they spend in the RFQ, a DC gradient towards the exit is created by applying appropriate voltages to the rod segments. The result is a beam with low transverse emittance and energy spread. In RFQs used for beam cooling and bunching, the DC voltages applied to the rod segments create a potential well and thus a three-dimensional ion trap (see Fig. 1), where a cold ion cloud is accumulated. The ion cloud is released in a short bunch having also low transverse emittance.
2. Overview of existing and future setups For beam emittance improvement and bunching of quasi-DC ISOL beams at energies of several 10 keV, RFQ beam coolers and bunchers are in use at ISOLTRAP [3] located at ISOLDE/CERN
[4], in Geneva and at the IGISOL facility [5] in Jyv€askyl€a [6]. At ISOLTRAP an incident beam of 60 keV is first electrostatically decelerated to a few 10 eV and then injected into the RFQ structure. Here, the remaining kinetic energy of the ions is dissipated in collisions with buffer gas atoms. The ions are accumulated in the potential well close to the exit and then extracted in a bunch short enough, i.e. a few microseconds, to be energy adapted in a pulsed cavity and so prepared for injection in a Penning trap at a relatively low energy of about 2 keV [3,7]. By the installation of this device the transfer efficiency of the ISOLDE ion beam into the Penning trap was significantly enhanced compared to earlier solutions. Additionally, the time needed for stopping the continuous 60 keV ion beam was shortened. By this, mass measurements of nuclei as short lived as 74 Rb (T1=2 ¼ 65 ms) and as rare as 32 Ar (yield of about 100 atoms/s) became possible [8,9]. In Jyv€askyl€a an ion beam with a kinetic energy of 40 keV is sent to the RFQ cooler. It is used to create ion pulses that are either extracted at low energy and transfered to a Penning trap [10], or that are reaccelerated and transported to a collinear laser spectroscopy setup [11,12]. The short pulse duration together with the knowledge about the arrival time provides an excellent possibility for efficient background suppression. Additionally, sensitivity and resolution are enhanced due to the considerably reduced longitudinal energy spread. The Jyv€askyl€a RFQ cooler is also able to deliver continuous beams with reduced longitudinal energy spread and transverse emittance at the original energy of 40 keV. The use of the RFQ cooler permitted on-line isotope shift and hyperfine structure measurements on several radionuclides [6,13]. One experimental highlight is certainly the successful investigation of 175 Hf [11]. Systems similar to those at ISOLTRAP and JYFL are planned or under construction at: LPC/ Caen to be installed at SPIRAL; for the mass spectrometer MISTRAL installed at ISOLDE; at the NSCL/MSU within the LEBIT facility; at ISOLDE as a general purpose device for beam improvement; at Munich for the nuclear reactor based MAFF facility [14]; and at TRIUMF/Van-
F. Herfurth / Nucl. Instr. and Meth. in Phys. Res. B 204 (2003) 587–591
couver for an advanced Penning trap mass spectrometer [15]. In all these cases the linear RFQ trap will be used to improve the emittance of, and in some cases to bunch incoming continuous beams. Within the LPC project this will allow for an efficient transfer of 6 He to a transparent Paul trap where the b–m angular correlation in nuclear b decay will be measured to test the electroweak sector of the standard model [16]. The system has been setup and underwent first test. The RFQ beam cooler operated in DC mode upstream of MISTRAL will reduce the emittance of the ISOLDE ion beam so that on the one hand, the transmission through the MISTRAL mass spectrometer will be improved and on the other hand, systematic errors due to different beam paths of reference ions and ions of interest will be reduced. A prototype of this device is in operation and comparative emittance measurements were performed [17]. The linear RFQ at the NSCL/MSU will be used for further cooling of the incoming beam by operating the whole device at liquid nitrogen temperature as well as for bunch preparation at variable energy [18]. These bunches will be injected into a Penning trap system or be distributed to other experiments. The general purpose RFQ beam cooler and buncher at ISOLDE [19], to be installed directly after the high-resolution mass separator, will provide all experiments with a low emittance beam that can be bunched if needed. This device will be of benefit for several experiments performed at ISOLDE ranging from collinear laser spectroscopy to solid state physics. At the facilities CPT/Argonne and SHIPTRAP/ GSI Darmstadt, ions with kinetic energies of typically MeV/u are stopped in a gas cell. RFQs are used to guide the ions from the high-pressure region close to the exit nozzle of the gas cell towards the high vacuum region. Due to the presence of the gas emerging from the gas cell into the linear segmented RFQ, it can be used to form a lowemittance ion beam or bunch well suited for injection into Penning or Paul trap systems. At the CPT the ions from the ATLAS accelerator are guided through an Enge spectrometer and then stopped in a gas cell. The ion bunch extracted through the linear RFQ trap is send first to another linear Paul trap and then to a Penning trap
589
for mass measurements. The complete system is in operation and first mass measurements were already done [20–22]. The SHIPTRAP system is designed for stopping heavy-ion beams available after the SHIP velocity filter at GSI. After the gas cell, an extraction RFQ [23] guides the ions in a continuous manner from the high-pressure region close to the gas cell to a low pressure region. There, another RFQ [24] is used for bunching the ions. The ion bunches are then sent to a Penning trap system. The first successful test of the gas cell RFQ buncher system was conducted in December 2001 [25,26]. Similar linear RFQ traps will be used in the trap facility LEBIT which is under construction at NSCL/MSU (East Lansing) [18,27] or are foreseen in the RIA project for handling radioactive beams after stopping relativistic beams of projectile fragments in a gas cell at high pressure (1 atm). Non-segmented RFQ traps, using external electrodes for creation of the longitudinal DC potential, have been realized at the LISOL facility (Louvain-la-Neuf) [28], at RIKEN (Tokyo) [29] and at HRIBF (Oak Ridge) [30], as well as within the LEBIT facility.
3. Performance and typical parameters All of the segmented linear RFQ traps described here are filled with buffer gas that is lighter than the ion species to be cooled. Therefore, helium is most commonly used since it is inert and light enough for almost all ion species. If faster cooling is required argon is preferred since the energy dissipation process goes faster with increased mass. In the case of the LPC system hydrogen is used since helium ions are to be cooled. The purity of the buffer gas is especially important in bunched mode devices were longer accumulation times are needed. This is because impurities cause ion loss through charge-exchange reactions. Resonant charge exchange is the strongest channel which is why helium can only be cooled by hydrogen. Buffer gas pressures required to achieve sufficient cooling range from 0.01 to 1 mbar depending on the mode (bunched or continuous) and the pumping speed available.
590
F. Herfurth / Nucl. Instr. and Meth. in Phys. Res. B 204 (2003) 587–591
Linear RFQs provide a considerable reduction of energy spread in beam direction. The energy spread after cooling is almost equal to the buffer gas temperature, i.e. well below 1 eV [11]. This is compared to the energy spread of a typical IGISOL source of about 100 eV at 40 keV beam energy. Used as a buncher, the linear RFQ trap can provide ion bunches as short as a few microseconds, with energy spreads around 1 eV [3], i.e. a longitudinal emittance of less than 10 eV ls. The transverse emittance also depends on the buffer gas temperature but even more on the extraction procedure. Care has to be taken to avoid a reheating of the beam due to buffer gas collisions where no confinement is present and due to RF field effects during extraction. However, output emittances in the order of 1p mm mrad at 40–60 keV are reached compared to input emittances of several 10p mm mrad at 40–60 keV [3,17]. Another important parameter is the ion storage capacity of such a device, i.e. the number of particles that can be confined without changing significantly the properties of the extracted ion beam or bunch or at least without decreasing the efficiency. The geometrical dimensions together with the depth of the pseudo potential determine the capacity perpendicular to the beam direction. In the longitudinal direction for bunched mode devices one has to choose between short pulses or higher storage capacity, determined by the length of the longitudinal potential well. In DC mode already beams of several nanoamperes have been cooled [19]. Bunched-mode devices have shown capacities of 104 particles without any change in the extracted beam. If a slight increase of beam size is acceptable, more than 105 ions can be stored and extracted. However, to reach bunch lengths of only a few microseconds the maximum number of ions per pulse is rather 103 . One very important aspect is the transfer efficiency through the beam cooling device. In DC as well as in bunched mode efficiencies as high as about 50% have been reached for devices that use an existing low energy (several 10 keV) beam [6]. For devices coupled directly to a gas cell even higher efficiencies have been reported [22]. However, the fact that several new projects are employing such devices will assure continued im-
provement in performance of ion traps in the service of nuclear physics. References [1] P. Dawson, Quadrupole Mass Spectrometry, Elsevier, Amsterdam, 1976. [2] H. Dehmelt, Adv. At. Mol. Phys. 3 (1967) 53. [3] F. Herfurth, J. Dilling, A. Kellerbauer, G. Bollen, S. Henry, H.-J. Kluge, E. Lamour, D. Lunney, R.B. Moore, C. Scheidenberger, S. Schwarz, G. Sikler, J. Szerypo, Nucl. Instr. and Meth. A 469 (2001) 254. [4] E. Kugler, Hyperfine Interact. 129 (2000) 23. € yst€ [5] J. A o, Nucl. Phys. A 693 (2000) 477. [6] A. Nieminen, P.C.J. Billows, D. Forest, J.A.R. Griffith, J. Huikari, A.J.I.D. Moore, R.B. Moore, G. Tungate, J. € yst€ A o, Phys. Rev. Lett. 88 (2002) 094801. [7] K. Blaum et al., Nucl. Instr. and Meth. B, these Proceedings. doi:10.1016/S0168-583X(02)01942-0 and 10.1016/ S0168-583X(02)02115-8. € yst€ [8] F. Herfurth et al., in: J. A o (Ed.), Proc. 3rd Int. Conf. on Exotic Nuclei and Masses, H€ameenlinna, Finland 2001, Eur. Phys. J. A 15 (2002) 17. [9] K. Blaum, F. Herfurth, et al., in press. [10] V.S. Kolhinen et al., Presented at 14th Int. Conf. on Electromagnetic Isotope Separators and Techniques Related to their Applications (EMIS-14), Victoria, BC, Canada, May 2002. € yst€ [11] A. Nieminen, J. Huikari, A. Jokinen, J. A o, P. Campell, E.C.A. Cochrane, Nucl. Instr. and Meth. A 469 (2001) 244. [12] A. Nieminen et al., Nucl. Instr. and Meth. B, these Proceedings. doi:10.1016/S0168-583X(02)02133-X. [13] P. Campbell et al., Eur. Phys. J. A 15 (2002) 45. [14] D. Habs et al., Nucl. Instr. and Meth. B, these Proceedings. doi:10.1016/S0168-583X(02)00496-8. [15] J. Dilling et al., Nucl. Instr. and Meth. B, these Proceedings. doi:10.1016/S0168-583X(02)02118-3. [16] P. Delahaye, G. Ban, D. Durand, A.M. Vinodkumar, C.L. Brun, E. Lienard, F. Mauger, O. Naviliat, J. Szerypo, B. Tamain, Hyperfine Interact. 132 (2001) 479. [17] S. Henry, C. Bachelet, Z. Djouadi, C. Gaulard, M. de Saint € yst€ Simon, D. Lunney, in: J. A o (Ed.), Proc. 3rd Int. Conf. on Exotic Nuclei and Masses, H€ameenlinna, Finland 2001, Springer. [18] S. Schwarz et al., Nucl. Instr. and Meth. B, these Proceedings. doi:10.1016/S0168-583X(02)02114-6 and 10.1016/S0168-583X(02)02122-5. [19] A. Jokinen et al., Nucl. Instr. and Meth. B, these Proceedings. doi:10.1016/S0168-583X(02)01894-3. [20] M. Maier, C. Boudreau, F. Buchinger, J.A. Clark, J.E. Crawford, J. Dilling, H. Fukutani, S. Gulick, J.K.P. Lee, R.B. Moore, G. Savard, J. Schwartz, K.S. Sharma, Hyperfine Interact. 132 (2001) 521. [21] G. Savard et al., Nucl. Instr. and Meth. B, these Proceedings. doi:10.1016/S0168-583X(02)02134-1 and 10.1016/ S0168-583X(02)00501-9.
F. Herfurth / Nucl. Instr. and Meth. in Phys. Res. B 204 (2003) 587–591 [22] J. Clark et al., Nucl. Instr. and Meth. B, these Proceedings. doi:10.1016/S0168-583X(02)02117-1. [23] O. Engels, L. Beck, G. Bollen, D. Habs, G. Marx, J. Neumayr, U. Schramm, S. Schwarz, P. Thierolf, V. Varentsov, Hyperfine Interact. 132 (2001) 505. [24] D. Rodrıguez, M. Tarisien, J. Dilling, H.-J. Kluge, G. Marx, W. Quint, J. Sch€ onfelder, G. Sikler, C. Weber, in: J. € yst€ A o (Ed.), in: Proc. 3rd Int. Conf. on Exotic Nuclei and Masses, H€ ameenlinna, Finland 2001, Springer. [25] G. Marx, D. Ackermann, J. Dilling, F.P. Hessberger, S. Hoffmann, H.-J. Kluge, et al., Hyperfine Interact. 132 (2001) 463. [26] G. Sikler et al., Nucl. Instr. and Meth. B, these Proceedings. doi:10.1016/S0168-583X(02)02116-X.
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[27] S. Schwarz, M. Baird, G. Bollen, D. Lawton, P. Lofy, D. Morrissey, J. Ottarson, R. Ringle, P. Schury, T. Sun, V. € yst€ Varentsov, in: J. A o (Ed.), Proc. 3rd Int. Conf. on Exotic Nuclei and Masses, H€ameenlinna, Finland 2001, Springer. [28] P.V. den Bergh, S. Franchoo, J. Gentens, M. Huyse, Y. Kudryavtsev, A. Piechaczek, R. Raabe, I. Reusen, P.V. Duppen, L. Vermeeren, A. W€ ohr, Nucl. Instr. and Meth. B 126 (1997) 194. [29] S. Fujitaka, M. Wada, H. Wang, J. Tanaka, H. Kawakami, I. Katayama, K. Ogino, H. Katsuragawa, T. Nakamura, K. Okada, S. Othani, Nucl. Instr. and Meth. B 126 (1997) 386. [30] Y. Liu, J.F. Liang, G.D. Alton, J.R. Beene, Z. Zhou, H. Wollnik, Nucl. Instr. and Meth. B 187 (2002) 117.
Segmented linear RFQ traps for nuclear physics F. Herfurth
*
CERN EP/ISOLDE, 1211 Geneva 23, Switzerland
Abstract Segmented linear radio-frequency quadrupole (RFQ) ion traps are nowadays widely used in experimental nuclear physics. Within recent years this type of device proved to be very valuable for an improved matching of the requirements of nuclear physics experiments with the boundary conditions dictated by the production technique of radioactive beams. Due to the success of the systems already in operation at on-line facilities, a number of new systems are designed or under construction. Ó 2003 Elsevier Science B.V. All rights reserved. PACS: 29; 41.85.)p Keywords: Beam manipulation; Linear Paul trap; Ion trap; Nuclear physics
1. Introduction and principle of operation Radio frequency quadrupole (RFQ) or Paul traps, devices for three-dimensional confinement of charged particles, are widely applied in physics and chemistry. The two-dimensional (linear) version of the RFQ trap, also called Paul mass filter, has turned out to be the most promising device for beam manipulation. The reason is that in this device the ion motion in direction of beam propagation is independent of amplitude, frequency and phase of the applied radio frequency (RF). The radial confinement is achieved by applying RF voltages to four rods in quadrupolar arrangement as shown in Fig. 1. The ion motion in a quadrupolar RF field can be described by the Mathieu equations [1] and is subject to the known
*
Tel.: +41-22-767-2780; fax: +41-22-767-8990. E-mail address: [email protected] (F. Herfurth).
stability conditions. It can be split into two characteristic motions, the micromotion that is the direct response to the applied RF and the macromotion, which is the oscillation in the harmonic pseudo-potential created by the RF. The macromotion can be damped by collisions with buffer gas atoms if the ion is heavier than the buffer gas atom while the micromotion is repeatedly fed with new energy by the RF [2]. In other words, the macromotion is random in nature and can thus be assigned a temperature while the micromotion is not random since it is strongly correlated with the driving RF field and can therefore not be expressed in temperature terms. This is one of the limits for the application of three-dimensional RFQ traps as ion motion cooling devices since the lowest temperature reachable by cooling is then not only determined by the buffer gas temperature. Longitudinal confinement in linear RFQ traps is generally realized by electrodes at appropriate DC potentials placed at both ends of the linear
0168-583X/03/$ - see front matter Ó 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0168-583X(02)02135-3
588
F. Herfurth / Nucl. Instr. and Meth. in Phys. Res. B 204 (2003) 587–591 buffer gas
ions
ions
U DC
trapping
z
r0
ejection/DC mode
U = UDC( z) + URF sin(wRF t ) U = UDC( z) - URF sin(wRF t )
Fig. 1. Principle of a segmented linear RFQ trap. The upper part shows a side view of such a system as well as a possible DC potential along the axis of beam propagation, the z-axis. The lower part shows how the RF voltage is applied to the four rods to create the oscillating quadrupolar field.
structure. Another possibility is to segment the rods and apply varying DC voltages to different segments. This is the most flexible solution that allows for two different modes of operation: the continuous beam cooling mode and the beam cooling and bunching mode. In both cases the device is filled with a buffer gas to achieve cooling of ion motion via ion-buffer-gas-atom collisions. For continuous-mode beam cooling the ion beam is continuously extracted. During the passage through the linear RFQ the radial motion will be damped and the longitudinal energy spread will be reduced. To avoid the ions drifting back as well as to shorten the time they spend in the RFQ, a DC gradient towards the exit is created by applying appropriate voltages to the rod segments. The result is a beam with low transverse emittance and energy spread. In RFQs used for beam cooling and bunching, the DC voltages applied to the rod segments create a potential well and thus a three-dimensional ion trap (see Fig. 1), where a cold ion cloud is accumulated. The ion cloud is released in a short bunch having also low transverse emittance.
2. Overview of existing and future setups For beam emittance improvement and bunching of quasi-DC ISOL beams at energies of several 10 keV, RFQ beam coolers and bunchers are in use at ISOLTRAP [3] located at ISOLDE/CERN
[4], in Geneva and at the IGISOL facility [5] in Jyv€askyl€a [6]. At ISOLTRAP an incident beam of 60 keV is first electrostatically decelerated to a few 10 eV and then injected into the RFQ structure. Here, the remaining kinetic energy of the ions is dissipated in collisions with buffer gas atoms. The ions are accumulated in the potential well close to the exit and then extracted in a bunch short enough, i.e. a few microseconds, to be energy adapted in a pulsed cavity and so prepared for injection in a Penning trap at a relatively low energy of about 2 keV [3,7]. By the installation of this device the transfer efficiency of the ISOLDE ion beam into the Penning trap was significantly enhanced compared to earlier solutions. Additionally, the time needed for stopping the continuous 60 keV ion beam was shortened. By this, mass measurements of nuclei as short lived as 74 Rb (T1=2 ¼ 65 ms) and as rare as 32 Ar (yield of about 100 atoms/s) became possible [8,9]. In Jyv€askyl€a an ion beam with a kinetic energy of 40 keV is sent to the RFQ cooler. It is used to create ion pulses that are either extracted at low energy and transfered to a Penning trap [10], or that are reaccelerated and transported to a collinear laser spectroscopy setup [11,12]. The short pulse duration together with the knowledge about the arrival time provides an excellent possibility for efficient background suppression. Additionally, sensitivity and resolution are enhanced due to the considerably reduced longitudinal energy spread. The Jyv€askyl€a RFQ cooler is also able to deliver continuous beams with reduced longitudinal energy spread and transverse emittance at the original energy of 40 keV. The use of the RFQ cooler permitted on-line isotope shift and hyperfine structure measurements on several radionuclides [6,13]. One experimental highlight is certainly the successful investigation of 175 Hf [11]. Systems similar to those at ISOLTRAP and JYFL are planned or under construction at: LPC/ Caen to be installed at SPIRAL; for the mass spectrometer MISTRAL installed at ISOLDE; at the NSCL/MSU within the LEBIT facility; at ISOLDE as a general purpose device for beam improvement; at Munich for the nuclear reactor based MAFF facility [14]; and at TRIUMF/Van-
F. Herfurth / Nucl. Instr. and Meth. in Phys. Res. B 204 (2003) 587–591
couver for an advanced Penning trap mass spectrometer [15]. In all these cases the linear RFQ trap will be used to improve the emittance of, and in some cases to bunch incoming continuous beams. Within the LPC project this will allow for an efficient transfer of 6 He to a transparent Paul trap where the b–m angular correlation in nuclear b decay will be measured to test the electroweak sector of the standard model [16]. The system has been setup and underwent first test. The RFQ beam cooler operated in DC mode upstream of MISTRAL will reduce the emittance of the ISOLDE ion beam so that on the one hand, the transmission through the MISTRAL mass spectrometer will be improved and on the other hand, systematic errors due to different beam paths of reference ions and ions of interest will be reduced. A prototype of this device is in operation and comparative emittance measurements were performed [17]. The linear RFQ at the NSCL/MSU will be used for further cooling of the incoming beam by operating the whole device at liquid nitrogen temperature as well as for bunch preparation at variable energy [18]. These bunches will be injected into a Penning trap system or be distributed to other experiments. The general purpose RFQ beam cooler and buncher at ISOLDE [19], to be installed directly after the high-resolution mass separator, will provide all experiments with a low emittance beam that can be bunched if needed. This device will be of benefit for several experiments performed at ISOLDE ranging from collinear laser spectroscopy to solid state physics. At the facilities CPT/Argonne and SHIPTRAP/ GSI Darmstadt, ions with kinetic energies of typically MeV/u are stopped in a gas cell. RFQs are used to guide the ions from the high-pressure region close to the exit nozzle of the gas cell towards the high vacuum region. Due to the presence of the gas emerging from the gas cell into the linear segmented RFQ, it can be used to form a lowemittance ion beam or bunch well suited for injection into Penning or Paul trap systems. At the CPT the ions from the ATLAS accelerator are guided through an Enge spectrometer and then stopped in a gas cell. The ion bunch extracted through the linear RFQ trap is send first to another linear Paul trap and then to a Penning trap
589
for mass measurements. The complete system is in operation and first mass measurements were already done [20–22]. The SHIPTRAP system is designed for stopping heavy-ion beams available after the SHIP velocity filter at GSI. After the gas cell, an extraction RFQ [23] guides the ions in a continuous manner from the high-pressure region close to the gas cell to a low pressure region. There, another RFQ [24] is used for bunching the ions. The ion bunches are then sent to a Penning trap system. The first successful test of the gas cell RFQ buncher system was conducted in December 2001 [25,26]. Similar linear RFQ traps will be used in the trap facility LEBIT which is under construction at NSCL/MSU (East Lansing) [18,27] or are foreseen in the RIA project for handling radioactive beams after stopping relativistic beams of projectile fragments in a gas cell at high pressure (1 atm). Non-segmented RFQ traps, using external electrodes for creation of the longitudinal DC potential, have been realized at the LISOL facility (Louvain-la-Neuf) [28], at RIKEN (Tokyo) [29] and at HRIBF (Oak Ridge) [30], as well as within the LEBIT facility.
3. Performance and typical parameters All of the segmented linear RFQ traps described here are filled with buffer gas that is lighter than the ion species to be cooled. Therefore, helium is most commonly used since it is inert and light enough for almost all ion species. If faster cooling is required argon is preferred since the energy dissipation process goes faster with increased mass. In the case of the LPC system hydrogen is used since helium ions are to be cooled. The purity of the buffer gas is especially important in bunched mode devices were longer accumulation times are needed. This is because impurities cause ion loss through charge-exchange reactions. Resonant charge exchange is the strongest channel which is why helium can only be cooled by hydrogen. Buffer gas pressures required to achieve sufficient cooling range from 0.01 to 1 mbar depending on the mode (bunched or continuous) and the pumping speed available.
590
F. Herfurth / Nucl. Instr. and Meth. in Phys. Res. B 204 (2003) 587–591
Linear RFQs provide a considerable reduction of energy spread in beam direction. The energy spread after cooling is almost equal to the buffer gas temperature, i.e. well below 1 eV [11]. This is compared to the energy spread of a typical IGISOL source of about 100 eV at 40 keV beam energy. Used as a buncher, the linear RFQ trap can provide ion bunches as short as a few microseconds, with energy spreads around 1 eV [3], i.e. a longitudinal emittance of less than 10 eV ls. The transverse emittance also depends on the buffer gas temperature but even more on the extraction procedure. Care has to be taken to avoid a reheating of the beam due to buffer gas collisions where no confinement is present and due to RF field effects during extraction. However, output emittances in the order of 1p mm mrad at 40–60 keV are reached compared to input emittances of several 10p mm mrad at 40–60 keV [3,17]. Another important parameter is the ion storage capacity of such a device, i.e. the number of particles that can be confined without changing significantly the properties of the extracted ion beam or bunch or at least without decreasing the efficiency. The geometrical dimensions together with the depth of the pseudo potential determine the capacity perpendicular to the beam direction. In the longitudinal direction for bunched mode devices one has to choose between short pulses or higher storage capacity, determined by the length of the longitudinal potential well. In DC mode already beams of several nanoamperes have been cooled [19]. Bunched-mode devices have shown capacities of 104 particles without any change in the extracted beam. If a slight increase of beam size is acceptable, more than 105 ions can be stored and extracted. However, to reach bunch lengths of only a few microseconds the maximum number of ions per pulse is rather 103 . One very important aspect is the transfer efficiency through the beam cooling device. In DC as well as in bunched mode efficiencies as high as about 50% have been reached for devices that use an existing low energy (several 10 keV) beam [6]. For devices coupled directly to a gas cell even higher efficiencies have been reported [22]. However, the fact that several new projects are employing such devices will assure continued im-
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