The design and fabrication of 81.25 MHz RFQ for Low Energy Accelerator Facility

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To provide high shunt impendence with low power losses, an 81.25 MHz continuous wave (CW) radio frequency quadrupole (RFQ) accelerator has been designed and machined as parts of the Low Energy Accelerator Facility (LEAF). In this paper, the mechanical structure and the main processing technology of the RFQ cavities are described according to the physical and geometric parameters requirements of the RFQ. The fabrication of the RFQ has been completed and the test results agree well with the design requirements. The RFQ accelerator will work in Institute of Modern Physics, Chinese Academy of Sciences in 2018.

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Nội dung Text: The design and fabrication of 81.25 MHz RFQ for Low Energy Accelerator Facility

Nuclear Engineering and Technology 51 (2019) 556e560 Contents lists available at ScienceDirect Nuclear Engineering and Technology journal homepage: www.elsevier.com/locate/net Original Article The design and fabrication of 81.25 MHz RFQ for Low Energy Accelerator Facility* Bo Zhao a, b, Shuping Chen a, *, Tieming Zhu b, Fengfeng Wang b, Xiaofeng Jin b, Chenxing Li b, Wei Ma b, Bin Zhang b a Lanzhou University of Technology, 287 Langongping Road, Lanzhou 730050, China b Institute of Modern Physics, Chinese Academy of Sciences, 509 Nanchang Road, Lanzhou 730000, China a r t i c l e i n f o a b s t r a c t Article history: To provide high shunt impendence with low power losses, an 81.25 MHz continuous wave (CW) radio Received 2 February 2018 frequency quadrupole (RFQ) accelerator has been designed and machined as parts of the Low Energy Received in revised form Accelerator Facility (LEAF). In this paper, the mechanical structure and the main processing technology of 10 September 2018 the RFQ cavities are described according to the physical and geometric parameters requirements of the Accepted 4 October 2018 RFQ. The fabrication of the RFQ has been completed and the test results agree well with the design Available online 9 October 2018 requirements. The RFQ accelerator will work in Institute of Modern Physics, Chinese Academy of Sciences in 2018. Keywords: RFQ © 2018 Korean Nuclear Society, Published by Elsevier Korea LLC. This is an open access article under the Design CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Fabrication Temperature Test 1. Introduction change the power source frequency to tune the cavity. The method of tuning RFQ is based on transmission line model; a tuning code The low energy high intensity accelerator is one of the hottest based on transmission line model [4] has developed, and its per- research points in the accelerator around the world. As the core formance has been proved in the C-ADS project [5] of IMP. RFQ device of the low Energy highly-charged ion Accelerator Facility accelerator consists of six cavities, twenty-four p-mode stabilizing (LEAF) undertaken by the Institute of Modern Physics, Chinese loops (PISLs), forty-eight tuners, two couplers and a series of Academy of Sciences, a high power high intensity heavy ion Radio components. The layout of the RFQ is shown in Fig. 2. The me- Frequency Quadrupole (RFQ) Accelerator has been designed. This chanical design, manufacture, assembly, and tests for the RFQ are device will provide unprecedented conditions of low energy highly described in this paper. charged and high intensity heavy ion beam for the basic research of nuclear science. 2. Parameters of RFQ This beam line consists of an ECR ion source, a low energy beam transport line (LEBT), a CW radio frequency quadrupole accelerator The RFQ is one of the core equipments in the LEAF project; its (RFQ) of 81.25 MHz, a medium energy beam transport line (MEBT) structure directly inﬂuences the beam injection quality from RFQ to and an experimental platform, as shown in Fig. 1. The RFQ as a CW the MEBT, as shown in Fig. 1. The RFQ cavity is a typical four-vane injector will accelerate the ion species from proton to uranium from structure. The operating frequency of the RFQ cavity is 14 keV/u up to 0.5MeV/u [1,2]; it employs sinusoidal modulation to 81.25 MHz, the vane voltage is 70 kV, and main parameters of the obtain high transmission efﬁciency and stable operation [3]. Based RFQ are summarized in Table 1 [1,2]. on cooling, LLRF system could follow the RFQ resonant frequency to 3. Mechanical design and 3D thermomechanical analysis * Work supported by the NSF (contract No. 11427904). An octagonal type cavity is applied to meet the need of weld- * Corresponding author. ability. The mechanical structure of the cavity is shown in Fig. 3. E-mail address: chensp@lut.cn (S. Chen). This cavity includes walls, vanes, p-mode stabilizing loops (PISLs), https://doi.org/10.1016/j.net.2018.10.003 1738-5733/© 2018 Korean Nuclear Society, Published by Elsevier Korea LLC. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/ licenses/by-nc-nd/4.0/). B. Zhao et al. / Nuclear Engineering and Technology 51 (2019) 556e560 557 Fig. 1. Beam layout of LEAF project. Fig. 4. The ﬂow distribution of RFQ cavity. entrance and exit end covers, coupler ports and vacuum pump ports, tuner ports and monitoring ports, cooling water channels, etc. The coupler port is connected with a coupler in order to realize power feed-in from the power source through feed tube. The vac- uum port is located at the wall of the cavity, with a cryogenic pump; Fig. 2. Layout of a single RFQ cavity. the opening port of the wall adopts fence structure to mitigate the RF detuning. The tuner is used to correct and compensate the cavity Table 1 frequency shift so that the ﬁnal resonant frequency is stabilized Main parameters of the cavity. near the design frequency; when the depth of the tuner is certain, Parameters Values the tuner is connected to the tuner port with ﬂange. The water- cooling method is used to tune the cavity online. The tuner is Frequency (MHz) 81.25 Input energy (keV/u) 14 cooled by contact with separate water-cooled plate and the cavity is Output energy (MeV/u) 0.5 cooled by water via water-cooling channels. Inter-vane voltage (kV) 70 Kilpatrick factor 1.55 Beam current (emA) 2 Input transversal emittance (mm.mrad) 0.3 p Input longitudinal emittance (mm.mrad) 0.098 p Output transversal emittance (mm.mrad) 0.293 p Output longitudinal emittance (mm.mrad) 0.098 p Transmission efﬁciency (%) 97.2 Power loss (kW) 53.2 Total length of the vane (mm) 5946.92 Fig. 3. The mechanical structure of cavity. Fig. 5. The ﬂuid model in ANSYS. 558 B. Zhao et al. / Nuclear Engineering and Technology 51 (2019) 556e560 Table 2 Main parameters of ﬂuid. w1~w8 V (3/6/9/12) The other v PISL Velocity(m/s) 2.0 1.5 1.5 2.0 Temperature( C) 20 20 20 20 Diameter (mm) 10 10 12 8 Reynolds number 19797.87 14843.9 17812.68 15833.5 Fig. 8. Changes of the cavity structural caused by the changes of the water temperature. Fig. 6. Temperature distribution of the RFQ cavity. or turbulent. y, r, m respectively are ﬂuid ﬂow velocity, ﬂuid density and viscosity coefﬁcient, d is the channel diameter. The port ﬂanges are made of stainless steel; other parts of the cavity are made of oxygen-free copper. Under operating condition, the rising temperature of the cavity will cause the deformation and rnd Re ¼ stress change. The yield stress after oxygen-free copper welding is m 60e70 MPa; the cavity should avoid permanent deformation. The Calculated result shows that Re > 4000, the turbulent ﬂow is in cavity model with the water cooling channels was built using cooling channels. The k-epsilon model of high Reynolds number is ANSYS. The ﬂow distribution of the RFQ cavity is shown in Fig. 4. adopted considering the ﬂuid ﬂow. Both ambient temperature and The ﬂuid model and grid division are established based on the cavity temperature are set to 20 C; the convective heat transfer cavity cooling channels structure, as shown in Fig. 5. coefﬁcient with air is 10 W/(m^ 2*K). The main parameters of ﬂuid The power loss of the whole cavity was calculated through the are indicated in Table 2. MWS with 53.2 kW [2]. We obtain the actual loss by multiplying Fig. 6 shows the cavity temperature distribution at the 20 C with an experienced coefﬁcient of 1.2 [2]. The distribution of power water temperature. Fig. 7 shows the deformation and stress dis- loss on the cavity internal surfaces is simulated with the High tribution of the cavity. The maximum deformation at vane tip is Frequency Structure Simulator (HFSS), which will be loaded into 0.0032 mm in the micrometer range. The maximum stress is the inner walls of the RFQ cavity as the boundary condition. It is 3.45 MPa below the yield strength. As shown in Fig. 8, the rela- very rare to study the cooling of RFQ combination with ﬂuid [6e12]. tionship between the maximum temperature of the cavity and The RFQ thermal response is obtained by coupled heat transfer in water temperature is linear; the maximum deformation and the paper. maximum stress of the cavity will change with the change of water Reynolds number is the characterization for the dimensionless temperature. The closer the water temperature is to 20 C, the less number of ﬂuid ﬂow situation, which is used to determine laminar stress changes. Fig. 7. Deformation (a) of RFQ cavity and stress (b) of vane tip and deformation (c) of vane tip. B. Zhao et al. / Nuclear Engineering and Technology 51 (2019) 556e560 559 Fig. 9. Soldering ﬂux gaps on vane. Fig. 11. The ﬁnal depth of the tuners. 4. Fabrication and cavity test bolted together by the stainless steel connecting block. 4.1. Fabrication 4.2. Cavity test Oxygen-free copper is selected for cavity material in order to meet high conductivity and high thermal conductivity. 304 stain- To make sure that no leak in any part of the RFQ cavity, the less steel is adopted as the ﬂanges material to guarantee sealing. vacuum leak testing must be carried out after welding is completed. Due to the high power and the beam loss, the cavity temperature is The leakage rate is less than 1 109 Pal/s. When leak testing is increasing when the RFQ is in operation, and so the water cooling completed, the cavity must pass the low power test. Fig. 10 shows structure is necessary for the RFQ cavity. According to the param- the test bench. Each cavity is equipped with eight tuners where two eters optimized by the CST, the inscribed circle diameter of the tuners in per quadrant. A total of 48 tuners are installed in full octagonal structure for the cavity is 721 mm; there are two cooling cavity. The nominal depth value of the tuner is 25 mm [1]; the ﬁnal channels on each wall and three cooling channels on each vane, as depth of each group of tuners is plotted in Fig. 11. The results of 4 shown in Fig. 4. In consideration of the installation space of the full cavity tests are revealed in Table 3 [3]. The test frequency is near ﬂange bolts, the length of transition tube between the ﬂanges and the design frequency and the measured Q factor 16230 is about 90% the cavity is 36 mm. Meanwhile, considering the assembly needs of the simulated value 17963. The ﬁnal assembly of the RFQ online for the related components of other systems, CF35, CF150, CF200, have been completed，as shown in Fig. 12. and CF100 ﬂanges are used to connect with the probes, coupler, vacuum pump and tuner, respectively; the outer transition component was composed of the ﬂange and transition tub. 5. Conclusions The RFQ accelerator consists of six cavities. A single cavity has four p-mode stabilizing loops (PISLs), four vanes, four walls, and An 81.25 MHz RFQ cavity has been designed and processed for the outer transition components. The nickel-plated ﬂange and the Low Energy Accelerator Facility. The test work has been copper tube are welded together to form the outer transition completed; and the test results of the RFQ cavity agree very well component. Soldering ﬂux gaps were made by milling on two side with the design requirements for frequency and Q factor. At pre- surfaces of the vane, as shown in Fig. 9. Before the cavity is welding sent, the assembled and tested of the cavity in the Institute of in the brazing furnace, the PISLs, vanes, walls and the outer tran- Modern Physics, Chinese Academy of Sciences is completed; and it sition components are assembled. The two adjacent cavities are will be put into work in 2018. Fig. 10. Test bench (a) of each cavity test and (b) of full cavity test. 560 B. Zhao et al. / Nuclear Engineering and Technology 51 (2019) 556e560 Table 3 The measured quadrupole mode frequencies and Q factors. Test 1 Test 2 Test 3 Test 4 Frequency Measured value (MHz) 81.189 81.245 81.227 81.254 Simulated value (MHz) 81.25 Q factor Measured value 13520 13150 14320 16230 Simulated value 17963 Acknowledgements One of the authors Bo Zhao would express her sincere acknowledgement to the colleagues in IMP (Tieming Zhu, Fengfeng Wang, Xiaofeng Jin, Wei Ma, etc.). This work is supported by Na- tional Natural Science Foundation of China (11427904). References [1] Wei Ma, Liang Lu, et al., Design of an 81.25 MHz continuous-wave radio- frequency quadrupole accelerator for low energy accelerator facility, Nucl. Instrum. Methods Phys. Res. (2017) 130e135. [2] Wei Ma, Liang Lu, et al., Three-dimensional multi-physics analysis and commissioning frequency tuning strategy of a radio-frequency quadrupole accelerator, Nucl. Instrum. Methods Phys. Res. (2017) 190e195. [3] Wei Ma, Study on the Low Energy High Current High Charge State RFQ, University of Chinese Academy of Sciences, 2018. PhD Dissertation. [4] G. Pan, Research on Tuning Four-vane RFQ, University of Chinese Academy of Sciences, 2013. Dissertation of Master Degree. [5] Z.L. Zhang, et al., Development of the injector II RFQ for China ADS project, in: Proceedings of IPAC2014, Dresden, Germany, 2014, pp. 3280e3282. [6] Zhouli Zhang, et al., Design of a four-vane RFQ for China ADS project, in: Proceedings of LINAC, Tel-Aviv, Israel, 2012, pp. 942e944. [7] R. Tiede, et al., A Coupled RFQ-ih-dtlcavity for Franz: a Challenge for RF Technology and Beam Dynamics, Proceedings of HB, Malmo €, Sweden, 2016, pp. 404e408. [8] Jing Wang, Jian-Long Huang, et al., Frequency tuning with RFQ temperature in China ADS Injector II, Chin. Phys. C 40 (3) (2016), 037003-1-037003-5. [9] Jing Wang, Jian-Long Huang, et al., Multi-physics analysis of the RFQ for Injector II Scheme of C-ADS driver linac, Chin. Phys. C 38 (10) (2014), 107005- 1-107005-5. [10] Hua-Fu OUYANG, Y.A.O. Yuan, Thermal analysis of CSNS RFQ, High Energy Phys. Nucl. Phys. 31 (12) (2007) 1116e1121. [11] A. Pepato et al. Engineering design and ﬁrst prototype tests of the IFMIF- EVEDA RFQ. Proceedings of IPAC’10, Kyoto, Japan. pp 600-602. [12] S.C. Joshi, N.K. Sharma, et al., 3D thermal-ﬂuid coupled analysis for 350 MHz RFQ for indian SNS programme, in: Proceedings of APAC, 2004, pp. 314e316. Gyeongju, Korea. Fig. 12. The ﬁnal assembly.