The REX-ISOLDE charge breeder as an operational machineF. Wenander, P. Delahaye, R. Scrivens, R. Savreux, and REX-ISOLDE Collaboration Citation: Review of Scientific Instruments 77, 03B104 (2006); doi: 10.1063/1.2149384 View online: http://dx.doi.org/10.1063/1.2149384 View Table of Contents: http://scitation.aip.org/content/aip/journal/rsi/77/3?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Status of charge breeding with electron cyclotron resonance ion sources (invited) Rev. Sci. Instrum. 77, 03B101 (2006); 10.1063/1.2149300 REXEBIS operation and developments Rev. Sci. Instrum. 75, 1607 (2004); 10.1063/1.1695644 Status of the REX-ISOLDE project AIP Conf. Proc. 610, 987 (2002); 10.1063/1.1470269 Electron cyclotron resonance charge breeder (invited) Rev. Sci. Instrum. 71, 617 (2000); 10.1063/1.1150331 Status of the REX-ISOLDE project AIP Conf. Proc. 475, 309 (1999); 10.1063/1.59247

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The REX-ISOLDE charge breeder as an operational machineF. Wenander, P. Delahaye, and R. ScrivensCERN, 1211 Geneva-23, 1211 Switzerland

R. SavreuxSektion Physik, Ludwig-Maximilians-Universität, München, D-85748 Garching, Germany

�REX-ISOLDE Collaboration�1211 Geneva-23, 1211 Switzerland

�Presented on 16 September 2005; published online 23 March 2006�

The charge breeding system of radioactive beam experiment at ISOLDE �REX-ISOLDE�,consisting of a large Penning trap in combination with an electron-beam ion source �EBIS�, is nowa mature concept after having delivered radioactive beams for postacceleration to a number ofexperiments for three years. The system, preparing ions prior to injection into a compact linearaccelerator, has shown to be versatile in terms of the ion species and energies that can be delivered.During the experimental periods 2004 and 2005 a significant part of the ISOLDE beam time wasdedicated to REX-ISOLDE experiments. Ion masses in the range between A=7 and 153 have beenhandled with record efficiencies. High-intensity as well as very-short-lived isotope beams wereproven to be feasible. Continuous injection into the EBIS has also been successfully tested. Twomeans of suppressing unwanted beam contaminations were tested and are now in use. In addition,the experience gained from the trap-EBIS concept from a machine operational point of view will bediscussed and the limitations described. © 2006 American Institute of Physics.�DOI: 10.1063/1.2149384�

INTRODUCTION AND EXPERIMENTAL SETUP

At radioactive beam experiment-isotope separator online1,2 �REX-ISOLDE� a unique combination of a Penningtrap �REXTRAP� and an electron-beam ion source �REX-EBIS� for the bunching and charge breeding �1+–n+ conver-sion� of radioactive ions prior to their acceleration has beenput into operation. Radioactive nuclides are produced at theCERN-ISOLDE on-line mass separator facility and injectedas 1+ ions into the large gas-filled REXTRAP,3,4 where theyare accumulated and phase-space cooled before being trans-ferred as an ion bunch to the REXEBIS.5,6 Inside the EBISthe ions are further ionized to an A /q�4.5 before beingextracted at a variable potential to match the velocity accep-tance �5 keV/u� of the radio frequency quadrupole �RFQ�the first element in the postaccelerating linear accelerator�LINAC�.7 Between the EBIS and LINAC a mass separatorselects the correct A /q for the desired radioactive ion fromionized stable residual gas contaminations coming mainlyfrom the trap and EBIS. The task of REXEBIS is to increasethe charge state of injected singly-charged radioactive ions toenable an efficient acceleration in the following LINAC. Theproperties such as ultrahigh vacuum �small residual gas con-tamination in the extracted beam�, small emittance, fastbreeding �minimizing ion losses due to radioactive decay�,pulsed extraction, and high extracted charge states make theEBIS well suited as charge breeder for radioactive ions andLINAC injection.8 Earlier results for the cooling-bunching-breeding systems are presented in Refs. 9–11.

RESULTS

Charge-bred elements

The REXTRAP is able to bunch and cool ions from Li+

to U+. For the noble-gas ion He+, charge-exchange processeswith the Ne buffer gas yield a poor efficiency �see Ref. 12�.Charge breeding in the EBIS has been performed with sev-eral stable ions. A total of 15 radioactive elements and 30isotopes, ranging from 9Li to 153Sm, have been delivered foracceleration to the experiments �see Table I�. A mass spec-trum of 110Sn, after 98 ms of charge breeding, can be seen inFig. 1. The spectrum is not representative as in most casesthe radioactive-beam peaks are not visible in the scan due tothe low current coming from ISOLDE.

Efficiencies

A high efficiency of the system is important as the exoticradioactive ions have low production yields. The bunchingefficiency of the trap, defined as the number of ions extracteddivided by the number of injected ions, is regularly 45%–55% for injected beam currents of less than 50 pA, a valuethat is very reproducible. The EBIS efficiency comprises thebeam transport from the trap to the EBIS, the injection-breeding-extraction cycle, and the transmission through themass separator. The EBIS efficiency for the selected chargestate has been determined to be around 10% for most ele-ments at average ion currents of a few picoamperes �seeTable I�. The so far best-obtained efficiency is 22% for K10+

compared with the theoretical maximum value of 25%–30%.The combined trap-EBIS efficiency was �10%. Higher effi-ciencies were expected for 7Li3+, where all the ions could be

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pushed into the highest charge state. However, this could notbe obtained during a Li run, the reason may be that Li servesas a coolant for the heavier residual gas �C, N, and O� and isevaporated away.

The EBIS efficiency is dependent on the effectiveness ofthe phase-space cooling in the trap. The optimization is doneby tuning the cooling cyclotron frequency while observingthe beam extracted from the EBIS. Surprisingly, the effi-ciency also varies strongly with the electron-beam energy.When the energy is changed by some 100 eV around a meanvalue of 4500 eV, the efficiency can increase/decrease by afactor of 2. This is believed to be due to a changed electron-beam radius in the suppressor-collector region where the ionbeam is radially trapped by the Coulomb force from the elec-tron beam. The electron-beam radius variation in thesuppressor-collector region is in turn caused by the alteredaxial position of the crests of the scalloping electron beam.

For radioactive elements the efficiencies may be loweras the beam intensities are often below the detection thresh-old for the Faraday cups �in our setup 0.1 pA�, which pre-vents beam tuning. Moreover, the beam intensity often fluc-

tuates with the proton impact on the primary target.Furthermore, laser-ionized beams inherently fluctuate in in-tensity.

High-intensity beams

Until now the limited throughput of the REX-ISOLDEcharge breeder system has not restricted the current for anyexperiment. In the rare cases where a high radioactive cur-rent ��50 pA� is being produced in the primary target-ionsource of ISOLDE, the limitations have been set by radiationlevel around the machine, mainly near the roughing pumpsfor the Penning trap.

The Penning trap has an intrinsic space-charge limita-tion, but can confidently handle 107 ions/bunch, translatinginto currents of up to 100 pA with a repetition rate of 50 Hz.The space-charge limitation of the REXEBIS is sufficientlyhigh, with 3�109 charges at 10% electron-beam compensa-tion for the present operation conditions. Nevertheless, highcurrents will be of interest at future generations of ISOLfacilities, and already now the stable isobaric contaminationfrom the primary target superimposed on the radioactivebeam may cause saturation problems in the trap.

Thus, a measurement series to investigate the bunching-cooling-breeding functions for higher beam currents was car-ried out. A stable K+ beam with variable intensity �up to3.6 nA� was provided by the local test ion source in front ofthe trap. The system was operated with a repetition rate of50 Hz �20 ms bunching and cooling times and approximately15 ms breeding time�. The individual efficiencies for the trapand the EBIS are presented in Fig. 2. As some Ne+, from thetrap cooling gas, tend to leave the trap together with the K+

at higher beam intensities, the trap efficiencies may be over-estimated to some extent while the EBIS efficiencies becomeunderestimated. However, the efficiency for the total system,decreasing from 13% to 2% with increased injected beam, inthe graph is reliable. Other tests have shown that the REX-TRAP has a saturation of around 1.5�108 charges per pulseof extracted beam. It has already been discovered that thespace-charge effects lead to a changed cyclotron coolingfrequency4,13 and a deficient phase-space cooling.4 The latter

TABLE I. Charge state, breeding time, and efficiency �one selected chargestate� for some of the stable and radioactive ions charge bred by the REX-EBIS.

ElementCharge

state RadioactiveBreedingtime�ms�

Efficiency�%�

7Li 2+ No 5 137Li 3+ No 18 12

23Na 7+ No 15 1826Na 7+ Yes 18 1128Mg 9+ Yes 18 17.539K 10+ No 14 2268Ni 19+ Yes 98 �768Cu 19+ Yes 98 �1766Zn 20+ No 78 370Sea 19+ Yes 58 6.5108Sn 30+ Yes 198 6110Sn 27+ Yes 98 9128Xe 32+ No 330 4133Cs 27+ No 38 14133Cs 32+ No 158 7.5

aInjected as SeCO+ into the EBIS.

FIG. 1. Mass spectrum obtained after injection of 48 pA of 110Sn1+ into theEBIS and 98 ms charge breeding. The trap cooling efficiency was 44%.

FIG. 2. REXTRAP and REXEBIS efficiencies �left scale� and combinedefficiencies �right scale� for high currents of K+ injected from the ion sourcein front of the trap. Trap cooling time of 20 ms and K10+ selected after15 ms breeding time. The lines are given as guides for the eye.

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leads to an increased trap emittance and thereby reducedEBIS injection efficiency. Nevertheless, even if the effi-ciency decreases, the overall ion throughput increases forinjected currents up to 4 nA.

Low-intensity beams and residual gas contamination

Extremely low-intensity beams have also passed thecharge breeding system, for instance, 11Li with only2500 ions per second out of the ISOLDE ion source. This ismade possible by the good vacuum in the EBIS. A residualgas mass spectrum from the EBIS is presented in Fig. 3 forthe interesting region of 3�A /q�4.5. Large Ne peaks fromthe REXTRAP are identifiable, as well as Ar which is usedfor venting the EBIS and thereafter residing in the nonevapo-rable getter �NEG� strips. In between the peaks the current isnot measurable with the Faraday cups and is hence less than0.1 pA. For most elements an A /q ratio within the accep-tance of the accelerator can be found without backgroundexcept for certain isotopes of low-Z elements such as Li, Be,B, etc. By using He as buffer gas inside the trap the Ne peaksdisappear, and in the future drift tubes with NEG-coated in-terior are an option to improve the pressure.

Suppression of beam contaminations

Isobaric contaminations from the ISOL target can beavoided by the use of molecular sideband beams. Accordingto their chemical properties, the radioactive elements cancombine themselves with the impurities present in theISOLDE target. For example, instead of selecting 70Se di-rectly from the target and primary ion source, with a 70Gecontamination several orders of magnitude higher in inten-sity, the molecule 70Se12C16O is mass selected. Germaniumdoes not preferably bind to CO and is therefore suppressed.The SeCO molecule is transferred to REX where it can eitherbe split inside the trap or the EBIS. The method was de-scribed in Ref. 14, and the results from this year’s run showthat the molecule can be kept fully intact inside the trap�normal 50% trap efficiency� with a consecutive moleculardisassociation in the EBIS. A breeding efficiency �includingmolecular disassociation� of 6.5% for 70Se19+ was achievedinside the EBIS. The efficiency is expected to depend on the

electronegativity of the desired ion in the molecule, disfavor-ing radioactive ions with high electronegativity values, sincethey tend to become neutral or negatively charged when themolecule is broken and escape the confining potential of theelectron beam.

Another method to suppress certain beam contamina-tions is to add a stripper foil ��50 �g/cm2 carbon foil� atthe end of the LINAC in front of the last bending magnet. Inthe case of radioactive 17F, the beam was superimposed by17O, with indistinguishable A /q values for the REX massseparator having an A /q resolution of �150. A stripper foilafter the LINAC shifted the charge state distribution of oxy-gen to 8+ while 40% of the fluorine ended up in 9+, and wasselected by the following bending magnet. The method hasalso been effectively used to move the stable contamination22Ne6+ away from 11Li3+, then with a near 100% transmis-sion efficiency for the radioactive ion. In a future extensionof the REX postaccelerator, two stripper foils with bendingmagnets are foreseen to improve the suppression factor.

Short-lived ions

At REX-ISOLDE the entire system is designed for anominal cycle time of 20 ms, i.e., the ions are cooled andbunched in the trap for 20 ms, thereafter sent to the EBIS forbreeding. To reach the acceptance of the entire accelerator ofA /q�4.5 the breeding time is variable from a few millisec-onds for light elements up to several hundred millisecondsfor heavy ions. In the REXEBIS one reaches a mass-to-charge ratio of 3–4 for A�50 within less than 20 ms. Whenthe breeding time exceeds 18 ms, the repetition rate has to belowered and the trap collects ions for a longer time. Radio-active Li �Z=3� has been charge bred to 2+ and 3+ withbreeding times of 5 and 9 ms. 11Li, with a half-life of only8.5 ms was charge bred and postaccelerated, and in the fu-ture the trap cooling time could, in principle, be reduced to10 ms by increasing the buffer gas pressure with less optimalcooling and reduced losses due to decay.

The REXEBIS is designed for a 500 mA electron beamat a density of 250 A/cm2 in the warm bore of the supercon-ducting 2 T magnet. Above 350 mA the electron losses in-crease rapidly and result in unstable conditions.6 For mostmeasurements about 200–250 mA current is used, corre-sponding to a theoretical current density of 100–125 A/cm2.The real current density has been derived by comparing mea-sured charge state distributions with calculated ones.15 Forshorter breeding times �typically �50 ms� the theoretical andexperimental values agree quite well, while for longer timesthe effective current density decreases, for example, to60 A/cm2 for Xe charge bred to 31+/32+ for 330 ms. Thedecrease is due to the electron-ion collision heating makingthe ions leave the electron beam for part of the confinementtime, and/or at high beam intensities, the electron beam be-coming partially compensated, thus permitting some of thehigh-energy ions to temporarily move outside the potentialwell of the electron beam. The ion injection conditions intothe EBIS also affect the effective current-density value,meaning that a poor injection results in longer breedingtimes. With an upgraded electron-beam gun and increased

FIG. 3. Analyzed extracted residual gas beam from the EBIS for a 50 msbreeding time and with 170 mA electron beam.

03B104-3 REX-ISOLDE charge breeder Rev. Sci. Instrum. 77, 03B104 �2006�

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electron current density we aim to reduce the breeding timeby 50%.

Continuous injection

With continuous EBIS injection �accumulation mode�,the 1+ ions are continuously introduced into the EBIS duringthe confinement period. The potential scheme for this modeis shown in Fig. 4. This method is, in principle, well adaptedfor primary ion sources with very low intensity �i.e., ISOLion sources� where the collection of ions inside the EBIS cancontinue for the whole breeding period without running intospace-charge limitations �see also Ref. 16�. Subsequently, thespace-charge-limiting Penning trap would be redundant sincethe need for an efficient bunching disappears. This, however,requires a high-quality emittance from the on-line separatorto be successful as continuous-injection mode has a reducedphase-space acceptance when the injected beam passes overthe EBIS outer barrier compared with pulsed injection �fordetailed discussion see further Ref. 5�. In addition, a rapidionization from 1+ to 2+ or higher charge state during oneround trip inside the EBIS trapping region is essential toobtain a high efficiency. Lighter ions may be more difficultto trap due to high ionization potential and short round-triptime �a similar effect has been seen for lighter ions in acharge-breeding electron cyclotron resonance ion source�ECRIS��.

The continuous-injection mode was tested at REX with astable K beam produced from the surface ion source of thetrap. It has a calculated transverse emittance of 5� mm mrad�95% at 30 keV�, but its true value is believed to be at leasttwice this number. The beam was transferred through REX-TRAP without cooling, i.e., no trapping potentials nor cool-ing gas but with the solenoidal magnetic field still present.Only 75% of the beam intensity passed the trap so the emit-tance of the beam entering the EBIS is unknown. The con-tinuous beam was transported to the EBIS and injected intothe trapping region over a low outer barrier �see Fig. 4 forbeam parameters and voltage settings�. The system was run-ning with 50 Hz repetition rate and during 18 ms of time thebeam was injected �2 ms was the dead time left for rampingvoltages between injection and extraction�. The beam extrac-tion out of the EBIS was pulsed, emptying the trapping re-gion after 18.5 ms by completely lowering the outer trappingbarrier.

A dc of 75 pA left the trap, while 14 and 13.7 e pA wasmeasured after the REX mass separator for K9+ and K10+,

giving EBIS efficiencies of 1.9% and 2%, respectively. Thecorresponding efficiency for K10+ in pulsed mode was at thismoment 15% �not including the 50% efficiency for the trap�.The peak charge state was at 9+, one charge lower than forpulsed injection, see Fig. 5 for a comparison. Two brief testswith increased residual gas pressure and 10% higher andlower electron-beam current were performed without anymajor improvement in efficiency. Further tuning would haveincreased the efficiency for the higher current though. Previ-ously published numbers for continuous-injection efficiencyquote subpercent figure.17

The attained efficiency was most likely limited by agradual buildup of the electron-beam space-charge compen-sation in the trapping region. With the low outer barrier thetrap could only hold 10 pC, a space charge that is typicallyoccupied by the residual gas ions after 10–20 ms breeding.Nevertheless, a higher efficiency could possibly still bereached since there remains a large parameter space to scan.The losses in the transfer section were not recorded and theoptimum relations between injection energy, outer barrier po-tential, and trap potential remain to be found.

Future tests would involve notably higher electron-beamcurrents leading to a larger beam acceptance and faster ion-ization, verification of the injection efficiency for high in-jected currents, and injection with lighter and heavier ele-ments. The continuous-injection mode could be interestingfor relatively heavy ions �A�20� with half-lives shorter than20 ms, and in cases where the Penning trap starts to becomesaturated �a few nanoamperes of injected beam current, seeFig. 2�.

Operation and outlook

Up to now the charge breeding system has been runningfor nearly four beam-time periods at ISOLDE and is nowdelivering beams for approximately nine weeks/year �ex-cluding setup, etc.�. As have been already mentioned the in-tensities of the radioactive beams are often so small and un-stable in intensity that beam tuning is impossible. Instead thewhole machine is set up with a stable pilot beam with adetectable intensity �10–100 pA�. Thereafter the machine�i.e., beam optics elements, separator magnet, timing, cyclo-

FIG. 4. EBIS potential settings and ion energy for continuous injection.

FIG. 5. Charge state distributions for continuous and pulsed injections. Forcontinuous mode a dc current of 60 pA K+ was extracted from the trap,while for pulsed injection the average current was 23 pA.

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tron frequency, etc.� is scaled for the radioactive beam, anoperation that takes �1/2 h and is very reproducible. Allelements between the Penning trap, EBIS, and analyzingmagnet are electrostatic and therefore charge and mass inde-pendent. The reliability of the machine is constantly ad-dressed, and now the REX low-energy part runs stable for acomplete experiment, typically one week, without any needfor interventions, except to change the isotope.

The weakest part is still the EBIS cathode with a lifetimeof three to four months operation. We are investigating alter-natives to the monocrystalline LaB6 �310� oriented cathode,with IrCe cathodes being considered. In addition, simulationsof an electron gun design using a postanode to suppress theelectron-beam scalloping, and guns with larger compressionto obtain a faster breeding time, are carried out with the aimof producing and testing a low-loss high-current gun whichshould reach the original design goal of 500 mA. The higherelectron-beam current will lead primarily to an enlarged in-jection acceptance, and a shorter breading time for heavierisotopes, as there are several demands for heavier REX-ISOLDE beams �the machine was designed for A�50 in afirst stage�. Presently the control system is being modified toallow for a slow ion extraction out of the EBIS �some milli-seconds instead of �40 �s full width at half maximum�FWHM� for a self-extracted pulse�. The trapping tubes willslowly be raised allowing some of the ions to leak out overthe outer barrier. This is requested by some experiments asthey suffer from a large dead time in the detectors during thenormal self-extracted pulse.

ACKNOWLEDGMENTS

Delahaye would like to acknowledge the support fromthe EURONS JRA, contract No. RII3-CT-2004-506065 andEURISOL DS, contract No. RIDS 515768.

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