RAON, Korean Heavy Ion Accelerator Facility

「加速器」Vol. 17, No. 4, 2020（293–301）

特集大強度不安定核ビーム世界で動き出す重イオン加速器 RAON, Korean Heavy Ion Accelerator Facility

M. KWON*, Y. S. CHUNG*, Y. K. KWON*, T. S. SHIN* and Y. U. SOHN*

Abstract RAON, being constructed as the Rare Isotope Science Project (RISP) by the Institute for Basic Science (IBS) since 2011 is a ﬂag- ship heavy ion accelerator facility in Korea to promote fundamental science and application of isotope nuclei and related science.1–3) Civil construction of the RAON site in Shindong, Daejeon of Korea, is going to ﬁnish in 2021 and installation of the heavy ion accelerator systems including injector, rare isotope (RI) production systems, and experimental systems are currently being progressed toward to commissioning of RAON. The overview RAON accelerator facility and status of RISP are reported in this paper.

low energy beam transport (LEBT), a radio frequency Introduction quadrupole (RFQ), and a medium energy beam transport The RAON heavy ion accelerator facility is to acceler- (MEBT). ate both stable and rare isotope beams up to the power SCL3 uses two different families of superconducting of 400 kW with an energy higher than 200 MeV/u and to resonators, i.e., a quarter wave resonator (QWR) and a half produce rare isotopes. It is worthwhile to mention that rare wave resonator (HWR). The first half of SCL3, SCL31 isotope (RI) production system of RAON is planned to consists of 22 QWR’s whose geometrical β is 0.047 with have both Isotope Separation On-Line (ISOL) and In-Flight 81.25 MHz of resonance frequency. Each QWR cryo- (IF) fragmentation method combined to produce rare iso- module bears one superconducting cavity. Meanwhile, the tope beams far from the valley of stability. The construc- second half of SCL3, SCL32 consists of 102 HWR’s with tion project for RAON facility, which will be located at 0.12 of β and 162.5 MHz of frequency. All 102 HWRs of Shindong area near the city of Daejeon, is progressed with the same cavities are installed in two different cryo-modules a great expectation on the commissioning of low energy depending on the number of cavities in a cryo-module, facilities including injector, Superconducting Linac 3 and where the 13 Type-A cryo-modules bear two cavities each KoBRA. as the 19 Type-B cryo-modules with four cavities each. The Superconducting Linac 2 (SCL2) accepts a beam Overview of RAON Accelerator at 18.5 MeV/u and accelerates it to 200 MeV/u. The SCL2 The Superconducting Linac 3 (SCL3) in Fig. 2 is the uses two types of single spoke resonators, as SSR1 and post-accelerator of ISOL facility, which is to produce high- SSR2. The SCL2 consists of the SCL21 and the SCL22, intensity and high-quality beams of neutron-rich isotopes each with geometric β of 0.30 (SSR1) and 0.51 (SSR2), with masses in the range of 80–160 by means of a 70 MeV respectively, with resonance-frequency of 325 MHz. The proton beam directly impinging on uranium-carbide thin- single-spoke-resonator type is chosen mainly because it disc targets. It accelerates stable or rare isotopes drawn can have a larger bore radius compared with the half-wave- from ISOL up to about 20 MeV/nucleon for low-energy ex- resonator type, which is very important for reducing the periments at Korea Broad acceptance Recoil spectrometer uncontrolled beam loss in the high-energy linac section. and Apparatus (KoBRA). Another consideration for SSR is the robust design of The injector system comprises 14.5 GHz and 28 GHz mechanical structure and less severe multipaction. They electron cyclotron resonance ion sources (ECR-IS), a would provide the advantages during beam operation. The

* Institute for Basic Science, Daejeon, Korea

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Figure 1: The site photograph of RAON accelerator complex.

states, for example, of uranium beams (238U33+ and 238U34+ of 12 pµA) for high-intense ion beams, simultaneously. The MEBT is designed to transport and match ion beams from the RFQ to the superconducting linac, which consists Figure 2: The conﬁguration of RAON accelerator complex is of four 81.25 MHz re-buncher cavities and eleven quadru- shown. ISOL which is not a part of injector system is poles. connected to LEBT for acceleration of rare isotopes through SCL3. The integrated beam test of injector system is being carried out using argon and oxygen beam from 14.5 GHz numbers of cavities of the SSR1 and the SSR2 is 69 and ECR to MEBT and will be an operating mode in an early 150, and the cryo-modules for these cavities bear 3 and 6 part of 2021. In the meantime, the SC 28 GHz ECR-IS for cavities, respectively. The SCL2 provides a beam into the the Uranium ion beam will go through a series of perfor- in-ﬂight fragmentation (IF) system via a high-energy beam mance tests for the preparation of extraction of Uranium transport (HEBT). ion beams in 2023.

Injector System SRF Cavity and Cryo-module

The RAON injector system produces ions and acceler- SRF Cavities ated ion beams to the desired beam structure for SCL3. The In the optimization of the superconducting cavity, the layout of injector system is shown in Fig. 3. resonant frequency, modes, and mechanical constraints in The 14.5 GHz, ECR-IS is a compact permanent magnet the cavity should be considered to design a high-efficiency, ion source manufactured by the Pantechnik™, France. The superconducting cavity. Figure of merits related with the superconducting 28 GHz, ECR-IS, which is dedicated to cavity performance should be optimized. For particle extract Uranium ion beam, is in the process of performance beams passing through the cavity to get maximum energy test after reinstallation from the test site. The LEBT is de- gain, the acceleration gradient should be maximized while signed to transport and to match ion beams from the ECR minimizing the peak electric and magnetic fields. Table 1 ion source to the RFQ, The RFQ is designed to accelerate summarizes the parameters of four different superconduct- beams from proton to uranium from 10 to 500 keV/u. One ing cavities required for the RISP superconducting linac. feature is that this RFQ can accelerate two different charge To confirm the stable beam operation with SRF cavities,

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Table 2: Calculated thermal load for each SRF linac converted with 4.5 K. Period SCL1 SCL2 SCL3 Total (W) Dynamic (W) 1,274 6,023 1,274 8,571 Static (W) 650 1,265 650 2,565 Sum (W) 1,924 7,289 1,924 11,136 SCL1,11) which is not described in this paper, is postponed after RISP.

one of the critical issues is to avoid the multipacting effect inside the cavity. It can make surface heating, then cause to cavity quench directly soaring vacuum pressure to end up with a cavity fault. The multipaction is a phenomenon in which secondary electron emission occurs exponentially by electrons repeatedly hitting surface due to electron existing on the surface of the metal of the radio-frequency devices and resonating with the RF electromagnetic field without scattered out. Hence, all the cavities are optimized geo- metrically to reduce the multipaction as much as possible. Cryo-module The two linacs have five types of cryo-modules for four different kinds of cavities. The main roles of the cryo- modules are maintaining the operating condition of the superconducting cavities and aligning the cavities along the beam line. The design of cryo-modules is based on the standard design configuration for 2 K and 4 K supercon- ductivities with insulation vacuum, thermal shield, and magnetic shield. The 2 K modules for HWR, SSR1 and SSR2 bear a 4 K subcooled helium reservoir supplied by an Figure 3: On the top, the layout of injector system is shown with external valve boxes, a heat exchanger and Joule–Thomson the photograph of RFQ, which pre-commissioned in expansion valve in each cryo-module, leading to the case 2017.4) On the bottom, photograph shows that ECR-IS and all other injector components are installed and being that the 2 K environment for cavities is made in the module commissioned. itself. While a QWR cryo-module is operated at 4.5 K with the simpler design, but no prominent advantage compared Table 1: Design parameters of RAON SRF cavities. to the 2 K operation. The thermal shield of QWR, HWR, Parameter Unit QWR HWR SSR1 SSR2 SSR1 and SSR2 cryo-module are cooled with 40 K helium Frequency MHz 81.25 162.5 325 325 gas, and 4.5 K thermal intercepts are added to the HWR, β 0.047 0.12 0.30 0.51 g SSR1 and SSR2 cryo-module to minimize external heat Leff=βg λ m 0.173 0.221 0.277 0.452 Q 109 0.24 2.6 3.2 3.2 load input. The cold mass, including the cavity string, cou-

QRs Ω 21 42 98 112 pler and tuner, is installed on a strong back. The estimated R/Q Ω 468 310 246 296 thermal loads of the superconducting linacs are summa- E MV/m 6.1 6.5 8.5 8.7 acc rized in Table 2. The dynamic and the static loads are tak- Epeak/Eacc 5.6 5.0 4.4 3.9

Bpeak/Eacc mT/(MV/m) 9.3 8.2 6.3 7.2 ing 70% and 30% of the total budget of the thermal load, receptively. The independent two cryo-plants serve the linacs op-

J. Particle Accelerator Society of Japan, Vol. 17, No. 4, 2020 295 M. KWON, et al. eration. The cryo-plant with 4.2 kW @4.5 K cool capacity more complicated than that of TM mode i.e., an elliptical for SCL3 cools 22 QWR and 33 HWR cryo-modules. The cavity. Nevertheless, the basic recipe of surface treatment other plant with 13.5 kW for SCL2 does 1 HWR, 23 SSR1 would be similar to that of elliptical cavity. However, the and 25 SSR2 modules and the superconducting magnets in detail parameters and procedures are adapted to RAON’s the IF zone. Estimated from Table 2, the capacity margins own recipe, shown in Fig. 4. For the mass production, the are reserved sufficiently by considering the uncertainty. industry constructed a dedicated facility for the necessary SRF Test Facility treatment, but cavities failed in VTs with simple causes are Total 335 SRF cavities with 104 cryo-modules in RAON re-treated by using the on-site facility. accelerate heavy ion and proton beams to the target for var- QWR Cryo-module ious experiments. The final performance tests for all cavi- All the fabrication process of the QWR cryo-module and ties and cryo-modules from the prototyping such as cavity procedure of surface preparation were setup basically dur- vertical test (VT), module horizontal test (HT) and coupler ing the prototyping. At the beginning much time was spent conditioning and test are implemented by IBS staffs at the to find the sources of performance failures or degradation SRF test facility. Two test facilities, independently operat- and to clear out the causes. The major causes of VT fail- ing, one is on site, the Shindong campus and the other is ure were the multipacting (MP) in the power coupler, the in the Munji campus which is 20 km apart from RAON premature field emission (FE), the vacuum leakage, non- site. In both facilities, VT pits, HT bunkers, ISO class 4~7 optimization of VT RF circuit and cryogenic system and cleanrooms, buffered chemical polishing (BCP) system, also human errors due to less-experience. It took about one high-pressure rinsing (HPR) system, high- and low-temper- years to find proper remedies of MP and FE. The coupler ature furnaces and a helium liquefier are furnished. Three MP was mitigated by the change of cleaning & assembly VT pits and 3 HT bunkers are in the Shindong campus and method, as adopting a solution that the RF flowing surface two VT pits and two HT bunkers in Munji. Throughput of was decontaminated with HPR. To erase the premature FE two SRF test facilities is enough to cover all the cryo-mod- by the particle contamination the quality and procedure ules in the given milestone. At completion of accelerators, control of BCP, HPR and cavity string assembly was kept the Munji facility will be moved and merged to the Shin- to the extremely severe level. Also the assembly procedure dong facility. for vertical test was improved to prevent a particle intake. Preparation of Cavity Table 3: VT acceptance rate of QWR. Acceptance criteria is The cavities chosen for RAON accelerate ion beams Q0>2.4E8 @6.1 MV/m. using by TEM modes, of which mechanical structure is Period 1st 2nd >3rd Total # of VTs 19 10 3 32 **Accepted 9 7 3 19 Acceptance (%) 47 70 100 56

Figure 4: The procedures of cavity surface preparation and vertical test for RAON. Figure 5: The result of VT for 22 QWR cavities.

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Table 4: VT acceptance rate of HWR. Acceptance criteria is

Q0>2.4E9 @6.5 MV/m. Period Before Aug Sept Oct Nov Total # of VTs 12 11 13 7 *43 **Accepted 5 10 12 7 34 *Since total of 106 cavities are required, production is going on as of 16 Nov. 2020. ** Accepted number is the number of cavities passed at the 1st VT trial. Figure 6: Performance criteria of all QWR cryo-modules.

Figure 8: VT results of HWR cavities. Plot includes the result of Figure 7: Photograph of QWR cryo-modules installed inside bare cavities. All the failed cavities were retreated by SCL3 tunnel. HPR and then the performances are recovered.

Table 3 shows the acceptance history from VTs. With those and field emission from cavities. In June–July, 2020 the adopted processes, the performance of QWR cavities were recipe of surface preparation was optimized as much as 4 obtained as shown in Fig. 5. out of 5 accepted at the VT trials. But in the beginning of All 22 certified cavities were furnished to 22 cryo- August all VT trials of 4 cavities couldn’t be reached to the modules which were passed through the integrated module acceptance target. By checking the treatment procedures test, so-called HT. The major parameters checked were the and parameters, the TOC (total organic contents) of the HPR thermal load, the frequency deviation against helium pres- water was found to be contaminated. During the mid-June, sure fluctuation, df/dp, where f is frequency in MHz and an exceptional heavy rains made the water supply contami- p is helium pressure in mbar, the Lorentz-force-detuning nated. After a systematic control of water quality, the accep- (LFD) in Hz/(MV/m)2. Figure 6 shows the whole module’s tance rate shows no severity as seen from Table 4 and the performance. VT performance of HWR cavities is shown in Fig. 8. At present, all 22 QWR cryo-modules are installed in The production of HWR cryo-modules are on the way, the tunnel, as shown in Fig. 7. The connection activities and will be completed until May in next year. Figure 9 for control cables, high power transmission and cryogenic shows the fabrication process of type-B HWR cryo-mod- cooling lines are on the way, and then the pre-commission- ule. At present, only two type-A HWR cryo-modules are ing of each components and the commissioning of the inte- tested successfully, compatible with nominal values such grated linac of SCL3 will begin in 2021. as thermal load of 14.1 W @6.5 MV/m at 2 K and df/dp of HWR Cryo-module 15 Hz/mbar. The cryo-modules will be installed in the The production of HWR cavities is going on smoothly, tunnel one by one from next January. All the installation after a half year’s R&D with the first 3 production cavities. works of SCL3 and preparation of pre-commissioning will The main issues were also multipaction from power coupler be completed by May next year. All cryo-modules in SCL3

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in between coupler port and cavity cell made during BCP. The effects of these imperfect surface treatment5) will be analysed and assessed with successive VT trial. The VT data obtained so far for the prototype cavities is shown in Fig. 10. The pressing mould of SSR2 cavity is in the process of optimization. The prototyping R&D of SSR will be completed by middle of 2021 and the call for tender for the production will follow. Cryogenic plant and cold distribution system To operate and test stably the super-conducting accel- Figure 9: Assembly process of type-B HWR Cryo-module. erator systems, two cryo-plants with a helium recovery system, a cryogenic distribution system, two super-conducting RF test facilities are necessary. For the linacs and IF separator, helium flows of 2.05 K, 4.5 K, and 35 K are required. By using two independent cryo-plants and a cryogenic distribution system, the required helium flows with given conditions are stably supplied and distributed to each superconducting system. A smaller cryo-plant (SCL3 cryo- plant) is for cooling SCL3 and has the cooling capacity of 4.2 kW as the equivalent heat load @ 4.5 K. A larger cryo- plant (SCL2 cryo-plant) is for cooling mainly SCL2 and IF Figure 10: VT data of SSR1 prototype cavities. The #5 separator (LTS and HTS magnets) and has the cooling ca- (TRIUMF) fails absolutely due to particle contamina- pacity of 13.5 kW as the equivalent heat load @ 4.5 K. The tion. Pattern of #1 (Vitzro) looks well, but Q0 is too low, it seems to be caused by high surface resistance. SCL2 cryo-plant is also being designed to supply the helium flows to SCL3, SCL2, and IF separator, simultaneously will be cooled down from next June. After few months of when the RAON accelerator operates with low RF powers pre-commissioning, SCL3 with ISOL will be integrated A cryogenic distribution system is to supply gaseous and and commissioned late 2021. liquid helium to superconducting systems and recover gas- Single spoke resonators for SCL2 eous helium back. The cryogenic distribution system has a The SSR1 and SSR2 for SCL21 and SCL22 respectively dedicated valve box for each superconducting system and are on the prototyping process before the mass production. it should satisfy the thermal and hydraulic requirements. Five prototype SSR1 cavities were fabricated and now in Prototype tests of QWR and HWR valve boxes for the ther- the VT phase. One bare cavity and one jacketed cavity get mal and hydraulic checks were performed and QWR valve the nominal values of Eacc>8.5 MV/m and Q>3.2e9. Now boxes are being installed in SCL3. All procedures related to the facilities for BCP and HPR are improved and the im- installation, connection, and inspection are reviewed. In or- proved coupler control mechanism are adopted for the re- der to minimize risks for SCL3 commissioning and to opti- producibility of better surface preparation. The main cause mize control logic of cryogenic distribution and supercon- of VT failure is the field emission (FE). It is well known ducting systems, simulation models with control logic are that the FE is arisen from foreign particles and/or irregu- being developed and all designed components and control lar scratches on the cavity surface. For the SSR cavities logic will be checked by a dynamic simulation. Ecosim- of RAON has shown two possibilities. One is the non- Pro™ is being used to develop all models of cryogenic and accessible zone for HPR spray because of a deep concave superconducting systems. All control schemes are being of balloon and narrow joint area of spoke and cavity cell. demonstrated in EcosimPro™ and checked the functions The second is a deep comb-shaped scratches at the surface at transient and steady state conditions. Communication

298 J. Particle Accelerator Society of Japan, Vol. 17, No. 4, 2020 RAON, Korean Heavy Ion Accelerator Facility between EcosimPro™ and EPICS, EPICS and PLC system, hazard. RAON is planned to use 70 MeV proton cyclotron and EcosimPro™ and PLC system will be demonstrated of maximum ~1 mA of current for a target driver, a surface to check control programs in PLC system and EPICS. Two ion source and laser ion source for target/ion source, and plants and cold distribution systems are on the construction mass separators and transport lines to transport ion beams. phase at present, and then the plant for SCL3 will be com- Also, in order to accelerate ion beams from ISOL, there is missioned on the coming February and the 13.5 kW plant an EBIS charge breeder system including RFQ CB (cool- for SCL2 will begin service from next June. er/buncher) in ISOL beam line in Fig. 11. ISOL target system is planned to develop up to 10 kW of RI Production System Uranium target within RISP and also preparing for deploy- ISOL and IF (in-flight fragment separation) methods ments of non-fissile material targets for commissioning and have been played major roles to shine on the uncharted area early stage of RI production runs. After obtaining experi- 6, 7) of nuclear landscape. RAON as a rare isotope production ences of LaC2 fabrication in the early stage of target devel- facility designed to accelerate ion beams produced from opment, with a help from CERN ISOLDE group,9) 5 kW of ISOL to IF to search for wider spectra of isotope produc- Uranium target (UCx) are already developed and 10 kW tion.8) ISOL system is being installed and stable ion beam UCx target will be fabricated by 2021. The development of test will start 2021. IF system, which is not discussed here SIS and RILIS method for target ion source (TIS) has been in this paper, is being fabricated and will start to assemble demonstrated in 2016 by reporting on the extraction of sta- and install on 2021. ble Na, K, Cs, Sn isotopes successfully.10) For commission- ISOL system ing and early operation of ISOL system, 2 kW of non-fissile The RAON ISOL system designed to utilize high power material targets, such as SiC, BN, etc., are prepared by ap- target system capable with full remote handling system for plying the same fabrication process as UCx and tested us- target change, high radiation protection, minimization of ing proton beam at KOMAC in Korea as shown in Fig. 12.

Figure 11: Layout of ISOL system in installation is shown with photographs.

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Employment of the target and target ion source using 70 kW of beam is a very complex matter considering radiation hard environments on ISOL target including adjacent auxiliary systems. Radiation hard environments require an isolated bunker and it will have key components including the front-end beam line and the target/ion source system, beam diagnostic systems including interface with cyclotron beam line. These systems in Fig. 13 are divided into RI beam, target/ion and proton module system to remotely handle for operation and maintenance purposes.

Experiment System

RAON experiment system11) is designed for pure and applied sciences using rare isotope (RI) beams. The facilities for pure sciences are to be designed to carry out experiments for few keV to hundreds MeV experiments for fundamental science researches. They consist of the recoil-spectrometer KoBRA for nuclear structure and nuclear astrophysics study, multi-purpose large acceptance spectrometer LAMPS for an intermediate energy of hun- Figure 12: Fabrication process and composite analysis of 5 kW UCx target (Up) and test results of SiC target (Down) dreds MeV/u to study symmetry energy and nuclear mat- are shown. ter, MMS (mass measurement system) using MR-TOF-MS for high precision mass measurement, and CLS (collinear laser spectroscopy) for an investigation of fundamental properties of nuclei. Applied sciences include BIS for a bio- medical facility and NDPS for neutron science facility, µSR systems for material science and applications. KoBRA is about to ﬁnish installation on 2020 and will be beam commissioned on 2021. Other experimental systems are being constructed and will be commissioned on 2022. KoBRA Facility KoBRA is going to be a main experimental facility for low-energy nuclear science at RAON accelerator complex. The stable and radioactive ion beams up to 30 MeV/u are delivered from superconducting linear accelerator SCL3 at RAON. Various interesting topics of nuclear astrophysics, nuclear structure of exotic nuclei, study of very heavy nuclei, so on, are expected to be studied with KoBRA spectrometer. As an addition to these, KoBRA will be fa- cilitated as an isotope production system via a low-energy in-ﬂight method with high-intense stable ion beams with Figure 13: The module systems inside ISOL bunker (Up) and re- high mass resolving power (m/Δ m ~300), large angular ac- mote handling system to operate modules (Down) with ceptance (>100 mSr), high-resolution by dispersion match- interface are shown. ing technique.

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Figure 14: Schematic drawing of KoBRA recoil spectrometer is shown and Wien ﬁlter to be added when stage-2 ready. The small panel is expected RI beam intensities using 40Ar (100 pnA) beam with 0.3 mm-thick 12C target (LISE++ with EPAX2).

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