Archive | 2019
Long-term 0peration with Beam and Cavity Performance Degradation in Compact-ERL Main Linac at KEK
Abstract
We developed ERL main linac cryomodule for Compact ERL (cERL) in KEK. The module consists of two 9-cell 1.3 GHz superconducting cavities. After construction of cERL recirculation loop, beam operation was started in 2013 Dec. First electron beam of 20 MeV successfully passed the main linac cavities [1]. Beam current increased step by step and currently reached to 1 mA (CW) [2]. Energy recovery has successfully achieved. However, field emission was one of the problems for long term operation [3]. Therefore, the performance of the SRF cavities through long term beam operation has been investigated. In this paper, details of SRF beam operation, degradation, applied recovery methods are described. INTRODUCTION FOR COMPACT ERL Compact ERL Accelerator Compact ERL (cERL) [1] is a test facility, which was constructed on the ERL Test Facility in KEK. Its aim was to demonstrate technologies needed for future multi GeV class ERL light source [4]. Recently, the future light source project in KEK was shifted to the high-performance ring accelerator. KEK directorates kept the importance of the R&D for industrial application based on ERL technologies like EUV-lithography [5] and so on. R&D of cERL was shifted to the industrial application from 2017. cERL consists of 500 kV DC photocathode gun, which made high charge and low emittance electron beam, the injector cavities, the main linac cavity, which made energy recovery, recirculation loop and the beam dump. Detailed design beam parameters are shown in Table 1. Table 1: Design Parameters of the cERL Nominal beam energy 35 MeV Nominal injection energy 5 MeV Beam current 10 mA (initial goal) 100 mA (final goal) Normalized emittance 0.1 – 1 mm-mrad Bunch length (bunch compressed) 1-3 ps (usual) 100 fs (short bunch) Main Linac Cryomodule The left of Fig. 1 shows a schematic view of the main linac cryomodule [3], which contains two 9-cell KEK ERL model-2 cavities [6] mounted with He jackets. In order to achieve strong HOM damping for high-current ERL, iris diameter is increased to 80 mm. Epeak/Eacc becomes high and to be 3.0. Beampipe-type ferrite HOM absorbers [7] are connected at both sides of cavities, to strongly damp HOMs. The HOM absorbers are placed on 80 K region. Coaxial input couplers [8] with double ceramic windows feed RF power to the cavities. Frequency tuners [9] control cavity resonant frequencies. Cooling pipes of 80K, 5K and 2K are extended throughout the cryomodule. The 80K line was cooled by Nitrogen, and 5K and 2K lines were cooled by Helium. After filling with 4K liquid He, insides of the He jackets were pumped down and the cavities were cooled down to 2K. RF amplitude and phase on the main linac cavities are stabilized by the digital feedback system. RF stability of 0.01% R.M.S. for amplitude and 0.01 degree R.M.S. for phase were achieved [10]. Unfortunately, main linac cavity performance was not so good. Severe field emission was observed from low fields, for both cavities [3]. Operation voltage was limited to 8.6 MV for each cavity, to avoid the problem caused by the heavy radiation. Therefore operation energy of cERL beam was limited to 20 MeV; 2.9 MeV at injector part and 8.6 + 8.6 MeV at main linac part. Figure 1: Schematic view of ERL main linac cryomodule (left) and the one placed inside the cERL radiation shielding room (right). Beam Operation History until 2016 We briefly summarize our beam operation history. After first beam commissioning at December of 2013, we did the energy recovery with 6.5 \uf06dA. CW beam in 2nd and 3rd beam operation phase. In this phase, we learned the careful beam tuning without beam loss [11]. We started 100 \uf06dA current beam operation at 4th and 5th phase in 2015. During summer shutdown in 2014, we installed Laser Compton Scattering (LCS) beamline to demonstrate the future high-flux gamma-ray source and advanced X-ray imaging technology [12]. By using this high beam current, we successfully obtain clear narrow-band X-ray image come from LCS [13]. During summer shutdown in 2015, we upgraded the DC Gun and added the radiation shield to be operated for 1 mA energy recovery condition. In 2016, we started the 6th phase beam operation for 7 weeks. Finally, energy recovery has successfully achieved with about 1 mA (CW) beam [2]. Q-values of cavities were several times measured until 2016. Results are shown in Fig. 2. Main linac 1 (ML1) and Main linac 2 (ML2) represent the KEK-ERL model-2 ___________________________________________ † [email protected] 29th Linear Accelerator Conf. LINAC2018, Beijing, China JACoW Publishing ISBN: 978-3-95450-194-6 ISSN: 2226-0366 doi:10.18429/JACoW-LINAC2018-THPO008 Electron Accelerators and Applications Energy recovery linacs THPO008 695 Co nt en tf ro m th is w or k m ay be us ed un de rt he te rm so ft he CC BY 3. 0 lic en ce (© 20 18 ). A ny di str ib ut io n of th is w or k m us tm ai nt ai n at tri bu tio n to th e au th or (s ), tit le of th e w or k, pu bl ish er ,a nd D O I.