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「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 ac- celerator 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 J. Particle Accelerator Society of Japan, Vol. 17, No. 4, 2020 293 M. KWON, et al. 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, 294 J. Particle Accelerator Society of Japan, Vol. 17, No. 4, 2020 RAON, Korean Heavy Ion Accelerator Facility 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.

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