Overview of the Rare isotope Accelerator complex for ON-line experiments (RAON) project

A new rare-isotope beam (RIB) accelerator complex, RAON, is being constructed in the Institute for Basic Science (IBS) in Korea. RAON is equipped with both isotope separation online (ISOL) and in-flight fragmentation (IF) systems and ultimately combines them to provide more neutron-rich (than any single mode operation) ion beams to the users. However, due to the delay in developing high-energy superconducting cavities and cryomodules, it was decided to proceed with the RAON construction project in two phases. In the first phase, the low-energy accelerator system, ISOL, and all experimental setups will be completed by the end of 2022. In the second phase, the high-energy superconducting system will be completed by 2029 or later. In this contribution, the status of the accelerators, RIB production systems, and experimental instruments for RAON is summarized.


Introduction
The origin of elements, the limit of existence for nuclei in all directions in the nuclear landscape, the detailed structure of exotic nuclei, and the way how the neutron stars can be stabilized in the Universe are some of the critical questions in nuclear physics.These questions have not only fascinated people in general as they sound, but also played the key role in understanding the nature of interactions and the evolution of the Universe.Several RIB accelerators have been built to answer these questions, and indeed provided a lot of essential data that help to descibe the nature in many aspects [1,2].
However, technical challenges have been prevalent in producing isospin asymmetric ion beams for the experiments and more beamtime has been desired to test various novel ideas.
To contribute to such a worldwide endeavor and push forward for the advancement of RIB accelerator techniques, IBS in Korea launched the Rare Isotope Science Project (RISP) in 2011 right after it's establishment.
The goal of RISP is to build a novel RIB accelerator complex, called RAON, in Korea [3,4], which comprises a high-energy (200 MeV/u) and high-power (400 kW) heavy-ion accelerator.RAON plans to provide high-quality rare-isotope beams via both ISOL and IF for isotope science and applications.ISOL uses direct fission processes induced by intense proton beams, and IF uses projectile fragments or in-flight fission products of high-intensity heavy beams like uranium.In the original design the independent operation of ISOL and IF were proposed; however, due to the shortage of budget, the first superconducting Linac (SCL1) was canceled and the implementation of the simultaneous operation of the two RIB production systems is a future mission.Recenly, RISP was determined to separate into the two phases because of the delay in the development of the high-energy superconducting Linac (SCL2) system.SCL2 uses for the first time the SSR-type cavities, which was developed by the Fermi National Accelerator Laboratory (FNAL) for proton beams, in accelerating ion beams.As it requires significant more time than originally expected, the R&D and construction of the SCL2 cavities and cryomodules will be completed by 2029 or later in the second phase.As a result, the first phase, scheduled by the end of 2022, includes the installation and beam commissioning of the injector system, lowenergy superconducting Linac system SCL3, and ISOL.The installation and commissioning of all experimental instruments and IF separator will also be completed within the period of the first phase.
Figure 1 shows the schematic of the overall RAON facility.It also shows the diagram for RIB production and acceleration scheme and the beam extraction positions for various experimental setups.The recent drone picture for the RAON site is given in Fig. 2. The possible beam acceleration methods at RAON are summarized as follows: • KoBRA: In this method, the stable primary ion beams from SCL3 impinge the production target at the first focal point of KoBRA.By direct or multi-nucleon transfer reactions, the secondary light ions are produced up to a few tens of MeV/u.Because this method uses stable ions as primary beams, RISP will attempt it in the beginning of RAON operation.• ISOL: This method requires the completion of the ISOL system.RIBs are produced in the target ion source (TIS) module by high-intensity proton beams from a cyclotron.In this method SCL3 is a post-accelerator, increasing the energy of RIBs to a few tens of MeV/u.The details about SCL3 and ISOL are given in Secs. 2 and 3, respectively.• IF: This method requires the completion of the SCL2 system.This method provides a wide spectrum of beam species based on different combinations of beam production systems and accelerator systems.One possible mode is to use the stable ion beams from the ECR ion source.These beams are accelerated by SCL3 and, continuously, SCL2 and bombard the IF target.The projectile fragmentations or in-flight fissions of heavy beams like uranium generate various RIB species to the experiments at the highest beam energies, for example, up to approximately 250 MeV/u for 132 Sn.Ultimately, RAON will attempt to use RIBs from the ISOL system to produce more neutron-rich ion beams by bombarding them to the IF target.The simulations have shown that the combination of ISOL and IF can produce new RIB species near the neutron dripline or enhance the yields of certain rare isotopes by large factors [5].

Status of accelerator system
Figure 3 shows the injector system of RAON, which consists of the two ECR ion sources, LEBT, RFQ, and MEBT.One of ECRs is the 14.5-GHz source using a permanent magnet for the production of relatively light ions, and another one is the 28-GHz source using a superconducting magnet for the production of heavy ions.RFQ accelerates the beams at 10 keV/u from LEBT to 500 keV/u with the frequency of 81.25 MHz, matching with the characteristics of QWR in SCL3.The beam commissioning of the injector system started in October 2020. 40Ar 8+ and 40 Ar 9+ beams with the pulse length of 100 µs and the repetition rate of 1 Hz were tested.The peak currents were larger than 30 eµA at LEBT for both beams.Using the beam current monitoring detectors at LEBT and MEBT, the transmission rate of RFQ was measured to be larger than 92%, which agrees with the simulation results.
The superconducting cavities and cryomodules should be tested in the onsite Superconducting Radio Frequency (SRF) test facility for their performances before assembly.The SCL3 modules that satisfied the criteria were assembled with the warm sections in the clean booth which was set up in the tunnel.In Fig. 4, the top left picture shows the assembly of the cryomodules and warm sections for SCL3, and the top right one shows the completed SCL3 system in 2021.
To operate the superconducting Linacs, the cryoplant system is needed.RAON has two cryoplants and the cryogenic distribution system for liquid helium (LHe).The smaller cryoplant for SCL3 has the cooling capacity of 4.2 kW as the equivalent heat load at 4.  The left panel of Fig. 5 shows the schematic of the ISOL system and beamlines for RAON.It consists of the cyclotron, TIS, pre-mass separator, RFQ cooler buncher (RFQ-CB), Electron Beam Ion Source (EBIS) charge breeder, and A/q separator.The installation of the driver cyclotron was performed in April 2022, and SAT is completed shortly after.The cyclotron can accelerate the protons up to 70 MeV with a maximum current of 0.75 mA.Several ISOL targets, such as SiC, BN, MgO, LaC 2 , and UC x ,, have been prepared for various RIB species.Among them SiC will be used in the beginning of the ISOL operation, and UC x will be used after 2025 as it requires extra care.The surface ion source, resonant ionization laser ion source (RILIS), and plasma ion sources are available for TIS.To avoid any radiation hazard, a full remote handling system was built for changing the target in TIS.The current ISOL system is expected to provide RIBs with 6 ≤ A ≤ 250, 10 ≤ K ≤ 80 keV and the purity higher than 90% for, e.g., 132 Sn beams.
To test the ISOL system before the cyclotron supplies proton beams, the 133 Cs + ions were produced from TIS in a hot-cavity Ta heater, as shown in the right-upper panel of Fig. 5.The optics elements of the ISOL beamline are divided into two (from TIS to RFQ-CB and from EBIS to LEBT) because the ion beams are manipulated at RFQ-CB and EBIS.The 133 Cs + beams were extracted from TIS with a current of 4.0 nA and transported down to the A/q separator of the ISOL beamline.From the measurement of the horizontal beam size in the pre-mass separator the mass resolving power was determined to be approximately 1,000 in 2σ.The Cs beams were bunched into 1.66×10 8 /sec at RFQ-CB, and, subsequently, EBIS measured 5.0×10 7 of 133 Cs 27+ ions.The breeding efficiency of all charge states for Cs was ∼82%, and that of a single charge state 133 Cs 27+ was measured to be approximately 19.4%.The right-bottom panel of Fig. 5 shows the measured A/q spectrum for the Cs beams, which shows clearly separated charge states after EBIS.From this attempt, the mass resolving power of the A/q separator was determined to be ∼250 in 2σ, but the mass resolving power can be improved to approximately 400 with more careful tuning of the system.In April 2022, Sn beams were also extracted by RILIS at TIS, and all natural abundances of the Sn isotopes were successfully reproduced.

Status of experimental systems 4.1. KoBRA
KoBRA is a dedicated apparatus for studying nuclear structure and nuclear astrophysics at low energies below 40 MeV/u.The schematic of the KoBRA setup is shown in Fig. 6 with the two images taken at the production target (F0) area and the dispersive focus (F1) area.KoBRA consists of a swinger magnet just before the target chamber, two curved-edge bending magnets, and several multipole magnets (two hexapole and fifteen quadrupole magnets).The installation of the KoBRA system was completed in June 2021 except for the Wien filter, which will be added at the end of 2023.As the ISOL system will not provide RIBs in the early stage of the RAON operation, the stable ion beams will bombard the production target at F0 of the KoBRA setup.Then, the subsequent part of the magnetic spectrometer will select the desired RIBs for the experiments.However, the true strength of KoBRA can be recognized when ISOL provides the exotic nuclei with high For the experiments at KoBRA, several detector systems, such as high-resolution γ system, Si detectors, Active-Target Time-Projection Chamber (AT-TPC) are being prepared.

NDPS
NDPS is a dedicated system for nuclear data production and other industrial applications.NDPS provides both white neutrons and monoenergetic pulsed neutrons, using the deuteron beams at 49 MeV/u and the proton beams at 83 MeV, respectively.A thick graphite target will be used for the former and a thin lithium target will be used for the latter.NDPS consists of four quadrupoles, one dipole magnet, two production target chambers, a proton beam dump for monoenergetic neutrons, and a 4-m-long neutron collimator.Figure 7 shows the schematic of the NDPS system at RAON with the two images of the target room and neutron collimator.The neutron beam dump, which is simply a concrete block with a thickness of 1 m, is located approximately 35 meters away from the exit of the neutron collimator.

LAMPS
LAMPS was originally designed to study the nuclear equation of state (EoS) and symmetry energy at supra-saturation baryon densities, nonetheless, it can also be useful for studying the nuclear structure.LAMPS at the high-energy experimental hall accepts intense high-energy RIBs, for example, 132 Sn beams at 250 MeV/u with an intensity up to 10 8 pps.The top panel of Fig. 8 shows the schematic of the LAMPS system [6], which consists of a beam diagnostic vacuum chamber, an azimuthally symmetric tracking system surrounded by a superconducting solenoid magnet with a maximum field strength of 1 T, and the forward neutron detector array.In the beam diagnostic vacuum chamber, the starting counters coupled with veto paddles and beam drift chambers are placed.For the charged particle tracking system, the TPC segmented into eight octant sectors in the transversal plane and the barrel and forward time-of-flight array (called BToF and FToF, respectively) are used.Most of the detector components and magnet were already developed, manufactured, and assembled.The integration and machine commissioning of the whole LAMPS system, including the LAMPS trigger and data acquisition system, is forseen by the end of 2023.

Summary
Since 2011 when RISP was launched, there have been a lot of important achievements for RAON, including the completion of the SCL3 superconducting Linac, installation, and operation of the cryoplants, installation of the ISOL system with successful transportation of the stable ion beams from the radiation sources, and machine commissioning of the KoBRA spectrometer.However, there will be much more to be done for the next years, including the delivery of the stable and radioactive ion beams to KoBRA, MMS, and CLS.In addition, the superconducting cavities and cryomodules for SCL2 should be completed as soon as possible.
After the completion of SCL2, RAON will aim at the stable operation of uranium beams at 200 MeV/u up to 80 kW, and gradually increasing the beam power to the design goal of 400 kW.Ultimately, RAON plans to combine the ISOL and IF systems to generate more neutron-rich radioactive ion beams for experiments.

Figure 1 .
Figure 1.(Top) Schematic of the RAON facility.The components written in blue are for accelerators or RIB production systems, while those in red are for experimental setups.(Middle) Schematic of RIB production system, the accelerating components and the beam extraction positions for seven experimental setups at RAON. (Bottom) The full names of the various acronyms used in this article.

Figure 2 .
Figure 2. Recent bird-eye-view image of the RAON site.

Figure 3 .
Figure 3. Schematic (left) and image (right) of the injector system.

Figure 4 .
Figure 4. Following clockwise from the top-left picture, the assembly of the cryomodules and warm sections for SCL3, the completed SCL3 system, and the cryoplant system (the liquid-He distribution box, the warm compressors, and cold box) are displayed.
5 K. Its installation was completed in 2021, and the Site Acceptance Test (SAT) was completed at the end of July 2022.Cooling down of the SCL3 cryoplant and RF conditioning of the coupler and cavity per module started in September 2022.The beam commissioning with the 40 Ar 9+ beam is in progress.The bigger cryoplant has the cooling capacity of 13.5 kW as the equivalent heat load at 4.5 K for SCL2 and IF separator.

3 .Figure 5 .
Figure 5. (Left) Schematic of the ISOL system and beamlines.On the right, Cs ion source installed in TIS (top) and the measured A/q spectrum for the Cs beams (bottom) are shown.

Figure 6 .
Figure 6.Schematic of the KoBRA system.The stable or rare isotope beams from SCL3 are entering from the right.The two images show the F0 (right) and F1 areas (left).

Figure 7 .
Figure 7. Schematic of the NDPS system.The proton or deutron beams from SCL3 are injected from the right.The two images show the target room (right) and the 4-m-long neutron collimator (left).

28thFigure 8 .
Figure 8. (Top) Schematic of the LAMPS system.The ion beams from IF enter from the left.(Bottom) Images of the superconducting solenoid magnet, TPC, BToF/FToF, and the forward neutron detector array.