Research application of RFQ linacs

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Nuclear Instruments and Methods in Physics Research B 99 (19951688-693

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Beam Interactions with Materials 6 Atoms

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Research application of RFQ linacs * A. Schempp

*

Institutfiir Angewandte Physik Johann Wolfgang Goethe-Unkersitiit, D-60054 Frankfurt/Main,

Germany

Abstract RFQ linacs are efficient, compact low energy ion structures, which have found numerous applications. They use electrical rf focusing and can capture, bunch and transmit high current ion beams. Some recent developments and new projects, like heavy ion injectors for tandems and for cyclotrons, RFQ post-accelerators, and the status of the work on high energy implanters and small neutron sources will be discussed.

1. Introduction The possibilities of the RFQ structure to bunch and accelerate low energy high current ion beams opens new parameter spaces for accelerator designs. The variety of RFQ-accelerators covers the full ion mass range from H to U, frequency range from 5-500 MHz and duty factors from below 0.01 up to 100% [l-3]. The physics of transport and acceleration of high current ion beams in RFQs have been solved to such extent, that the best beams, which can be produced by ion sources and transported in a LEBT, can be captured and transmitted with very small emittance growth by RFQs as shown schematically in Fig. 1. The RFQ is basically a homogeneous transport channel with additional acceleration. The mechanical modulation of the electrodes, as indicated in Fig. 1, adds an accelerating axial field component, resulting in a linac structure which accelerates and focuses with the same rf fields. For a given injection energy and frequency the focusing gradient G = Xl&/a2 (X < 1 for modulated electrodes) determines the acceptance in a low current application. A maximum voltage Uo has to be applied at a minimum beam aperture a, if the radial focusing strength is the limiting factor. The highest possible operating frequency should be chosen to keep the structure short and compact. Besides the choice of CJo and operating frequency f of the “RFQ design”, the values of aperture a, modulation m and the length L, along the RFQ determine the electrode shape (pole tips) and the beam properties. The optimum frequency can be determined by many factors. In smaller projects it is the availability of transmitters or a postaccelerator to match. Lower frequencies give

‘*‘Supported by the BMFT. * Corresponding author. Tel. +49 69 798 2802, fax +49 69 798 8510, e-mail [email protected]. Elsevier Science B.V. SSDI 0168-583X(94)00597-4

stronger focusing, less difficulties with power density and mechanical tolerances, and generally a higher current limit. Higher frequencies, for which 4-vane RFQ structures are employed, are favourable for compact designs with highest brilliance, e.g. because the charge per bunch and the frequency jump to a final linac stage are smaller. Fig. 2 shows the current limit in proton-RFQs, considering the maximum phase advance per cell and setting the maximum surface field to 2 X Kilpatrick as the sparking limit, for proton beams from 50 keV to 2 MeV. Similar curves describe optimization of structures at lower frequencies for heavy ions with respect to maximum acceptance or highest accelerating efficiency. Generally, the rf-power N needed is independent of the frequency, while the acceptance and the maximum ion current are proportional to the electrode voltage and the square of the rf-power N, which is not a big issue in pulsed injectors with low average power. High duty factor operation is the current area of development. A first class of structures, which might be used as SNS-linac injectors with duty factors of up to lo%, are being designed now, with even more difficulties to be expected for cw-RFQs 141.

2. Research applications The standard application is as a preinjector for an Alvarez linac feeding a synchrotron. These systems are

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Fig. 1. Scheme of a RFQ injector.

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comfortably matched to ion source and RFQ designs, because they have a low duty factor, which allows pulsed, high power density operation. Examples are the injectors at BNL, DESY and CERN. RFQs are very attractive for low energy ion accelerators as well. They cannot replace static injectors and Van de Graaff generators in terms of energy resolution and beam quality, but are favourable for applications with high current beams or in combination with sources such as an ECR because the source can be close to ground potential and is easy to operate and service. The RFQ concept of spatially homogeneous strong focusing proposed by Kapchinskij and Teplyakov employs strong focusing with rf-electrical focusing which is independent of velocity and therefore the acceleration can start at low energy with short cells. This allows an adiabatic capture of the dc-beam from the ion source.

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Heavy ion RFQs have been built e.g. at LBL, INS, ITEP, GSI, Saclay and IAPF for atomic and nuclear physics research. They can be classified by the lowest specific charge they can accelerate and by the duty factor of the operation. Storage ring and synchrotron injectors have a favourable low duty factor. New machines are the new HIMAC injector at NIRS [s], which is very similar to the TALL-RFQ at INS [6], the heavy ion linac at TIT [7]. operating at 80 MHz, and the new injectors at CERN [8] and Saclay [9]. The RFQ2 at the Saturne complex at Saclay is a four-rod structure (200 MHz) for ions with q/A > 0.25 from a EBIS ion source accelerating to 200 keV/u, which has been successfully brought to operation. The RFQ of the CERN Pb-injector is fed by a pulsed ECR source and operates at 101 MHz. It is designed to accelerate Pb2’+ ions from 3 to 250 keV. The RFQ built at IFN Legnaro is shown schematically in Fig. 3. This structure is a symmetric 4-rod RFQ with small vanes, such as investigated at

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Fig. 4. The RFQ tandem postaccelerator

at Sandia NL.

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Fig. 5. The MPI high current injector layout.

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CRNL and IAPF. Less complex but still more difficult is the high duty factor HLI-RFQ at GSI, which has been upgraded to 25% duty factor by improving the cooling and the alignment of the electrodes [lo]. The HLI-RFQ is 3 m long (designed for 108.5 MHz and U”‘+ ions q/A = 0.117), and it accelerates from 2.5 to 300 keV/u. The high rf-efficiency (figure of merit: shunt impedance) of the HLI-RFQ is important for high duty factor structures, for which technical problems like cooling and thermal stress control dominate, while for synchrotron injectors this is not a major concern. High current injectors studies at GSI are now concentrating on a injector design for U’+ ions. The layout consist of a RFQ (2.2-120 keV/u) [ll] and an IH-structure (0.12-1.4 MeV/u) operating at 36 MHz, without an intermediate stripper like in the previous designs for U2+, which had a spiral RFQ (27 MHz) injecting into the WidrerGe part of the UNILAC.

3. Tandem postaccelerator

RFQ

Van de Graaff tandem machines have been the work horse for nuclear physics research. Their limitations are low currents and low energy per nucleon for heavier ions, which has led to the various heavy ion rf-accelerators used as tandem postaccelerators. A RFQ, although a low velocity structure, can be applied as a postaccelerator for a tandem installation if very heavy particle and a fixed output energy/u are no restriction for the experiments. At Sandia NL an installation of this type is being set up as shown in Fig. 4 [12]. The PA-RFQ, which has been built by AccSys, is a 425 MHz structure very similar to their “standard” proton injectors. The rather low output energy of the tandem of 0.25 MeV/u (Au’~+) fits the velocity range of the RFQ (output energy 1.9 MeV/u). The PA-RFQ has a total accelerating voltage of 11.6 MV (total length of 6.2 m), and uses two equal length resonators. The experiments in the area of material modification and ion beam analysis make use of the k-beam, an advantage of the low emittance tandem beams, not spoiled by rf-postacceleration.

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especially important for laser cooling experiments. A prototype RFQ, which is based on the GSI HLI design, has been operated successfully at MPI [13]. The ISL at the HMI Berlin (the former VICKSI machine) is an isochronous cyclotron with four separate sectors. It has an external injection of beams with variable energy from either a CN-Van-de-Graaff or an 8UD-tandem. Fig. 6 shows the layout of the accelerator complex and its major facilities. A first proposal to use an RFQ to improve the axial injection system of a cyclotron was made by Hamm [14]. To inject into a Separated Sector Cyclotron, the RFQ has to provide a bunched beam at a well defined injection energy given by the inner radius of the SSC. The operating frequency of the RFQ must be synchronized with the cyclotron frequency, which for RFQs normally means a fixed output energy per nucleon. This would be a possible solution only for fixed energy cyclotrons. The fixed velocity profile is typical for RFQs. It can only be changed by varying the cell length L or the frequency f, The second possibility of changing the Wideriie resonance condition: L = R, A,,/2 = ~.&!f, is the way which has been used for RFQs with variable energy (VE-RFQ). For this reason it is possible to change the output energy using the same electrode system by varying the resonance frequency of the cavity: L‘~-f, T - ~1:. To change the frequency of the 4-rod-RFQ, a type of RFQ resonator which has been developed at Frankfurt, in which the resonator can be tuned capacitively or inductively [15]. Fig. 7 shows the latter way of tuning by a movable tuning plate, which varies the effective length of the stems. In Frankfurt the VE-RFQ was developed first for the application as a cluster postaccelerator at the 0.5 MV Cockroft-Walton facility at the IPNL in Lyon (France). It was designed for Ei, = 10 keV/u and an output energy between E,,, = 50 and E,,, = 100 keV/u for m = 50 u. Real heavy and low velocity particles are accelerated in the second VE-RFQ, with an energy range from 2 eV/u to

4. Tandem replacement RFQs have been regarded as possible replacements for tandem injectors for postaccelerators very early. But the first one to be built was a superconducting heavy ion linac bypassing the EN-tandem for uranium beams. At the MPI-Heidelberg many experiments at the TSR storage ring were limited by the low currents delivered by the MP tandem. A high current injector for singly charged light ions, consisting of a CHORDIS source, two RFQs and -/-gap resonators as shown in Fig. 5, will provide up to three orders of magnitude increase in intensity which is

Fig. 7. The VE-RFQ structure

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1 keV/u for singly charged metallic clusters up to mass m = 1000 u (frequency range from 5 to 7 MHz) [16]. The third VE-RFQ a first combination of an ECR ion source with a VE-RFQ, has been built for the IKF Frankfurt. The design values are a minimum specific charge of 0.15, an output ion energy of E,,, = 100-200 keV/u, a maximum electrode voltage of 70 kV and it has a structure length of 1.5 m. VE-RFQs have a fixed ratio of output to input energy given by the length of the first and last modulation cell. This is similar to the energy gain factor of a SS-Cyclotron which makes them well suited as injectors. To cover the energy range of 1.5-6 MeV/u, the injection energy of the ISL must be between Ei, = 90 and Ei, = 360 keV/u (maximum accelerating voltage V, = 2.9 MV, at cyclotron frequencies of 10 to 20 MHz). The ISL tandem injector will be replaced by a combination of an ECR ion source on a 200 kV platform, which will produce highly charged ions with charge-to-mass-ratios between l/8 and l/4, and a VE-RFQ [17]. To stretch the energy range the RFQ will be split into two RFQ stages. Each stage, with a length of 1.5 m, consists of a ten stem 4-rod-RFQ-structure. With an rf-power of 20 kW per stage, an electrode voltage of 50 kV is possible. In the first mode of operation both RFQs accelerate, the output energy of the cyclotron is between E,,, = 3 and 6 MeV/u with a harmonic number of 5 for the cyclotron. For low energy beams, only RFQ 1 accelerates while RFQ 2 is detuned to transport the beam. In this mode the energy range of the cyclotron is between E,,, = 1.5 and 3 MeV/u and the cyclotron works on the harmonic number 7. In both modes the RFQs are tuned to the eighth harmonic of the cyclotron. The RFQ-output emittance depends largely on the input conditions. For matched input beams with AE/E < l%, normalized emittance E, < 0.5 n mm mrad and a bunch length At < 1 ns, a transmission of 100% is expected. To obtain this beam quality, it is necessary to have a buncher-chopper system between the ECR and the RFQ. The ECR-source is mounted on the 200 kV platform, formerly used for the tandem. The vertical beam is bent by 90”, passes through the buncher-chopper system and is injected into the RFQs. The final matching into the RFQ is done by a triplet lens approximately 1 m before the RFQ

to allow for diagnostics and a Faraday cup. The beam from the RFQ is transported into the injection beamline of the cyclotron, to which a rebuncher has been added to make a proper time focus for the cyclotron. The RIKEN injector has been a variable frequency Wideroe-type linac. The 450 keV CW injector will now be replaced by an RFQ which is tunable from 8 to 28 MHz to match the RRC cyclotron. It is planned to use a asymmetric 4-rod type RFQ, excited by one h/4 resonator [18].

5. New developments There are a number of studies on the use of RFQ ion injectors, of which the proposal to built accelerators for radioactive ion beams seem to be of highest interest in nuclear physics research. The typical RIB facility starts from an ISOLDE type of ion source with singly charged ions. To get a reasonable amount of ions, cw operation is planned. This favours superconducting structures; however, the low energy section must use much lower frequencies for rf-acceleration than suitable for SC-cavities. This leaves room for considering nc-RFQ structures to form the beam, as shown in Fig. 8 for a possible ISL injector at LBL [19]. The various systems can be distinguished by the heaviest mass number, which is planned to be used and the accelator system which will be employed. INS, TRIUMPF, ANL have linac based system for masses between 30 u and 60 u. For ISL at LBL the full mass range of singly charged ions (m I 240 uf is planned to be used, resulting in a large installation. A very interesting RIB concept is being pursued by MPI [20], which starts with an EBIS ion collector-charger with pulsed extraction. A small compact accelerator of the MPI high current injector respecpectively postaccelerator type is to be installed at the CERN ISOLDE facility.

6. Conclusions There are a lot of new developments in the field of rf-ion accelerators, which will allow new experimental

A. Schempp / Nucl. Instr. and Meth. in Phys. Rex B 99 (19951688-693

parameters cyclotrons, accelerators

for atomic and nuclear physics. Injectors into tandem replacements and new compact ionare attractive application of RFQs.

References [I] I.M. Kapchinskiy and V. Teplyakov, Prib. Tekh. Eksp 119 (2) (1970) 17. [2] K.R. Crandall, R.H. Stokes and T.P. Wangler, Linac79, BNL 51134 (1979) p. 205. [3] A. Schempp, Adv. of Act. Physics and Technology (World Scientific. (1994) 194. 141 GE. McMichael, Linac90, LA-12004-C (1990) p. 518. [51 S. Yamada et al., Proc. Linac94, Tsukuba, Japan, Aug. 1994. [6] N. Ueda et al., Proc. Linac84 GSI 84-11 (984) p. 71. [7] M. Okamura et al.. Proc. Linac94. Tsukuba, Japan, Aug. 1994.

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[S] D. Warner, Proc. Linac94, Tsukuba, Japan, Aug. 1994. [9] J.L. Lemaire et al., Proc. Linac94. Tsukuba. Japan, Aug. 1994. [lo] J. Klabunde et al.. Proc. Linac94, Tsukuba, Japan, Aug. 1994. [ll] H. Deitinghoff et al., GSI-Report 94-10 (1994) p. 30. [12] H. Schone, presented at this Conference (13th Conf. on the Application of Accelerators in Research and Industry, Denton, TX, USA, 1994). [13] CM. Kleffner et al., EPAC92 (Editions Front&es, 1992) p. 1340. [14] R.W. Hamm et al.. Proc. 9th. Cyclotron Conf., Caen, France, 1981, p. 359 [15] A. Schempp, Nucl. Instr. and Meth. B 40/41 (1989) 937. [16] A. Schempp, Nucl. Instr. and Meth. B 88 (1994) 16. [17] A. Schempp et al., EPAC94, (World Scientific, 1994) p. 566. [181 Y. Yano et al., EPAC94, (World Scientific, 1994) p. 515. [19] M. Nitschke, LBL 3.5533, 1994. [20] D. Habs, REX-ISOLDE proposal, CERN/ISC 94-25.

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