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Status of the R&D for the rare isotope accelerator project
Nuclear Instruments and Methods in Physics Research B 204 (2003) 771–779 www.elsevier.com/locate/nimb
Status of the R&D for the rare isotope accelerator project G. Savard
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Physics Division, Argonne National Laboratory, 9700 South Cass Avenue, Argonne, IL 60439, USA
Abstract A next generation radioactive beam facility, the Rare Isotope Accelerator (RIA), is in preparation in the US. RIA aims at producing intense beams of radioactive isotopes and providing them to experimental stations with energy variable from ion source energy to a few hundred MeV/u. To perform this task, RIA will use standard ISOL and fragmentation techniques together with novel approaches combining advantages of both techniques to obtain high quality beams of the produced isotopes at all energy regimes. RIA will use these approaches in combination with a novel 400 kW superconducting heavy-ion driver linac to produce the activity and a very efficient post-acceleration scheme based on low-frequency RFQs injecting an ATLASlike superconducting linac to obtain maximum intensity and excellent beam quality at the experimental stations. The development of the RIA concept required new ideas and significant technical advances. The technical issues with this versatile high-power facility and their present solutions will be presented together with the present status of the R&D efforts and the performance obtained with various prototypes that have been completed. Published by Elsevier Science B.V. PACS: 29.17.+w; 29.25.Rm; 41.85.)p Keywords: Radio frequency quadrupole; Superconducting linac; Charge stripper; Source of radioactive nuclei
1. Introduction The Rare Isotope Accelerator (RIA) [1] is a next generation radioactive beam facility in preparation in the US. RIA combines the best of standard ISOL and in-flight fragmentation technology with novel approaches to handle high-primary beam power and remove existing limitations in the extraction of short-lived isotopes. The use of a versatile primary accelerator (superconducting linac designed to allow multiple charge state acceleration and capable of providing beams from
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Tel.: +1-630-252-4024; fax: +1-630-252-6210. E-mail address: [email protected] (G. Savard).
0168-583X/03/$ - see front matter Published by Elsevier Science B.V. doi:10.1016/S0168-583X(03)00501-9
protons at 900 MeV to uranium at 400 MeV/u at power levels up to 400 kW) allows various production and extraction schemes to be used to optimize production of specific isotopes. In particular, a novel approach where radioactive isotopes produced by fragmentation of a fast heavy-ion beam are stopped and cooled in a highpurity helium gas cell and extracted by electromagnetic fields as thermal singly-charged ions promises large increase in efficiency for very shortlived isotopes. These isotopes will then be available for studies at ion source energy or can be further accelerated by a second superconducting linac whose novel injection system, based on a combination of a CW low-frequency split-coax RFQ and two hybrid RFQs, allows the efficient acceleration
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of singly-charged heavy ions with masses up to 240 amu from ion source energy. The proposed facility relies heavily on new technologies to provide: (i) the primary beam power required for the production of the radioactive isotopes, (ii) versatile target/ion source systems that can efficiently and rapidly release and ionize the reaction products of interest while withstanding the high primary beam power and (iii) post-accelerate in a continuous fashion these radioactive ions with high-efficiency starting from singly-charged ions at ion source potential. Demonstrating the feasibility of the approaches selected for these three tasks required a vigorous R&D effort that began around 1995 and became a national effort in 1999 with the R&D funds distribution determined by the Marx committee. Although the funds available every year have been small on the scale of this project, with additional support from the laboratories it has been possible to make significant progress in the R&D for the project. The important results obtained this far, which confirm that the approach selected for RIA is valid, will be presented in the following.
2. The RIA project A block-diagram of the RIA facility is shown in Fig. 1. The facility can be separated in three sections: the driver linac, the isotope production complex, and the post-accelerator and experimental areas. The heart of the facility, the section
Fig. 1. Simplified schematic layout of the RIA facility.
that dictates what the other sections must provide, is the isotope production complex. It has been determined through extensive calculations [2] that to obtain optimum production of the various short-lived isotopes over the Segre chart it is necessary to use many production mechanisms and extraction techniques. These range from the standard ISOL spallation reactions with light beams on thick targets for isotope which diffuse out of targets rapidly, to two-step neutron generator techniques for isotopes close to the top of the fission peaks distribution, to in-flight fragmentation or fission of fast heavy beams followed by a gas catcher system or directly to experiments with fast beams. In all cases, these approaches must be able to handle the high primary beam power required for maximum production. It is also clear from these calculations that what matters is not only the raw production (in which case standard ISOL would always be best) but the amount that can be extracted, and that, for very short-lived or refractory species tends to favor other approaches such as the gas catcher technique. Asking for the facility to be able to use these multiple production methods imposes very demanding requirements on the driver linac. It must be able to deliver essentially all stable beams, from proton to uranium, with high current and efficiency. This forces the choice of superconducting RF technology for the driver linac. Although this choice implies the development of superconducting cavities spanning a huge range of velocities, including the medium beta region where no cavities previously existed, it is warranted by the many benefits, most notably the large velocity acceptance and the ability to accelerate multiple charge states simultaneously which greatly increases the driver linac efficiency for heavy ions. The post-accelerator section is itself also determined by the production methods. The most efficient production techniques for low-energy radioactive beams, whether from a standard ISOL source or a gas catcher system, produce singlycharged ions. For the post-accelerator the choices are then either to increase the charge state for acceleration by a standard post-accelerator or to start the acceleration process directly with the singly-charged ions until they have enough energy
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to be stripped efficiently. Although charge state breeders have made significant progress, they still yield a conversion efficiency in a single charge state averaging around 5%. This factor of 20 loss is far too important for a facility of the magnitude of RIA and the more expensive, more technically challenging, but also more efficient approach of accelerating the singly-charged ions from ion source potential is the logical choice. This choice implied that new accelerating structure had to be developed that could accelerate in a continuous fashion ions with a mass to charge ratio as high as 240. These low-frequency RFQ structures together with highly efficient non-equilibrium charge state stripping [3] in gases and existing low-beta superconducting cavities provide the most efficient postaccelerator design while providing excellent beam quality and energy variability from ion source energy to about 10 MeV/u. As seen above, RIA is a complex project that utilizes a number of new technologies and will push other existing technologies far beyond where they have been used so far. The following will succinctly present the facility and highlight important R&D developments in the three main systems presented above that make RIA possible.
3. Driver linac The driver linac produces the wide variety of high-power stable beams required for the different production mechanisms. The driver linac design has a total accelerating voltage of 1.4 GV and is capable of producing beams of essentially all stable species from 900 MeV protons and 2.3 GeV 3 He up to 400 MeV/u uranium. The wide range of energies possible comes from the large velocity acceptance of the short independently-phased superconducting cavities. The required intensity for the very heavy ions, in excess of 100 kW for uranium, can only be attained with existing ion source technology with a very efficient acceleration scheme. The design of the driver linac (Fig. 2) is therefore based on the simultaneous acceleration of multiple charge states of these heavy beams and an important simulation and R&D effort has been put forth to demonstrate this capability. In the
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Fig. 2. RIA driver linac details with the multiple charge states carried and energy in the three main sections of the accelerator for a uranium beam.
baseline design for the driver linac it was decided that to minimize the R&D effort required the 805 MHz elliptical cavities [4] currently being developed for the SNS project would be used for the highest-b section of the RIA driver linac. That constraint, together with the desire to have the highest acceptance for the lower velocity section of the linac and minimizing the cavity count (and hence the cost), determined the frequency choice for the initial section of the baseline design to be 1/14 of that frequency or 57.5 MHz. The frequency in the following sections then goes with the beta of the cavities. The intricacies of the acceleration scheme can best be explained by following a uranium beam through the machine since this one beam requires all the recent developments to reach the required beam power. The heavy ions are produced in one of two ECR sources. The beam is bunched in a 28.7 MHz buncher system which feeds a 57.5 MHz RFQ section. To increase the current available in the case of uranium, two charge states, 28+ and 29+, are carried [5] by the low-energy beam transport system and bunched in alternating RF buckets in the 57.5 MHz RFQ [6] so that one bucket accelerates 28+ and the next one 29+ and so on. This approach fills the RFQ with about twice as much beam as would be available from the maximum charge state alone and has been modeled in details [5]. Following the RFQ, both charge states are injected into the low-b section of the superconducting linac composed of a total of 85 cavities of four different types: three 57.5 MHz fork cavities
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with b of 0.024, 0.036 and 0.061, and one 115 MHz b ¼ 0:15 quarter-wave resonator. The fork type cavities are similar to those used in the PII injector [7] at the ATLAS superconducting linac at Argonne and are being prototyped together with a new cryostat design. The quarter-wave cavities are the result of a new development and incorporate beam steering compensation [8] to correct for the magnetic steering experienced when ions traverse the structure at non-zero RF phase. This is accomplished by tilting the faces of the center post of the cavity so that the perpendicular electric field component cancels out the magnetic force. A uranium beam leaves this section of the linac at an energy of 9.3 MeV/u. The beam is then stripped and after a short matching section 5 charge states [9] centered around 71+ are transferred to the medium-b section of the linac. This section contains two types of half-wave cavities, 172.5 MHz b ¼ 0:25 cavities and 345 MHz b ¼ 0:39 double-spoke type cavities, for a total cavity count of 195. Both cavity types are new developments being prototyped at ANL and the spoke cavities cover a velocity range where no superconducting cavities existed previously. Prototyping of the spoke cavities has already produced cavities which after chemical polishing and high-pressure water rinse [10] have far exceeded the RIA requirements in tests at both ANL and LANL [11]. A fast tuner for these 350 MHz cavities is being developed at LLNL. The simultaneous acceleration of multiple charge states in this section is a key component of the RIA design. Tests were carried out at Argonne to accelerate multiple charge states [12] through the booster section of the ATLAS linac and a 94% transmission was observed, in agreement with calculations, and over a mass-to-charge range exceeding that required for RIA. This warrants this component of the design that not only yields much larger beam intensities but just as importantly, also reduces the power requirements on the charge-state strippers for a given output current. The uranium beam leaves the medium b-section at an energy of 93 MeV/u and is further stripped to a mean charge state of about 89+ and four charge states are carried through to the high-b section. This section contains a total of 136 cavities, com-
posed of three types of 805 MHz 6-cell elliptical cavities with b of 0.49, 0.61 and 0.81 operating at 2.0 K. The two highest b cavities are similar to spallation neutron source (SNS) cavities developed for the same velocity range by Jefferson Laboratory. This choice was made to have maximum overlap with existing R&D efforts on-going in the US and result in not only cost savings, but just as importantly, time and manpower savings. The main difference with the SNS application of these cavities comes from the fact that RIA will be a continuous machine, not a pulsed one, and therefore the RF power feeding and control must be different. Both are being developed at Jefferson Laboratory for RIA. The cavity themselves are performing at a level of about 35 MV/m (newly adopted by the SNS as its target surface gradient) well above the initial RIA design surface gradient. In RIA these cavities will operate at a slightly lower level to allow locking at the much higher effective Q that will result from the CW operation. The lowest-b cavities in this section are a new development pushing the elliptical cavities to lower b than has been done before. Two single cell b ¼ 0:47 cavities have been tested at Jefferson Laboratory and MSU and reached very high Q. Multi-cell cavities are now being developed and microphonics (to which the CW machine is more sensitive) are being studied. The final component of the linac is a switchyard with RF switching to distribute the beam, bucket per bucket, among target stations. An ion optical design capable of splitting the multiple charge states of uranium among two targets while maintaining the mm beam size on both targets has been worked out. While the technical feasibility of the components of the driver linac is now well documented, R&D is still on-going to ensure that the planned construction schedule can be maintained and that potential savings identified in the early R&D efforts can be taken advantage off. For example, there are 276 low- and medium-b drift-tube resonators in 33 cryomodules. Procurement, assembly, installation, testing, and commissioning over a 4–5 year construction schedule requires complete development and prototyping prior to final engineering design. Similarly, being able to use a
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higher charge state out of the ECR ion source (for example from new sources such as the superconducting ECR source VENUS [13] being developed at LBL) or obtaining higher accelerating gradient as the R&D seems to indicate would yield a shorter and hence less expensive linac. These venues are being explored and the base parameters of the linac are being reevaluated periodically to see if the R&D indicates that such changes are warranted.
4. Production complex The production of the radioisotopes occurs by interaction of the primary beams on high-power targets. To obtain maximum yield over the widest range of unstable isotopes it is necessary to use different production mechanisms [2] and the versatile driver available at RIA enables four main mechanisms which are described below. The standard ISOL technique [14] where a highenergy light-ion beam impinges on a thick target producing short-lived isotopes via spallation or induced-fission reactions. The target is heated to release the activity quickly. The activity then diffuses to an ion source where it is ionized and extracted and made available for experiments at low energy or at higher energy after post acceleration. At RIA, the light ion beam will have power up to 400 kW and the main difficulty will be to handle the power deposited in the production target. R&D at ORNL and ANL is therefore concentrating on techniques to remove the heat from the target and on refractory compounds with enhanced thermal conductivity and release properties. Dependence of the extraction delay times on the geometry is also being studied with realistic full Monte-Carlo simulations [15]. A second approach is that of production of radioactive species by fragmentation [16] of a fast heavy-ion beam on a thin target capable of handling the high beam power. The reaction products are pushed forward by the kinematics and can be separated from the beam in a fragment separator. These radioactive beams are then available for experiments at high-energy but with poorer beam quality. The standard fragmentation target used at
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RIA will be a flowing windowless liquid lithium film for all heavy beams. A prototype of such a target is being developed at ANL [17] and a lower power version combining liquid lithium and beryllium is being developed by an ANL/NSCL collaboration and will be tested in on-line conditions at the NSCL. The isotopes produced in the fragmentation and in-flight fission processes must also be collected with maximum efficiency which requires the highest acceptance fragment separator. A design for an 18% momentum acceptance separator is being investigated and R&D on radiation resistant large aperture quadrupoles for the initial section of the separator is on-going at MSU. These two conventional approaches are implemented with unprecedented power at RIA and are supplemented by two new complimentary approaches that remove the main limitations of these standard approaches. The first new approach is the two-step neutron generator technique [17,18] where the high-energy light ion beam is converted to fast neutrons in a cooled converter which is surrounded by a production target where fissions induced by the fast neutrons produce the radioactive species which are then extracted in a fashion similar to that used in the standard ISOL technique. This yields significant gains since the power deposited by the light ion beam (mostly via electromagnetic interactions) is in a volume which can be cooled efficiently, while only the neutron and the fission power is deposited in the releasing target where power removal is much more difficult to perform without affecting the other functions of the device. Yields have been calculated in details [2] and the geometry optimized to obtain maximum yield for the most neutron-rich isotopes. Initial tests have been carried out at PNPI to compare direct and indirect production of neutron-rich isotopes and give excellent agreement with simulations [19]. The two-step generator has actually other benefits such as minimizing the isobaric contamination produced by spallation and as such is now being used routinely at ISOLDE to improve the purity of the most neutronrich beams. A second new approach brings together the advantages of the in-flight fragmentation approach
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with that of the ISOL techniques [20]. Radioactive isotopes are produced by fragmentation (or inflight fission) of fast heavy-ion beams on a ‘‘thick’’ thin target, the reaction products are pushed again forward by the kinematics and after selection by a large acceptance fragment separator are monochromatized, slowed down and stopped in a gas catcher system [21] where they are thermalized but remain singly charged and can be extracted by a combination of DC and RF fields to be further reaccelerated. This results in beams of quality similar to those obtained by ISOL techniques, available at any energy, without the chemical limitations encountered by the ISOL technique in the diffusion and release out of thick targets. A scaled down version of the gas catcher system has been investigated thoroughly on-line and off-line at ANL and both high efficiency and short extraction delay times have been obtained. Investigations of the expected most important limitations of such a system have also taken place and conditions for reliable on-line operation obtained. The next step is the construction of a full-scale prototype (see Fig. 3) RIA gas catcher which is on-going at ANL [21] where it is being fully characterized before being moved to GSI for tests at the full RIA energy behind the FRS separator. Preliminary experiments at GSI have been performed that demonstrated the achromatization of the reaction
Fig. 3. Picture of the full-scale RIA gas catcher prototype currently being assembled at ANL.
products required to minimize the range straggling [22] in the stopping gas catcher.
5. Post accelerator The most efficient techniques to produce the radioactive ions create them as singly-charged ions. The RIB accelerator system must therefore efficiently accept and accelerate singly-charged ions over the full mass range, up to m=q of 240, to an output beam energy up to 5–10 MeV/nucleon. This must be done while maintaining a small longitudinal emittance (typically 0.5 p keV/u ns) over the full range of energy and mass. The short independently-phased cavities developed for existing superconducting heavy-ion linacs provide a basis for all but a small portion of the RIB post-accelerator system. Only the initial section of the post-accelerator, a very low charge state injector section providing bunching and the first few MV of acceleration, can be made most efficiently by using a different technology namely sections of cw normal-conducting RFQs. The RIB linac therefore consists of the following main sections: • An injector with three sections of normal-conducting RFQs capable of accepting ions with q=m > 1=240 and 2 gas stripper locations for the heaviest ions. • A first section of superconducting linac that will accelerate ions of q=m > 1=66 to 680 keV/u or more. • A carbon-foil stripper to provide, when necessary, a q=m > 2=15 for the last stage of acceleration. The beam energy at this point depends on the particular charge-to-mass ratio. • A superconducting linac to accelerate ions of q=m > 2=15 to Coulomb barrier energy or higher. Following the radioactive ions from the ion source through the post-accelerator we first pass through a high-resolution (Dm=m ¼ 1=20 000) isobar separator before reaching the low-frequency RFQ injectors. The buncher, the first two sections of 12 MHz RFQ, and two He gas-stripper cells are
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placed on a 380 kV variable-voltage platform. Placing these elements on a variable-voltage platform allows operation with a fixed velocity profile for the full mass range of ions. This first section accelerates all ions up to 7 keV/u and the beams of 66 < mass 132 will be charge-stripped at this point. For best efficiency over the full mass range, nonequilibrium helium gas stripping [3] must be performed at different energies in the RFQ section for different mass ions. By stripping at 7 keV/u for example, an incident 132 Sn beam can be stripped into charge state 2+ with 55% efficiency and further accelerated. But for the heavier ions with Z > 54 higher charge states are required for which the best stripping efficiency is achieved at the higher energy of 20 keV/u. Whether stripped or not, ions of any mass and charge state, including mass 240 at charge state 1+, will be further accelerated by the next section of the 12 MHz RFQ to an energy of 20.3 keV/u. At this point the beams of mass > 132 will be stripped. The final RFQ operating at 24.25 MHz will accelerate the ions, now at a charge state q=m > 1=66, to an energy of 62.6 keV/u for injection into the superconducting linac. Although this first section of the post-accelerator is quite different and significantly more powerful than any proposed this far, it is based on existing technology or advanced R&D efforts. The bunching system is similar to the 12 MHz fourharmonic bunching system presently in use on the ATLAS accelerator. The first RFQ is operated at the lowest frequency feasible to maximize the transverse focusing strength. It has been demonstrated at ANL that a split-coaxial RFQ geometry is appropriate for cw operation at 12 MHz [23]. The RFQ is designed for a minimum charge-tomass ratio of 1/240 with ions of higher charge state simply accommodated by scaling both the platform voltage and the RFQ RF voltage to match. The proposed CW inter-vane voltage of 92 kV with a mean bore radius of 9 mm has been proven entirely practical in extensive tests with the existing prototype at ANL. The last two sections of the RFQ are based on a novel more effective accelerating structure, a hybrid RFQ [24]. It has been found that the concept of separated accelerating and focusing zones can be applied to the acceler-
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ation of heavy ions with q=m P 1=240 and at very low energies if the beam focusing is provided by RF quadrupoles. The DTL accelerating and RF focusing sections are integrated into a single resonant structure called the hybrid RFQ. The 12 MHz H-RFQ will be based on a cold 1:2 scale prototype that was completed at ANL where the structure is being studied for electrodynamic properties. Numerical simulations of the beam dynamics through the entire chain of RFQ sections, including RF bunchers between sections and transverse focusing, have been performed. The proposed design achieves longitudinal emittance as low as 0.2 p keV/u ns for 80% of the DC beam entering the buncher. The RIB beams then enter the superconducting section of the RIB linac. The lowest velocity section is a low-charge-state injector linac based on established interdigital drift-tube SC niobium cavity designs. Cavities in this velocity range provide typically 1 MV of accelerating potential per cavity [7]. This section is similar to the PII injector at ATLAS except for the lower charge state of the ions which then requires more cavities and stronger transverse focusing. For the charge states considered here (q=m ¼ 1=66) the proper focusing can be reached with the help of strong SC solenoid lenses with fields up to 15 T that are available from commercial vendors [25]. Additional space around these solenoids has been included in the design to allow for shielding of the cavities from these strong fields. The section of the superconducting linac consists of 54 interdigital cavities operating at )20 synchronous phase with each cavity followed by a SC solenoid. This linac can accelerate any beam with q=m P 1=66 over the velocity range 0:0011 6 b 6 0:04. Masses m 6 66 will not require any stripping and be provided with maximum intensity to the astrophysics area where most experiments will require the maximum beam intensity available. For the heavier ions, the high helium-stripping efficiency in the range of 30–45% for masses 240 P m P 66 will also provide unmatched accelerating efficiency to this energy range. In both cases, it will allow beam intensities higher by about an order of magnitude as
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compared with RIB accelerators based on the ECR charge breeder. After this second stripper, the desired charge state must be selected and further accelerated by eight SRF cavities of b ¼ 0:037 to bring the beam energy to approximately 1.4 MeV/u and match the velocity acceptance of the b ¼ 0:06 resonators in the final linac section. This linac section consists of 12 b ¼ 0:06 resonators, 30 b ¼ 0:105 resonators and eight quarter-wave resonators recently designed for b ¼ 0:15. This post-stripper section of the RIB linac is designed for the acceleration of multiple charge state beams to enhance the available beam intensities for experiments. As was shown in [12] a wide range of the charge spread Dq=q, about 20%, can be accepted and accelerated in an ATLAS-like accelerator. For the RIA postaccelerator, we have restricted the possible range of Dq=q to 6 11% in order to avoid emittance halo in phase-space. As a consequence of multiple charge state acceleration the total stripping efficiency is significantly higher than for the single charge-state beams. The transverse and longitudinal emittances of multi-q beams will however be larger by a factor of 3 as determined by beam measurements in ATLAS [12] and the options will be opened to the experimenters to choose either the highest beam intensity or the highest beam quality in which case the last section only accelerates one charge state. The post-accelerator efficiencies for different masses and energies are depicted in Fig. 4.
6. Conclusion The RIA facility is based on what seem at present the most efficient technologies for acceleration of the primary beam, production and extraction of the unstable isotopes and postacceleration of those isotopes. An important R&D effort has demonstrated the feasibility of the approach selected and further efforts aimed at optimizing the design and the implementation of the selected technologies is ongoing. The overall design of the facility has been extremely stable over the last few years but various subsystems are being reexamined at regular interval to incorporate the latest R&D findings. In particular, potential cost savings have been identified as a result of the R&D and are being further investigated. The high level of technical preparation for the RIA project that will be achieved with the current R&D efforts should lead to a rapid construction schedule which is key to staying within the cost estimates for the facility.
Acknowledgements The RIA concept has evolved from ideas generated within the US nuclear physics community and contributions from abroad and from other fields. The development work on the project has been performed by a national program for RIA R&D with current participation from ANL, JLAB, LANL, LBNL, LLNL, NSCL and ORNL. This work was supported by the US Department of Energy, Division of Nuclear Physics, under contract number W-31-109-ENG-38.
References
Fig. 4. Stripping efficiency for secondary beams of various mass as a function of the final energy. The first numbers are for a single charge state acceleration while the numbers in parenthesis are for acceleration of multiple charge state after the stripper following the lowest-b superconducting section.
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Status of the R&D for the rare isotope accelerator project G. Savard
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Physics Division, Argonne National Laboratory, 9700 South Cass Avenue, Argonne, IL 60439, USA
Abstract A next generation radioactive beam facility, the Rare Isotope Accelerator (RIA), is in preparation in the US. RIA aims at producing intense beams of radioactive isotopes and providing them to experimental stations with energy variable from ion source energy to a few hundred MeV/u. To perform this task, RIA will use standard ISOL and fragmentation techniques together with novel approaches combining advantages of both techniques to obtain high quality beams of the produced isotopes at all energy regimes. RIA will use these approaches in combination with a novel 400 kW superconducting heavy-ion driver linac to produce the activity and a very efficient post-acceleration scheme based on low-frequency RFQs injecting an ATLASlike superconducting linac to obtain maximum intensity and excellent beam quality at the experimental stations. The development of the RIA concept required new ideas and significant technical advances. The technical issues with this versatile high-power facility and their present solutions will be presented together with the present status of the R&D efforts and the performance obtained with various prototypes that have been completed. Published by Elsevier Science B.V. PACS: 29.17.+w; 29.25.Rm; 41.85.)p Keywords: Radio frequency quadrupole; Superconducting linac; Charge stripper; Source of radioactive nuclei
1. Introduction The Rare Isotope Accelerator (RIA) [1] is a next generation radioactive beam facility in preparation in the US. RIA combines the best of standard ISOL and in-flight fragmentation technology with novel approaches to handle high-primary beam power and remove existing limitations in the extraction of short-lived isotopes. The use of a versatile primary accelerator (superconducting linac designed to allow multiple charge state acceleration and capable of providing beams from
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Tel.: +1-630-252-4024; fax: +1-630-252-6210. E-mail address: [email protected] (G. Savard).
0168-583X/03/$ - see front matter Published by Elsevier Science B.V. doi:10.1016/S0168-583X(03)00501-9
protons at 900 MeV to uranium at 400 MeV/u at power levels up to 400 kW) allows various production and extraction schemes to be used to optimize production of specific isotopes. In particular, a novel approach where radioactive isotopes produced by fragmentation of a fast heavy-ion beam are stopped and cooled in a highpurity helium gas cell and extracted by electromagnetic fields as thermal singly-charged ions promises large increase in efficiency for very shortlived isotopes. These isotopes will then be available for studies at ion source energy or can be further accelerated by a second superconducting linac whose novel injection system, based on a combination of a CW low-frequency split-coax RFQ and two hybrid RFQs, allows the efficient acceleration
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of singly-charged heavy ions with masses up to 240 amu from ion source energy. The proposed facility relies heavily on new technologies to provide: (i) the primary beam power required for the production of the radioactive isotopes, (ii) versatile target/ion source systems that can efficiently and rapidly release and ionize the reaction products of interest while withstanding the high primary beam power and (iii) post-accelerate in a continuous fashion these radioactive ions with high-efficiency starting from singly-charged ions at ion source potential. Demonstrating the feasibility of the approaches selected for these three tasks required a vigorous R&D effort that began around 1995 and became a national effort in 1999 with the R&D funds distribution determined by the Marx committee. Although the funds available every year have been small on the scale of this project, with additional support from the laboratories it has been possible to make significant progress in the R&D for the project. The important results obtained this far, which confirm that the approach selected for RIA is valid, will be presented in the following.
2. The RIA project A block-diagram of the RIA facility is shown in Fig. 1. The facility can be separated in three sections: the driver linac, the isotope production complex, and the post-accelerator and experimental areas. The heart of the facility, the section
Fig. 1. Simplified schematic layout of the RIA facility.
that dictates what the other sections must provide, is the isotope production complex. It has been determined through extensive calculations [2] that to obtain optimum production of the various short-lived isotopes over the Segre chart it is necessary to use many production mechanisms and extraction techniques. These range from the standard ISOL spallation reactions with light beams on thick targets for isotope which diffuse out of targets rapidly, to two-step neutron generator techniques for isotopes close to the top of the fission peaks distribution, to in-flight fragmentation or fission of fast heavy beams followed by a gas catcher system or directly to experiments with fast beams. In all cases, these approaches must be able to handle the high primary beam power required for maximum production. It is also clear from these calculations that what matters is not only the raw production (in which case standard ISOL would always be best) but the amount that can be extracted, and that, for very short-lived or refractory species tends to favor other approaches such as the gas catcher technique. Asking for the facility to be able to use these multiple production methods imposes very demanding requirements on the driver linac. It must be able to deliver essentially all stable beams, from proton to uranium, with high current and efficiency. This forces the choice of superconducting RF technology for the driver linac. Although this choice implies the development of superconducting cavities spanning a huge range of velocities, including the medium beta region where no cavities previously existed, it is warranted by the many benefits, most notably the large velocity acceptance and the ability to accelerate multiple charge states simultaneously which greatly increases the driver linac efficiency for heavy ions. The post-accelerator section is itself also determined by the production methods. The most efficient production techniques for low-energy radioactive beams, whether from a standard ISOL source or a gas catcher system, produce singlycharged ions. For the post-accelerator the choices are then either to increase the charge state for acceleration by a standard post-accelerator or to start the acceleration process directly with the singly-charged ions until they have enough energy
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to be stripped efficiently. Although charge state breeders have made significant progress, they still yield a conversion efficiency in a single charge state averaging around 5%. This factor of 20 loss is far too important for a facility of the magnitude of RIA and the more expensive, more technically challenging, but also more efficient approach of accelerating the singly-charged ions from ion source potential is the logical choice. This choice implied that new accelerating structure had to be developed that could accelerate in a continuous fashion ions with a mass to charge ratio as high as 240. These low-frequency RFQ structures together with highly efficient non-equilibrium charge state stripping [3] in gases and existing low-beta superconducting cavities provide the most efficient postaccelerator design while providing excellent beam quality and energy variability from ion source energy to about 10 MeV/u. As seen above, RIA is a complex project that utilizes a number of new technologies and will push other existing technologies far beyond where they have been used so far. The following will succinctly present the facility and highlight important R&D developments in the three main systems presented above that make RIA possible.
3. Driver linac The driver linac produces the wide variety of high-power stable beams required for the different production mechanisms. The driver linac design has a total accelerating voltage of 1.4 GV and is capable of producing beams of essentially all stable species from 900 MeV protons and 2.3 GeV 3 He up to 400 MeV/u uranium. The wide range of energies possible comes from the large velocity acceptance of the short independently-phased superconducting cavities. The required intensity for the very heavy ions, in excess of 100 kW for uranium, can only be attained with existing ion source technology with a very efficient acceleration scheme. The design of the driver linac (Fig. 2) is therefore based on the simultaneous acceleration of multiple charge states of these heavy beams and an important simulation and R&D effort has been put forth to demonstrate this capability. In the
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Fig. 2. RIA driver linac details with the multiple charge states carried and energy in the three main sections of the accelerator for a uranium beam.
baseline design for the driver linac it was decided that to minimize the R&D effort required the 805 MHz elliptical cavities [4] currently being developed for the SNS project would be used for the highest-b section of the RIA driver linac. That constraint, together with the desire to have the highest acceptance for the lower velocity section of the linac and minimizing the cavity count (and hence the cost), determined the frequency choice for the initial section of the baseline design to be 1/14 of that frequency or 57.5 MHz. The frequency in the following sections then goes with the beta of the cavities. The intricacies of the acceleration scheme can best be explained by following a uranium beam through the machine since this one beam requires all the recent developments to reach the required beam power. The heavy ions are produced in one of two ECR sources. The beam is bunched in a 28.7 MHz buncher system which feeds a 57.5 MHz RFQ section. To increase the current available in the case of uranium, two charge states, 28+ and 29+, are carried [5] by the low-energy beam transport system and bunched in alternating RF buckets in the 57.5 MHz RFQ [6] so that one bucket accelerates 28+ and the next one 29+ and so on. This approach fills the RFQ with about twice as much beam as would be available from the maximum charge state alone and has been modeled in details [5]. Following the RFQ, both charge states are injected into the low-b section of the superconducting linac composed of a total of 85 cavities of four different types: three 57.5 MHz fork cavities
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with b of 0.024, 0.036 and 0.061, and one 115 MHz b ¼ 0:15 quarter-wave resonator. The fork type cavities are similar to those used in the PII injector [7] at the ATLAS superconducting linac at Argonne and are being prototyped together with a new cryostat design. The quarter-wave cavities are the result of a new development and incorporate beam steering compensation [8] to correct for the magnetic steering experienced when ions traverse the structure at non-zero RF phase. This is accomplished by tilting the faces of the center post of the cavity so that the perpendicular electric field component cancels out the magnetic force. A uranium beam leaves this section of the linac at an energy of 9.3 MeV/u. The beam is then stripped and after a short matching section 5 charge states [9] centered around 71+ are transferred to the medium-b section of the linac. This section contains two types of half-wave cavities, 172.5 MHz b ¼ 0:25 cavities and 345 MHz b ¼ 0:39 double-spoke type cavities, for a total cavity count of 195. Both cavity types are new developments being prototyped at ANL and the spoke cavities cover a velocity range where no superconducting cavities existed previously. Prototyping of the spoke cavities has already produced cavities which after chemical polishing and high-pressure water rinse [10] have far exceeded the RIA requirements in tests at both ANL and LANL [11]. A fast tuner for these 350 MHz cavities is being developed at LLNL. The simultaneous acceleration of multiple charge states in this section is a key component of the RIA design. Tests were carried out at Argonne to accelerate multiple charge states [12] through the booster section of the ATLAS linac and a 94% transmission was observed, in agreement with calculations, and over a mass-to-charge range exceeding that required for RIA. This warrants this component of the design that not only yields much larger beam intensities but just as importantly, also reduces the power requirements on the charge-state strippers for a given output current. The uranium beam leaves the medium b-section at an energy of 93 MeV/u and is further stripped to a mean charge state of about 89+ and four charge states are carried through to the high-b section. This section contains a total of 136 cavities, com-
posed of three types of 805 MHz 6-cell elliptical cavities with b of 0.49, 0.61 and 0.81 operating at 2.0 K. The two highest b cavities are similar to spallation neutron source (SNS) cavities developed for the same velocity range by Jefferson Laboratory. This choice was made to have maximum overlap with existing R&D efforts on-going in the US and result in not only cost savings, but just as importantly, time and manpower savings. The main difference with the SNS application of these cavities comes from the fact that RIA will be a continuous machine, not a pulsed one, and therefore the RF power feeding and control must be different. Both are being developed at Jefferson Laboratory for RIA. The cavity themselves are performing at a level of about 35 MV/m (newly adopted by the SNS as its target surface gradient) well above the initial RIA design surface gradient. In RIA these cavities will operate at a slightly lower level to allow locking at the much higher effective Q that will result from the CW operation. The lowest-b cavities in this section are a new development pushing the elliptical cavities to lower b than has been done before. Two single cell b ¼ 0:47 cavities have been tested at Jefferson Laboratory and MSU and reached very high Q. Multi-cell cavities are now being developed and microphonics (to which the CW machine is more sensitive) are being studied. The final component of the linac is a switchyard with RF switching to distribute the beam, bucket per bucket, among target stations. An ion optical design capable of splitting the multiple charge states of uranium among two targets while maintaining the mm beam size on both targets has been worked out. While the technical feasibility of the components of the driver linac is now well documented, R&D is still on-going to ensure that the planned construction schedule can be maintained and that potential savings identified in the early R&D efforts can be taken advantage off. For example, there are 276 low- and medium-b drift-tube resonators in 33 cryomodules. Procurement, assembly, installation, testing, and commissioning over a 4–5 year construction schedule requires complete development and prototyping prior to final engineering design. Similarly, being able to use a
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higher charge state out of the ECR ion source (for example from new sources such as the superconducting ECR source VENUS [13] being developed at LBL) or obtaining higher accelerating gradient as the R&D seems to indicate would yield a shorter and hence less expensive linac. These venues are being explored and the base parameters of the linac are being reevaluated periodically to see if the R&D indicates that such changes are warranted.
4. Production complex The production of the radioisotopes occurs by interaction of the primary beams on high-power targets. To obtain maximum yield over the widest range of unstable isotopes it is necessary to use different production mechanisms [2] and the versatile driver available at RIA enables four main mechanisms which are described below. The standard ISOL technique [14] where a highenergy light-ion beam impinges on a thick target producing short-lived isotopes via spallation or induced-fission reactions. The target is heated to release the activity quickly. The activity then diffuses to an ion source where it is ionized and extracted and made available for experiments at low energy or at higher energy after post acceleration. At RIA, the light ion beam will have power up to 400 kW and the main difficulty will be to handle the power deposited in the production target. R&D at ORNL and ANL is therefore concentrating on techniques to remove the heat from the target and on refractory compounds with enhanced thermal conductivity and release properties. Dependence of the extraction delay times on the geometry is also being studied with realistic full Monte-Carlo simulations [15]. A second approach is that of production of radioactive species by fragmentation [16] of a fast heavy-ion beam on a thin target capable of handling the high beam power. The reaction products are pushed forward by the kinematics and can be separated from the beam in a fragment separator. These radioactive beams are then available for experiments at high-energy but with poorer beam quality. The standard fragmentation target used at
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RIA will be a flowing windowless liquid lithium film for all heavy beams. A prototype of such a target is being developed at ANL [17] and a lower power version combining liquid lithium and beryllium is being developed by an ANL/NSCL collaboration and will be tested in on-line conditions at the NSCL. The isotopes produced in the fragmentation and in-flight fission processes must also be collected with maximum efficiency which requires the highest acceptance fragment separator. A design for an 18% momentum acceptance separator is being investigated and R&D on radiation resistant large aperture quadrupoles for the initial section of the separator is on-going at MSU. These two conventional approaches are implemented with unprecedented power at RIA and are supplemented by two new complimentary approaches that remove the main limitations of these standard approaches. The first new approach is the two-step neutron generator technique [17,18] where the high-energy light ion beam is converted to fast neutrons in a cooled converter which is surrounded by a production target where fissions induced by the fast neutrons produce the radioactive species which are then extracted in a fashion similar to that used in the standard ISOL technique. This yields significant gains since the power deposited by the light ion beam (mostly via electromagnetic interactions) is in a volume which can be cooled efficiently, while only the neutron and the fission power is deposited in the releasing target where power removal is much more difficult to perform without affecting the other functions of the device. Yields have been calculated in details [2] and the geometry optimized to obtain maximum yield for the most neutron-rich isotopes. Initial tests have been carried out at PNPI to compare direct and indirect production of neutron-rich isotopes and give excellent agreement with simulations [19]. The two-step generator has actually other benefits such as minimizing the isobaric contamination produced by spallation and as such is now being used routinely at ISOLDE to improve the purity of the most neutronrich beams. A second new approach brings together the advantages of the in-flight fragmentation approach
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with that of the ISOL techniques [20]. Radioactive isotopes are produced by fragmentation (or inflight fission) of fast heavy-ion beams on a ‘‘thick’’ thin target, the reaction products are pushed again forward by the kinematics and after selection by a large acceptance fragment separator are monochromatized, slowed down and stopped in a gas catcher system [21] where they are thermalized but remain singly charged and can be extracted by a combination of DC and RF fields to be further reaccelerated. This results in beams of quality similar to those obtained by ISOL techniques, available at any energy, without the chemical limitations encountered by the ISOL technique in the diffusion and release out of thick targets. A scaled down version of the gas catcher system has been investigated thoroughly on-line and off-line at ANL and both high efficiency and short extraction delay times have been obtained. Investigations of the expected most important limitations of such a system have also taken place and conditions for reliable on-line operation obtained. The next step is the construction of a full-scale prototype (see Fig. 3) RIA gas catcher which is on-going at ANL [21] where it is being fully characterized before being moved to GSI for tests at the full RIA energy behind the FRS separator. Preliminary experiments at GSI have been performed that demonstrated the achromatization of the reaction
Fig. 3. Picture of the full-scale RIA gas catcher prototype currently being assembled at ANL.
products required to minimize the range straggling [22] in the stopping gas catcher.
5. Post accelerator The most efficient techniques to produce the radioactive ions create them as singly-charged ions. The RIB accelerator system must therefore efficiently accept and accelerate singly-charged ions over the full mass range, up to m=q of 240, to an output beam energy up to 5–10 MeV/nucleon. This must be done while maintaining a small longitudinal emittance (typically 0.5 p keV/u ns) over the full range of energy and mass. The short independently-phased cavities developed for existing superconducting heavy-ion linacs provide a basis for all but a small portion of the RIB post-accelerator system. Only the initial section of the post-accelerator, a very low charge state injector section providing bunching and the first few MV of acceleration, can be made most efficiently by using a different technology namely sections of cw normal-conducting RFQs. The RIB linac therefore consists of the following main sections: • An injector with three sections of normal-conducting RFQs capable of accepting ions with q=m > 1=240 and 2 gas stripper locations for the heaviest ions. • A first section of superconducting linac that will accelerate ions of q=m > 1=66 to 680 keV/u or more. • A carbon-foil stripper to provide, when necessary, a q=m > 2=15 for the last stage of acceleration. The beam energy at this point depends on the particular charge-to-mass ratio. • A superconducting linac to accelerate ions of q=m > 2=15 to Coulomb barrier energy or higher. Following the radioactive ions from the ion source through the post-accelerator we first pass through a high-resolution (Dm=m ¼ 1=20 000) isobar separator before reaching the low-frequency RFQ injectors. The buncher, the first two sections of 12 MHz RFQ, and two He gas-stripper cells are
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placed on a 380 kV variable-voltage platform. Placing these elements on a variable-voltage platform allows operation with a fixed velocity profile for the full mass range of ions. This first section accelerates all ions up to 7 keV/u and the beams of 66 < mass 132 will be charge-stripped at this point. For best efficiency over the full mass range, nonequilibrium helium gas stripping [3] must be performed at different energies in the RFQ section for different mass ions. By stripping at 7 keV/u for example, an incident 132 Sn beam can be stripped into charge state 2+ with 55% efficiency and further accelerated. But for the heavier ions with Z > 54 higher charge states are required for which the best stripping efficiency is achieved at the higher energy of 20 keV/u. Whether stripped or not, ions of any mass and charge state, including mass 240 at charge state 1+, will be further accelerated by the next section of the 12 MHz RFQ to an energy of 20.3 keV/u. At this point the beams of mass > 132 will be stripped. The final RFQ operating at 24.25 MHz will accelerate the ions, now at a charge state q=m > 1=66, to an energy of 62.6 keV/u for injection into the superconducting linac. Although this first section of the post-accelerator is quite different and significantly more powerful than any proposed this far, it is based on existing technology or advanced R&D efforts. The bunching system is similar to the 12 MHz fourharmonic bunching system presently in use on the ATLAS accelerator. The first RFQ is operated at the lowest frequency feasible to maximize the transverse focusing strength. It has been demonstrated at ANL that a split-coaxial RFQ geometry is appropriate for cw operation at 12 MHz [23]. The RFQ is designed for a minimum charge-tomass ratio of 1/240 with ions of higher charge state simply accommodated by scaling both the platform voltage and the RFQ RF voltage to match. The proposed CW inter-vane voltage of 92 kV with a mean bore radius of 9 mm has been proven entirely practical in extensive tests with the existing prototype at ANL. The last two sections of the RFQ are based on a novel more effective accelerating structure, a hybrid RFQ [24]. It has been found that the concept of separated accelerating and focusing zones can be applied to the acceler-
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ation of heavy ions with q=m P 1=240 and at very low energies if the beam focusing is provided by RF quadrupoles. The DTL accelerating and RF focusing sections are integrated into a single resonant structure called the hybrid RFQ. The 12 MHz H-RFQ will be based on a cold 1:2 scale prototype that was completed at ANL where the structure is being studied for electrodynamic properties. Numerical simulations of the beam dynamics through the entire chain of RFQ sections, including RF bunchers between sections and transverse focusing, have been performed. The proposed design achieves longitudinal emittance as low as 0.2 p keV/u ns for 80% of the DC beam entering the buncher. The RIB beams then enter the superconducting section of the RIB linac. The lowest velocity section is a low-charge-state injector linac based on established interdigital drift-tube SC niobium cavity designs. Cavities in this velocity range provide typically 1 MV of accelerating potential per cavity [7]. This section is similar to the PII injector at ATLAS except for the lower charge state of the ions which then requires more cavities and stronger transverse focusing. For the charge states considered here (q=m ¼ 1=66) the proper focusing can be reached with the help of strong SC solenoid lenses with fields up to 15 T that are available from commercial vendors [25]. Additional space around these solenoids has been included in the design to allow for shielding of the cavities from these strong fields. The section of the superconducting linac consists of 54 interdigital cavities operating at )20 synchronous phase with each cavity followed by a SC solenoid. This linac can accelerate any beam with q=m P 1=66 over the velocity range 0:0011 6 b 6 0:04. Masses m 6 66 will not require any stripping and be provided with maximum intensity to the astrophysics area where most experiments will require the maximum beam intensity available. For the heavier ions, the high helium-stripping efficiency in the range of 30–45% for masses 240 P m P 66 will also provide unmatched accelerating efficiency to this energy range. In both cases, it will allow beam intensities higher by about an order of magnitude as
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compared with RIB accelerators based on the ECR charge breeder. After this second stripper, the desired charge state must be selected and further accelerated by eight SRF cavities of b ¼ 0:037 to bring the beam energy to approximately 1.4 MeV/u and match the velocity acceptance of the b ¼ 0:06 resonators in the final linac section. This linac section consists of 12 b ¼ 0:06 resonators, 30 b ¼ 0:105 resonators and eight quarter-wave resonators recently designed for b ¼ 0:15. This post-stripper section of the RIB linac is designed for the acceleration of multiple charge state beams to enhance the available beam intensities for experiments. As was shown in [12] a wide range of the charge spread Dq=q, about 20%, can be accepted and accelerated in an ATLAS-like accelerator. For the RIA postaccelerator, we have restricted the possible range of Dq=q to 6 11% in order to avoid emittance halo in phase-space. As a consequence of multiple charge state acceleration the total stripping efficiency is significantly higher than for the single charge-state beams. The transverse and longitudinal emittances of multi-q beams will however be larger by a factor of 3 as determined by beam measurements in ATLAS [12] and the options will be opened to the experimenters to choose either the highest beam intensity or the highest beam quality in which case the last section only accelerates one charge state. The post-accelerator efficiencies for different masses and energies are depicted in Fig. 4.
6. Conclusion The RIA facility is based on what seem at present the most efficient technologies for acceleration of the primary beam, production and extraction of the unstable isotopes and postacceleration of those isotopes. An important R&D effort has demonstrated the feasibility of the approach selected and further efforts aimed at optimizing the design and the implementation of the selected technologies is ongoing. The overall design of the facility has been extremely stable over the last few years but various subsystems are being reexamined at regular interval to incorporate the latest R&D findings. In particular, potential cost savings have been identified as a result of the R&D and are being further investigated. The high level of technical preparation for the RIA project that will be achieved with the current R&D efforts should lead to a rapid construction schedule which is key to staying within the cost estimates for the facility.
Acknowledgements The RIA concept has evolved from ideas generated within the US nuclear physics community and contributions from abroad and from other fields. The development work on the project has been performed by a national program for RIA R&D with current participation from ANL, JLAB, LANL, LBNL, LLNL, NSCL and ORNL. This work was supported by the US Department of Energy, Division of Nuclear Physics, under contract number W-31-109-ENG-38.
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Fig. 4. Stripping efficiency for secondary beams of various mass as a function of the final energy. The first numbers are for a single charge state acceleration while the numbers in parenthesis are for acceleration of multiple charge state after the stripper following the lowest-b superconducting section.
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